Bacterial Chemotaxis Introductory article

Michael Eisenbach, Weizmann Institute of Science, Rehovot, Israel Article Contents

. Introduction
Bacterial chemotaxis is the phenomenon in which bacteria actively modulate their . Varieties of Bacterial Behaviour
direction of movement so as to approach chemoattractants (favourable, usually nutritious . Bacterial Flagella
chemicals) and avoid chemorepellents (unfavourable, usually noxious chemicals). . The Flagellar Motor
. Link Between Flagellar Rotation and the Bacterial
Swimming Behaviour
Introduction . Genes Controlling Chemotaxis
. Signal Transduction Pathways of Chemotaxis
The phenomenon of bacterial chemotaxis was discovered
. Adaptation
by W. Engelmann and W. Pfeffer in the 1880s. Thorough
investigation of the phenomenon started in the 1960s with
the quantitative, genetic and biochemical studies of J.
Adler. 30 body lengths per second (20–60 mm s 2 1). As shown in
Bacterial populations may encounter a large spectrum of Table 1, there are various types of flagellar motility, which
environments during their life cycles. Due to their small depend on the location and number of flagella as well as on
size and relative simplicity, their ability to adjust the the species. In some species (e.g. Pseudomonas spp.,
environment to their needs is very limited. Instead, they Spirillum spp., Chromatium spp. and Halobacteria), the
apparently adopted a strategy of moving from one cells swim forward and backward, and reorientation
environment to another. Chemotaxis as well as other types appears to be passive by brownian motion. In other species
of taxis (e.g. thermotaxis and phototaxis; see below) thus (e.g. Escherichia coli, Salmonella typhimurium, Sinorhizo-
enable bacteria to approach (and remain in) beneficial bium meliloti, Rhodobacter sphaeroides, and Agrobacter-
environments and escape from hostile ones. (This phe- ium tumefaciens), the cells move in a rather straight line
nomenon, taxis, is prevalent not only in bacteria but also in and, occasionally, actively reorient themselves. It should
many other cell types and unicellular organisms that are be noted that combinations of the varieties mentioned in
capable of movement.) Chemotaxis also serves as a means Table 1 are also possible. For example, Vibrio alginolyticus
of cell-to-cell communication and cell recruitment under cells each have a sheathed flagellum at one pole that pushes
stress conditions. It is therefore not surprising that a very the cells forward or pulls backward. When on the surface of
large number of bacterial species are motile and chemo- a solid medium, the cells produce lateral flagella in addition
tactic. As a matter of fact, most rod-shaped bacteria are to the polar ones. The lateral flagella enable swarming.
motile, independently of their classification (e.g. Gram-
positive or Gram-negative, aerobes or anaerobes, spore Swarming
formers or not, etc.). In contrast, most round bacteria, Flagella are not only swimming tools; they also serve for
cocci, are nonmotile. swarming (Table 1). Swarming is an organized surface
motility of cells in a colony, which depends on massive
flagellation and cell-to-cell communication. This orga-
nized surface translocation has been demonstrated in both
Varieties of Bacterial Behaviour Gram-negative (primarily) and Gram-positive species.
Even bacteria such as E. coli and S. typhimurium with
As detailed below, bacterial species vary from each other in
well-characterized swimming motility, when on a hard
many aspects, including their modes of motility, their
surface, are able to differentiate into filamentous (up to
strategies of response to external stimuli, and the stimuli to
50 mm long), multinucleated, hyperflagellated cells that
which they are sensitive.
translocate together as a colony on the surface. Similar to
swarming bees, the differentiated bacteria in the colony are
Varieties of motility organized in such a way that the outer layer of the colony
moves like a swirl and expands outwardly, and the
Bacteria have acquired a number of motility strategies. evacuated space inside the colony is filled with newly
grown bacteria. The result is fast colony expansion (up to
Swimming  3 mm s 2 1 or 1 cm h 2 1). Quorum sensing appears to be
The most common strategy for motility is movement essential for swarming.
driven by flagellar rotation. By means of their rotation, the
flagella – external organelles that serve as ‘propellers’ –
exert thrust that drives the bacteria relatively quickly, up to

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 Bacterial Chemotaxis

Table 1 Varieties of flagellar motility in bacteria

Flagellation Appearance Species for example Description of motility
A single flagellum at Pseudomonas spp. The flagellum – depending on its direction of
(or near) one of the rotation – pushes or pulls the cell. Consequently,
cell poles the cell goes back and forth
A single flagellum Rhodobacter sphaeroides The flagellum either rotates clockwise or pauses.
roughly in the Consequently the cell swims in a rather straight
middle between the line and occasionally stops for reorientation.
poles During a pause the flagellum relaxes to a coil-like
form whose rotation reorients the cell
A bundle of flagella Chromatium okenii, some The bundle – depending on its direction of rotation
at one of the poles cells of Halobacterium – pushes or pulls the cell. Consequently, the cell
salinarium goes back and forth
A bundle of flagella Some cells of The bundles – depending on their direction of
at each of the two H. salinarium rotation – push or pull the cell. Consequently, the
poles cell goes back and forth or stops
A bundle of flagella Spirillum volutans Forward and backward swimming is carried out in
at each of the two the same manner, only that the bundles flip over
poles in Spirillum when the cell reverses. The helical cell body
spp. rotates in reaction to the rotation of the flagella and
this rotation produces the thrust for motility
5–10 flagella Escherichia coli, Most of the time the flagella rotate
randomly Salmonella typhimurium, counterclockwise and the cell swims in a rather
distributed around Bacillus subtilis straight line (a run). Intermittently, the flagella
the cell rotate clockwise or pause, as a result of which the
cell undergoes a vigorous angular motion (a
tumble)
5–10 flagella Sinorhizobium meliloti Most of the time the flagella rotate
randomly counterclockwise and the cell swims in a rather
distributed around straight line. Occasional changes in the speed of
the cell flagellar rotation cause the cell to turn (without
tumbles)
A polar tuft of 2 Agrobacterium Flagella rotate clockwise or pause; consequently the
flagella + 2–4 tumefaciens cell swims in a rather straight line or turns
lateral flagella

Excessive E. coli, S. typhimurium, Swarming – surface motility in a colony

flagellation around Serratia marcescens,
the cell Proteus mirabilis

One flagellum at one Spirochaetes The periplasmic flagella cause the cell to bend and
end, one or more gyrate. The cells exhibit smooth swimming,
flagella reversals, flexing and pausing. When the flagellar
subterminally at bundles at both cell poles rotate in opposite
each end. All the directions (one pulls and one pushes), the cell
flagella are swims in a rather straight line. When both bundles
contained within switch synchronously, the cell reverses. When
the periplasmic both bundles rotate in the same direction, the cell
space flexes

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 Bacterial Chemotaxis

Gliding without flagella or other visible appendages. The cells of

Other strategies of motility, which do not depend on these strains are rod-shaped and they move, without any
flagella, have also been recognized. Of these, the most apparent shape change, as fast as 25 mm s 2 1. The mechan-
abundant one is gliding motility. Gliding motility is the ism underlying this movement is obscure. It is known,
however, that this kind of motility requires extracellular
movement of an organism on a solid surface with no visible
external organelles for the movement and no shape change. Ca2 1 .
Unlike swarming motility, gliding requires a solid surface
covered with a liquid film. Gliding bacteria can be divided Twitching
into two classes according to their speed and possibly their Twitching is another kind of surface motility; it involves
motility mechanism: slow and fast gliders. intermittent and jerky movement of single bacterial cells or
group of cells in a colony, not necessarily along the long
Slow gliders axis of the cell. Due to the lengthy intermissions without
The most investigated class is the myxobacteria (e.g. movement, the progressive velocity, averaged over time, is
Myxococcus xanthus) – Gram-negative bacteria that live in very low (  2 mm min 2 1). At least in the case of
soil. They glide very slowly (1–20 mm min 2 1). The speed of Pseudomonas aeruginosa, polar pili are responsible for this
gliding within this range depends on the distance between motility. The mechanism involved, however, is not known.
the moving cell and its nearest cell: the greater the distance
the lower the speed. Chain-like aggregates, termed strands Propulsion by actin filaments
and found within the bacterial cell wall, are thought to be A unique mode of motility, first described in 1989, is the
one part of the myxobacterial gliding machinery (yet to be movement of bacteria such as Listeria, Shigella and
discovered). Myxobacteria have two independent motility Ricettsia in host eukaryotic cells. The bacteria use a
systems, ‘adventurous’ and ‘social’, which are genetically continuous actin filament assembly for propulsion in the
and functionally distinct. The adventurous motility is the cytoplasm of the infected host cell. The actin assembly at
motility of cells located more than a cell’s length from any the bacterial surface is asymmetrical, with the filaments
neighbouring cell. It is effective mainly on relatively hard growing like a comet tail at one end of the bacterial cell and
and dry surfaces (such as 1.5% agar). Social motility is pushing the cell in the other direction. Although factors
movement in groups involving continuous reorientation involved in this motility are gradually being revealed, the
and reassociation of the cells in the group. It is mostly mechanism is essentially still obscure.
effective on softer and wetter surfaces (such as 0.3% agar).
Social motility (but not adventurous motility) involves pili,
present in tufts at one or both poles of the cell. Each Varieties of response
motility system is sensitive (though in a different way) to A variety of responses to stimuli are observed among
the local cell density. different bacterial species, even when the compared
bacteria belong to the same type of motility, e.g. flagellar
Fast gliders
motility. Depending on the varieties of flagellar motility
The other class of gliding bacteria involves faster gliders (Table 1), some bacterial species react to changes in the
(1–10 mm s 2 1) such as cyanobacteria and Cytophaga. concentration of chemical stimuli by changing the direc-
(Some of the fast gliders, e.g. Deleya marina, have, tion of flagellar rotation, others by changing the speed of
depending on the conditions, both flagellar and gliding swimming (chemokinesis) or by stopping the rotation.
motility.) Although gliding organelles have not been Generally speaking, when a bacterial cell senses a positive
found, latex beads, artificially attached to Cytophaga cells, stimulus (an increasing chemoattractant gradient or a
were seen to move back and forth at the speed of cell decreasing chemorepellent gradient) it continues to swim
gliding, or, instead, rotate. Recently a carbohydrate- in the same direction. When it senses a negative stimulus (a
secreting organelle was identified in cyanobacteria, Cyto- decreasing chemoattractant gradient or an increasing
phaga and Flexibacter, suggesting that steady secretion of chemorepellent gradient) it ceases to move in the original
slime through this organelle generates the thrust required direction and reorients itself. A few examples are given in
for gliding motility. A recently-discovered, ordered array Table 2.
of parallel fibrils between the peptidoglycan layer and the
outer membrane of cyanobacteria may provide another
mechanism for gliding motility.
Varieties of stimuli
Bacteria respond to a variety of stimuli, including chemical
‘Swimming’ without flagella stimuli. The origin of the chemical stimuli may be the
Many strains of cyanobacteria exhibit gliding motility. environment itself or the neighbouring cells. Table 3 lists a
Those of the marine cyanobacteria Synechococcus exhibit a number of known stimuli and the corresponding beha-
mysterious kind of motility through liquid – ‘swimming’ vioural responses of the bacteria. (Note that in species that

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Table 2 Examples of responses to chemotactic stimuli in bacteria with flagellar motility

Species Response to positive stimuli Response to negative stimuli
Escherichia coli, Salmonella Increased probability of counterclockwise Increased probability of clockwise
typhimurium flagellar rotation. Consequently, runs are rotation and pausing. Conse-
prolonged quently the cell tumbles and re-
orients more frequently
Bacillus subtilis Increased probability of clockwise flagellar Increased probability of counter-
rotation. Consequently, runs are prolonged clockwise rotation. Consequently,
the cell tumbles and reorients
more frequently
Sinorhizobium meliloti Increased speed of flagellar rotation. Decreased speed of flagellar rota-
Consequently, runs are prolonged tion. Consequently the bundle of
rotating flagella separates to indi-
vidual filaments rotating at differ-
ent speeds, and the cell turns
Rhodobacter sphaeroides Increased speed and decreased Increased stopping probability.
stopping probability of flagellar Consequently, the cell reorients
rotation. Consequently, runs are itself
prolonged
Azospirillum brasilense Increased speed and decreased reversal Presumably increased reversal
probability of flagellar rotation. probability of flagellar rotation.
Consequently, runs are prolonged Consequently, the cell reorients
itself
Spirochaetes Flagella rotate without pausing, Flagella pause frequently and ex-
resulting in coordinated rotation of the tensively, disrupting the coordi-
two polar bundles. Consequently the cell nated rotation of the two polar
swims in a straight line bundles. Consequently the cell
flexes and pauses

Table 3 Known stimuli and behavioural responses in bacteria been found only in mycoplasma gliding upstream in a
moving fluid. Some behavioural responses (e.g. chemo-
Stimulus Behavioural response taxis, thermotaxis, phototaxis and osmotaxis) apparently
Chemical Chemotaxis share, at least partially, a common molecular mechanism.
Electrical field Galvanotaxis The chemical stimuli for bacteria are diverse, and
Gravity Geotaxis or gravitaxis depend on the habitat in which the bacteria live. Sometimes
Light Phototaxis
a certain stimulant may act as a chemoattractant for one
Magnetic field Magnetotaxis bacterial species and as a chemorepellent for another. A
Moving fluid Rheotaxis few examples are listed in Table 4.
Osmolarity Osmotaxis This entry mainly concentrates on chemotaxis of E. coli
Temperature Thermotaxis and S. typhimurium, the most studied bacterial species.
Touch Thigmotaxis
Common chemical stimuli for E. coli are listed in Table 5.

grow, rather than move, in a certain direction in response Bacterial Flagella

to a stimulus, the suffix is ‘tropism’ instead of ‘taxis’:
chemotropism, thermotropism, etc.) Not every species As mentioned above, flagella are organelles that enable
responds to all stimuli. Thus, while chemotaxis and bacteria to swim in an aqueous solution or swarm on a hard
thermotaxis are probably common in all bacterial species surface (Figure 1). In addition to their role in motility,
capable of movement, magnetotaxis is restricted to species flagella are involved in bacterial colonization, in many
that contain magnetosomes (intracellular structures con- cases they contribute to the bacterial virulence, and they
sisting of a crystal of a magnetic mineral, usually the iron are often targets for antibody response. The term
oxide magnetite, Fe3O4, or the iron sulfide greigite, Fe3S4, ‘flagellum’ (pl. flagella) comes from Latin, meaning a little
surrounded by a membrane), and rheotaxis has thus far whip. Although this term is adequate for the eukaryotic

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Table 4 Stimuli with different functions in different species

Reagent Chemoattractant for Chemorepellent for
Phenol Esherichia coli Salmonella typhimurium
Leucine Bacillus subtilis Escherichia coli, Salmonella typhimurium
Valine Bacillus subtilis Escherichia coli, Salmonella typhimurium
Tryptophan Bacillus subtilis, Chromatium vinosum Escherichia coli, Salmonella typhimurium, Rhodobacter sphaeroides
Acetate Chromatium vinosum Escherichia coli, Salmonella typhimurium, Rhodobacter sphaeroides
Benzoate Pseudomonas putida Escherichia coli

Table 5 Common stimuli for Escherichia coli

Class of stimuli Examples
Chemoattractants
Sugars d-Glucose, d-galactose, d-ribose, d-mannose, maltose
Amino acids l-Serine, l-aspartate, l-alanine
Dipeptides l-Proline-l-leucine, glycine-l-proline
Energy-linked chemicals Oxygen at  0.7 mmol L 2 1
Weak organic bases Trimethylamine

Chemorepellents
Alcohols Ethanol, isopropanol
Polyalcohols Glycerol, ethylene glycol
Hydrophobic amino acids l-Leucine, l-valine
Inorganic ions Co2 1 , Ni2 1
Energy-linked chemicals Oxygen at  1 mmol L 2 1
Weak organic acids Acetate, benzoate
pH Acid, alkali
Others S2 2 , mercaptans (e.g. 2-propanethiol), indole

flagellum, which acts like a whip, it is not adequate (and

essentially misleading) for the bacterial flagellum, which
acts by rotation. Bacterial flagella and eukaryotic flagella
are totally different organelles. Table 6 indicates the main
differences between them, with E. coli and human
spermatozoa representing bacterial and eukaryotic flagel-
la, respectively.

Structure of flagella
Bacterial flagella consist of three major parts (Figure 2): a
basal body, a hook and a filament. Although the structure
of bacterial flagella may vary in some respects between
species and families (e.g. Gram-positive and Gram-
negative bacteria), the main structural aspects are common
to all.

Basal body
The basal body of E. coli and S. typhimurium is composed
of a central rod surrounded by four rings: an M ring (M for
membrane, as this ring is located in the cytoplasmic
Figure 1 Flagella of Escherichia coli observed in transmission electron membrane), an S ring (S for supramembrane, as this ring is
microscope. Bar, 1 mm. located above the cytoplasmic membrane), a P (for

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Table 6 Comparison between eukaryotic and bacterial flagella

Property Sperm flagellum (human) Bacterial flagellum (Escherichia coli)
Diameter (mm) 0.3–1 0.023
Length (mm) 60 10–15
Structure Complex structure of tubules surrounded by an Naked filament consisting of subunits of a single
extension of the cytoplasmic membrane; the protein
flagellum consists of  250 proteins
Function Active beating Passive rotation, driven by a rotary motor embed-
ded in the cytoplasmic membrane
Energy source ATP Proton-motive force across the cytoplasmic mem-
brane

Cap
on which the functional components of the motor are
mounted. The P ring is built from the FlgI protein, and it is
linked by a cylindrical wall to the L ring, built from the
FlgH protein. (The L and P rings are apparently missing in
Gram-positive bacteria.) Another ring, the C ring (C for
cytoplasm), which contains the proteins FliM and FliN, is
attached via the FliG protein to the MS ring from beneath,
on the cytoplasmic side.
Filament
Hook
The hook, built of a single protein – FlgE, is a short (only
130 FlgE subunits,  55 nm long), curved structure that
connects the basal body to the flagellar filament (Figure 2).
Junction It is believed to serve as a flexible joint that converts the
torque, generated by the flagellar motor in the plane of the
cell surface, into a force having both vertical and
Hook
Central channel horizontal components.
Out
Filament
OM
L ring
Bushing The filament – built from  20 000 subunits of a single
P ring PL protein, flagellin (FliC) – is a highly rigid, helical structure,
MotB Periplasm
Rod
10–15 mm long, 23 nm in diameter. It is connected to the
Basal
body S ring (FliF) hook via a short junction composed of two hook-
M ring (FliF) CM associated proteins, HAP1 and HAP3. At the other end
MotA
Cytoplasm
of the filament there is a cap-like structure, composed of the
FliG Motor protein HAP2. The filament can be in a number of helical
Switch FliN
C ring
forms (nine such forms have been observed experimen-
FliM In tally), depending on the conditions. The default physiolo-
gical form is a left-handed helix. It can be converted to one
Figure 2 Escherichia coli or Salmonella typhimurium flagellum. The actual
of the other forms by a mechanical force (for example,
diameters of the rod, L, P, M, S and C rings are  15, 33, 26, 29, 27 and 47
nm, respectively. CM, cytoplasmic membrane; OM, outer membrane; PL, when the direction of flagellar rotation is changed – see
peptidoglycan layer. below) or by changing the pH or the ionic strength of the
suspending medium. The filament is passive and its
rotation is totally dependent on the flagellar motor
peptidoglycan) ring, and an L (for lipopolysaccharide) (Figure 2).
ring. The M and S rings constitute essentially one ring,
composed of a single protein, FliF. The MS ring is a
structural part of the flagellar motor (Figure 2; see below),

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 Bacterial Chemotaxis

Assembly of flagella role is to shift the direction of rotation of the motor

according to signals received from the receptors on the cell
The assembly of flagella is synchronized with the cell cycle surface (see below).
and depends on cell division and growth phase. The first
observable structure is the MS ring (Figure 2). Next the C
ring is assembled, followed by another structure at the Function
centre of the C ring – the ‘export apparatus’ – thought to
function in export of proteins that are necessary for H 1 -driven motors
flagellar assembly. Then, at the other (outward) side, the The flagellar motor can rotate extremely fast, up to 270
rod of the basal body is added, subunits of the P ring are revolutions per second! The driving force of the flagellar
exported to the periplasm and form the P ring, and motor is the proton-motive force across the cytoplasmic
subunits of the L ring are exported to the outer membrane membrane, not ATP. The proton-motive force is produced
and form the L ring. Subsequently the hook is assembled by respiration or, under anaerobic conditions, by the
with the help of a scaffolding protein, FlgD. This protein is proton ATPase at the expense of ATP hydrolysis. The
associated with the tip of the growing hook, and when the inwardly directed proton electrochemical potential drives
hook reaches the desired length, HAP1 replaces FlgD. an influx of  1000 protons per revolution through a
Following the completion of the junction (made of HAP1 proton-conducting channel made of MotA and MotB.
and HAP3), the filament is assembled at its distal end in a MotB anchors MotA to the peptidoglycan layer. It is not
process that requires the cap HAP2. The proteins that are known how the flux of protons through the proton channel
the building stones of the flagellum are synthesized within actually rotates the motor, but the mechanism appears to
the cell. They are then pushed outward by the export involve electrostatic interactions and to act in steps (i.e. the
apparatus (probably at the expense of adenosine tripho- flagellar motor is apparently a stepping motor). The
sphate (ATP)) through the central channel of the flagellum. molecular mechanism by which the switch reverses the
The sequence and timing of export of the proteins involved direction of rotation of the motor is also not known.
in flagellar assembly are tightly controlled. However, the
mechanisms underlying this control and determining the
length of the hook and the filament are poorly understood.
It is believed that the expression of flagellin is regulated by
an anti-sigma factor, FlgM. This protein is apparently Na 1 -driven motors
secreted from the cell through the central channel of the A few species possess flagellar motors that are driven by a
flagellum and then, depending on the conditions, accumu- flux of Na 1 ions. These include alkalophilic Bacilii and
lates within the cell. As FlgM accumulates in the cell, it Vibrio species. Interestingly, under certain conditions,
binds to the sigma factor, FliA, and consequently the Vibrio can possess two types of flagella, each driven by a
flagellin expression is suppressed. Interestingly, similarities different ion: lateral flagella driven by a flux of protons, and
have been found between some proteins involved in the polar flagella driven by a flux of Na 1 ions. Na 1 -driven
flagellar export apparatus and type III secretion proteins motors can rotate even faster than proton-driven motors,
involved in export of virulence factors. Furthermore, the up to 600 revolutions per second! The Na 1 ions flow
recently discovered supramolecular structure of the type through Na 1 -conducting channels, made of the MotX
III secretion machinery somewhat resembles the structure and MotY proteins or the PomA and PomB proteins.
of the flagellar basal body.

The Flagellar Motor Functional states of the motor

Structure The flagella of bacteria such as E. coli and S. typhimurium
can rotate counterclockwise and clockwise (the direction of
Like any other electric motor, the flagellar motor contains rotation defined for a flagellum viewed from its distal end
a rotor and a stator. The rotor is built from the MS ring towards the bacterial cell), and they can also pause. A
(FliF) and FliG, and possibly also FliM and FliN. The pause seems to result from a futile switching attempt from
stator is built from the proteins MotA and MotB (Figure 2). counterclockwise to clockwise. Under nonstimulated
The drive shaft of the motor, termed the rod, is built from conditions, the flagella rotate mostly counterclockwise
the proteins FlgB, FlgC, FlgF and FlgG. The rod is with brief intermissions of clockwise rotation and pauses.
surrounded and held by the L and P rings, probably serving Different flagella on a given cell seem to be independent of
as a bushing. The helical propeller is the filament, and the each other with respect to their direction of rotation under
universal joint that connects it to the rod of the motor is the nonstimulated conditions: they reverse and pause asyn-
hook. The motor also has a gearshift, termed a switch. Its chronously.

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 Bacterial Chemotaxis

Link Between Flagellar Rotation and the destabilizes the left-handed helix. Consequently, the
flagella undergo a transition from a left-handed helix to a
Bacterial Swimming Behaviour right-handed one, and the transition propagates from the
flagellar junction with the cell body towards the distal end
Modes of swimming behaviour of the filament. However, because the periods of clockwise
Bacteria such as E. coli and S. typhimurium have two main rotation are relatively short and because of the occasional
swimming patterns: smooth swimming in a rather straight pauses, the transformation from left- to right-handed helix
line (a run) and a brief but abrupt turning motion (a is usually not complete. The consequence is that some
tumble). In the absence of stimuli the tumbles usually occur flagella have segments of opposite handedness within the
once every 1–5 s (depending on the bacterial strain). very same filament, resulting in a large angle between the
Consequently, the bacterial cells execute a random walk, segments (Figure 3a). This angle, which provides angular
composed of runs and tumbles with essentially no net motion to the bacterial cell, prevents bundle formation and
vectorial movement (Figure 3a). forces each flagellum to act separately (each exerts force in
a different direction), thus causing tumbling. In view of the
A run lack of synchrony between the flagella, tumbling possibly
occurs when only a few flagella on a given cell reverse to
The run is the consequence of counterclockwise rotation of
clockwise rotation. It is also possible that nearby flagella
the flagella. Because of the flagellar left-handed helicity,
mutually affect each other, but this is still an open question.
counterclockwise rotation exerts a pushing force on the
cell. Since the flagella around the cell have different lengths
and their distribution is not symmetric, the net force is not Swimming behaviour under stimulated
zero. Consequently the cell moves in the direction of the net
force and, due to the viscous drag of the medium, the
conditions
flagella are swept to the rear of the cell, amplify the net Positive stimulation decreases the probability of clockwise
force in the direction of movement, and form a left-handed rotation, whereas negative stimulation increases it. Con-
bundle (aligned with the long axis of the cell) that pushes sequently, positive stimulation suppresses the frequency of
the cell forward. tumbles, whereas negative stimulation increases it, and the
bacterial cells execute a random walk biased towards the
A tumble chemoattractant (Figure 3b) or away from the chemorepel-
The tumble is the consequence of clockwise rotation of the lent. (Runs in the ‘right’ direction are prolonged, and runs
flagella. Unlike counterclockwise rotation, which stabilizes in the ‘wrong’ direction are very short.) The end result is
the left-handed form of the flagella, clockwise rotation migration towards higher chemoattractant concentrations

Run

Attractant
Tumble

(a) Random walk (b) Biased random walk

Figure 3 Swimming behaviour of Escherichia coli cells. (a) nonstimulated conditions; (b) stimulated conditions.

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 Bacterial Chemotaxis

and avoidance of chemorepellents. Thus the question of counterclockwise in the case of E. coli and S. typhimurium.
how the chemotaxis process is carried out in bacteria can be This means that the motor always rotates counterclock-
reduced to the regulation of the direction of flagellar wise (in the temperature range 20–378C), unless it receives
rotation. a signal to do otherwise. This also means that the function
of chemoattractants and chemorepellents is to inhibit and
activate the clockwise signal, respectively. The chemotactic
excitatory signal is transduced very fast: the response delay
Genes Controlling Chemotaxis time of the flagella is  120 ms or  50 ms in response to a
step increase in the concentration of a chemoattractant or a
The genes involved in controlling chemotaxis are listed in chemorepellent, respectively.
Table 7. The function of each of them is indicated in the
table and described in more detail below.
Two additional genes of E. coli and S. typhimurium, flhC How is a gradient of a stimulant sensed?
and flhD, are indirectly involved in controlling chemotaxis Bacteria like E. coli and S. typhimurium sense temporal
in the sense that they regulate the synthesis of the gradients of stimuli (gradients over time), as opposed to
chemotaxis machinery. These genes form a master operon spatial gradients (gradients over space). This means that
whose gene products control the expression of the genes bacteria compare, between sequential time points, the
involved in flagellar synthesis, motility and chemotaxis. occupancy of their chemotaxis receptors, i.e. they possess
This master operon is itself positively regulated by the kind of a short-term memory. This arrangement is optimal
intracellular levels of cyclic adenosine monophosphate for bacteria of this size and shape, taking into considera-
(cAMP) and its receptor. In this manner the metabolic tion that a change in receptor occupancy as small as 0.4%
state of the cell is linked to the expression of the motility elicits a detectable chemotactic response. It is not
and chemotaxis components. Thus, when the level of impossible, however, that bacterial species with larger
cAMP goes up (e.g. when glucose availability goes down), dimensions or different shapes sense spatial gradients.
the flhDC operon is rapidly transcribed, the motility and
chemotaxis machineries are synthesized, and the bacteria
can navigate themselves to better locations. The conventional signal transduction
pathway in E. coli
The components
Signal Transduction Pathways of The components of the conventional signal transduction
Chemotaxis pathway are chemotaxis receptors, proteins involved in
signal transduction and adaptation, and switch proteins
The locations of the chemotaxis receptors and the flagella that determine the direction of flagellar rotation. In
are different. For example, in E. coli the receptors are addition, there are components (e.g. Ca2 1 and fumarate)
clustered at the bacterial poles, whereas the flagella are that may be required for signal transduction, but it is not
randomly distributed around the cell. This prevents direct yet known how. There are two kinds of receptors:
interaction between the receptors and the flagella, and the chemotaxis-specific receptors, and dual-function receptors
communication between them is carried out by a sophis- involved in both chemotaxis and transport of the ligand.
ticated signal transduction system (Figure 4), which belongs
to the large family of two-component regulatory systems. Chemotaxis-specific receptors
The end result of this signal transduction is a change in the The chemotaxis-specific receptors, dipeptides termed
direction of flagellar rotation. The flagellar motor has a MCPs (for methyl-accepting chemotaxis proteins), are
preferred direction of rotation, a default direction, expressed by the aer, tap, tar, tsr and trg genes (Table 7).
They are clustered at the bacterial poles (one or both of
CCW CW them). The MCPs are closely related to each other both in
amino acid sequence and in structure. They are, however,
CheY different with respect to their abundance (Table 7), the
Receptor

CheW sequence of their periplasmic part, and the presence of the

CheZ
CheA

Motor
Switch

CheW binding site for CheR in their cytoplasmic part. (CheR is a

specific methyltransferase having a role in adaptation; see
CheY~P
below. The CheR-binding site is a specific pentapeptide at
the MCP’s C-terminus.) Thus, the most abundant MCPs,
Figure 4 Signal transduction in bacterial chemotaxis. For simplicity, the Tsr and Tar, possess this CheR-binding site and therefore
proteins CheR and CheB, which are involved in adaptation at the receptor can function independently of the other MCPs. The other
level, are not shown; CCW, counterclockwise; CW, clockwise. MCPs, which do not possess this binding site, depend on

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 Bacterial Chemotaxis

Table 7 Genes controlling chemotaxis in Escherichia coli

Gene product
Monomeric
Gene Polymeric form size (kDa) Location Molecules per cella Function
aer Dimer 55 Cytoplasmic Presumably 150 Receptor that mediates the
membrane chemotactic response to oxygen and
to changes in the cell’s energy level;
a flavoprotein
cheA Dimer 73 and 67b Cytoplasm, 3000 Histidine kinase
receptor-bound
(via CheW)
cheB Monomer 36 Cytoplasm 1700 Methyl esterase
cheR Monomer 32 Cytoplasm 850 Methyl transferase
cheW Monomer 18 Cytoplasm, 3000 Linker?
receptor-bound
cheY Monomer 14 Cytoplasm 3000–17 500 Response regulator
cheZ Dimer 24 Cytoplasm 1200 Phosphatase
fliG In a complex 38 Cytoplasm, 35 per flagellumc Switch protein
motor-bound
fliM In a complex 38 Cytoplasm, 35 per flagellumc CheY-binding switch protein
motor-bound
(via FliG)
fliN In a complex 17 Cytoplasm, 100 per flagellumc Switch protein
motor-bound
(via FliG)
tap Dimer ∼60 Cytoplasmic 150 Receptor for dipeptides; also mediates
membrane the response to temperature changes
tar Dimer ∼60 Cytoplasmic 900 Receptor for some amino acids
membrane (e.g. aspartate, glutamate) and for
maltose-bound, periplasmic binding
protein; also mediates the response to
some chemorepellents (Ni2+, Co2+)
and to temperature changes
tsr Dimer ∼60 Cytoplasmic 1600 Receptor for some amino acids
membrane (e.g. serine, alanine, glycine,
cysteine); also mediates the response
to some chemorepellents (indole,
leucine, benzoate), to changes in the
cell’s energy level and to temperature
changes
trg Dimer ∼60 Cytoplasmic 150 Receptor for some sugar-bound,
membrane periplasmic binding proteins
(e.g. galactose, ribose); also mediates
the response to some
chemorepellents (phenol) and to
temperature changes
a
Approximate values. Values calculated from the protein concentration in the cell are based on a cell volume of 10–15 L (the cell volume depends
on the strain and growth conditions).
b
The gene cheA encodes for two polypeptides, long and short (cheAL and CheAS, respectively), as a consequence of translational initiation at two
distinct in-frame initiation sites.
c
The data are for Salmonella typhimurium.

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 Bacterial Chemotaxis

the presence of Tsr and Tar to function in adaptation. linker protein, CheW (Figure 4). CheY is thus one part of
Therefore, to be functional in adaptation, the minor MCPs the quaternary complex receptor: CheW:CheA:CheY.
must interact with major MCPs, and this may be one of the
reasons for the organization of the receptors in clusters. Signal transduction in response to negative stimulation
Under nonstimulated conditions, the phosphorylation
Dual-function receptors level of CheY is relatively low. Accordingly, the extent of
Chemoattractant sugars do not bind to the MCPs directly. CheY  P binding to the switch and, consequently, the
They either bind to a specific periplasmic binding protein probability of clockwise rotation are low. This situation
involved in both chemotaxis and transport of the sugar results in predominant counterclockwise rotation and
(e.g. the galactose-, maltose- and ribose-binding proteins), occasional clockwise rotation, and the bacterial swimming
or they bind to a specific Enzyme II (for glucose, mannose, behaviour mainly consists of runs with occasional tumbles.
mannitol and others) of the phosphoenolpyruvate-depen- A negative stimulus shifts the receptor to an active form
dent sugar phosphotransferase system (PTS). The re- that, together with CheW, stimulates the autophosphor-
sponses to both types of sugar chemoattractants are, ylation of CheA. This activation of CheA appears to
however, mediated by MCPs. The periplasmic binding involve a high-order interaction between receptor dimers
proteins bind to a specific MCP (Tar or Trg) and thus elicit in a structure that involves  7 receptor dimers per each
a chemotactic signal. In the case of a PTS sugar, the PTS CheA dimer. When CheA autophosphorylates, it instan-
Enzyme I modulates the kinase activity of the complex taneously phosphorylates CheY which, in turn, dissociates
MCP-CheW-CheA. Like the MCPs, at least some of the from the quaternary complex. CheY  P has a relatively
dual-function receptors (e.g. the periplasmic maltose- high affinity for the switch protein FliM and for the
binding protein) are clustered at the bacterial poles, phosphatase CheZ. Binding to the switch results in
probably in order to allow direct interaction with the increased probability of clockwise rotation and, therefore,
MCPs. the cell tumbles frequently (Figure 4). Binding to CheZ
sequentially results in CheZ oligomerization and delayed
Switch proteins activation of its phosphatase activity. Consequently
The switch, a complex of three proteins – FliG, FliM and CheY  P is dephosphorylated after a delay and the
FliN (Figure 2) – is the target of the signal from the clockwise signal is terminated. This termination may be
receptors. It is known that each protein in the complex required to avoid lengthy tumbling events (a brief tumble is
interacts with the other two proteins, but the structure of sufficient for reorientation), to avoid persistent clockwise
the complex is still obscure. The complex is linked to the rotation (persistent clockwise rotation leads to formation
MS ring of the flagellar motor via FliG, forming the of a bundle of right-handed flagella, resulting in a run), or
switch–motor complex. for adaptation (see below).
It should be pointed out that it is not known how a
Proteins mediating receptor–switch communication within chemorepellent activates the receptor. No binding of any
the cell chemorepellent to any chemotaxis receptor has been
demonstrated although, in vivo, the responses to most
The molecule that delivers the clockwise signal to the chemorepellents are mediated by one or more MCPs. It
switch–motor complex is the chemotaxis protein CheY was proposed that the MCPs are low-affinity receptors for
(Figure 4). This protein – a response regulator of a two- chemorepellents.
component regulatory system – can be in two main states,
phosphorylated and nonphosphorylated. The phosphory- Signal transduction in response to positive stimulation
lated site is Asp57. CheY is phosphorylated by CheA, an
autophosphorylatable histidine kinase. The autopho- Positive stimulation (e.g. binding of a chemoattractant to
sphorylation site on CheA is His48. It is only present in the receptor) shifts the receptor to a form that, together
the long form of CheA, CheAL (Table 7). Phosphorylated with CheW, inhibits the autophosphorylation of CheA.
CheY (CheY  P) is dephosphorylated spontaneously or (Under these conditions the complex unit is thought to
in enhanced manner by a specific phosphatase, CheZ involve an MCP dimer per each CheA dimer.) Therefore,
(Figure 4). (The term phosphatase is used here in the the steady-state level of CheY  P drops down, and the
broader sense and it does not imply a specific mechanism of probability of clockwise rotation decreases. The outcome
CheZ action.) The short form of CheA, CheAS, forms of this situation is prolonged runs and rare tumbles. Under
complexes with CheW and CheZ and activates the latter, at certain conditions, CheZ might be involved in lowering the
least in vitro. CheY can also be phosphorylated by small CheY  P level.
phosphodonors that reside in the cell (e.g. acetyl phos-
phate), but their contribution is negligible relative to that Signal transduction in response to multiple inputs
of CheA. When nonphosphorylated, CheY is bound to Generally speaking, when a cell is exposed to a number of
CheA, which itself is bound to the MCP receptor via a chemotactic stimuli, there appears to be only one type of

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 Bacterial Chemotaxis

response, i.e. the different inputs are integrated (though Variations on signal transduction pathways in
they are not necessarily additive). This applies also to cases other bacterial species
in which bacteria are exposed to a chemoattractant and a
chemorepellent together; however, when the response is More than one signal transduction pathway
analysed in fast kinetics, the response to the chemorepel- As the genome sequences of more and more bacterial
lent precedes the response to the chemoattractant. species become available, it turns out that, unlike E. coli,
The mechanism by which all the stimulated receptors which has only one set of che genes, a significant
function in concert is still a mystery. Thus, at least in vitro, proportion of the bacterial strains have two or more sets
the kinase CheA is linked to an MCP receptor at a ratio of of che genes. This suggests that these strains possess at least
1:1 when the former is not active, and at a ratio of 1:  7 two signal transduction pathways. For example, R.
when active (Figure 4). It is not at all clear how signalling by sphaeroides has 12 MCPs (some of which are in the
receptors, which constitute only a small fraction of the cytoplasm), three CheA, two CheR, three CheW and four
total number of receptors, can affect a sufficient number of CheY (but only one CheB and no CheZ). By studying
kinase molecules so that the observed swimming response proper mutants of R. sphaeroides it was demonstrated that
is extreme, rather than in proportion to the receptor this species indeed has two (or more) pathways. It is not yet
abundance. The same question holds for partial occupancy known whether the different pathways work in parallel or
of any chemotaxis receptor, bearing in mind that whether each is functional (or expressed) under different
occupancy as small as 0.4% is sufficient to elicit a conditions.
detectable chemotactic response. It is reasonable that the
receptor clustering provides an amplification mechanism, Lack of CheZ
or that it enables interreceptor communication, which
Many bacterial species do not contain CheZ. Such species
temporarily stops conflicting signalling from nonstimu-
usually have more than one CheY, one of which may fulfil a
lated receptors.
role analogous to that of CheZ. This was demonstrated in
the case of S. meliloti, where one of the CheY proteins may
assume the role of a ‘phosphatase’ by acting as a phosphate
sink.
Nonconventional signal transduction Different inputs
pathways in E. coli In contrast to E. coli, where chemoattractants are sensed
It was recently found that E. coli strains, lacking most of on the bacterial surface, there are bacterial species in which
the MCPs and the known chemotaxis machinery but chemoattractants or their metabolites are detected intra-
containing high levels of CheY, have a chemotactic-like cellularly. For example, in the case of R. sphaeroides the
response to conventional chemoattractants and chemor- sugars mannitol and fructose have to be transported into
epellents. These findings raise the possibility that, at least the cell and perhaps metabolized to be detected. This may
when the conventional signal transduction components are explain the finding that in this species some of the MCPs
missing, a nonconventional chemotactic signal transduc- reside intracellularly.
tion pathway might be functional in E. coli. The identity of
Different outputs
the components involved in this pathway is not known.
In other studies it was found that E. coli strains, lacking In some species the outcome of CheY  P interaction with
most of the conventional signal transduction components the switch is different from the outcome in E. coli. In H.
but expressing CheY, are able to respond to the salinarium, for example, CheY  P appears to increase the
chemorepellents indole and benzoate. This phosphoryla- switching probability rather than the clockwise probability
tion-independent signal transduction involves inhibition of the motor. In Bacillus subtilis, phosphorylation of CheY
of the enzyme fumarase by these chemorepellents, resulting apparently decreases (rather than increases) the clockwise
in elevation of the intracellular level of fumarate. Fumarate probability. In S. meliloti, an interaction of CheY  P with
interacts with the switch–motor complex and, by an the flagellar motor appears to slow down the rotation
unknown mechanism, reduces the free energy difference instead of changing its direction (the flagella of S. meliloti
of the counterclockwise-to-clockwise transition. Thereby rotate only in one direction; Table 1).
it increases the probability of the clockwise state.
Other studies demonstrated that CheY can undergo Signal transduction in large bacterial species
acetylation (on lysine residues 92 and 109) by the enzyme The signal transduction pathways discussed above are
acetyl coenzyme A (AcCoA) synthetase and directly by essentially networks of interacting enzymes, resulting in a
AcCoA. The physiological role of this acetylation, which relatively short signalling range. They are, therefore, not
results in CheY activation probably at a postswitch- suitable for large (longer than 20 mm) bacterial species.
binding step, is not known. Indirect evidence suggests that in such species (e.g.

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 Bacterial Chemotaxis

Spirillium volutans, Rhodospirillum rubrum, Thiospirillum consequent increased methylesterase (demethylation) ac-
jenense and cyanobacteria) the signal is electrical in nature. tivity.
Perhaps the most convincing evidence was obtained in Thus, negative stimulation results in enhanced autopho-
spirochaetes, where neurotoxins, which affect the action sphorylation of CheA, which, in turn, increases the steady-
potential in excitable eukaryotic cells, were found to inhibit state phosphorylation level of CheY and, more slowly, of
chemotaxis, and where clamping the membrane potential CheB. Upon phosphorylation, CheB is activated, the
at  0 mV had a similar inhibiting effect. MCPs are demethylated, and the probability of clockwise
rotation decreases to the prestimulus level. Positive
stimulation inhibits CheA autophosphorylation. CheR,
more slowly, methylates the MCP. The methylated MCP
enhances CheA autophosphorylation and the end result is
Adaptation increased probability of clockwise rotation and restoration
of the prestimulus level. (In B. subtilis, in contrast to E. coli
Adaptation is the process of recovery from a stimulated and S. typhimurium, the methyltransferase CheR is
behaviour when the stimulus is still present. Adaptation is involved in adaptation to negative stimulation and the
essential for every behavioural system because it allows methylesterase CheB in adaptation to positive stimula-
detection of small changes in the stimulus level on top of a tion.)
constant stimulation level. In the case of bacterial
chemotaxis, adaptation enables bacteria to respond to
new stimuli in the presence of constant levels of chemoat- Methylation-independent adaptation
tractants and/or chemorepellents. Bacterial adaptation is
There is evidence that, although methylation-defective
precise, in the sense that the postadaptation swimming
mutants (cheB cheR mutants) of E. coli are defective in
behaviour is exactly like the prestimulus behaviour.
adaptation, they can still adapt to a certain extent. This
Furthermore, this precision is robust; namely, it is
suggests that there is an additional, methylation-indepen-
independent of the exact level of the proteins involved in
dent adaptation mechanism. Such a mechanism may be
adaptation. However, the steady-state tumbling frequency
provided by CheZ. As indicated above, both the activation
and the adaptation time do vary with the protein
and deactivation of the phosphatase function of CheZ are
concentrations. In E. coli, there appear to be at least two
delayed. The apparent consequence of the delay is that the
adaptation mechanisms: methylation-dependent and
modulation of the phosphatase activity occurs only after
methylation-independent.
the excitatory signal is complete. Therefore, the delayed
activation and deactivation appear to constitute an
Methylation-dependent adaptation adaptation mechanism, which ensures that the phosphor-
ylation level is partially set back close to the prestimulus
The cytoplasmic domain of each MCP contains 4–6 level. Accordingly, cheZ mutants adapt slower than wild-
methylatable glutamate residues. The side-chain of each type mutants. It is not known whether the delayed
of these glutamate residues can be methylated by CheR – a activation and deactivation of CheZ is actually the
specific methyltransferase. (Some of these methylation methylation-independent adaptation mentioned above.
sites are encoded as glutamine residues which, posttran- One of the possibilities is that CheZ mediates the first step
slationally, are converted to glutamate residues by CheB.) of adaptation, while the second, slower step, which
The formed methyl ester bond can be hydrolysed by CheB includes the precise tuning of the direction of flagellar
– a specific methylesterase. A methylated MCP transmits a rotation, is mediated by the methylation system.
clockwise signal to the flagella, whereas demethylated
MCP transmits a counterclockwise signal. These signals
are presumably caused by conformational changes in the Further Reading
cytoplasmic, signalling domain of the MCP.
Adler J (1976) The sensing of chemicals by bacteria. Scientific American
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methylation reaction is enhanced by chemoattractants Eberl L, Molin S and Givskov M (1999) Surface motility of Serratia
and inhibited by chemorepellents, but the mechanism liquefaciens MG1. Journal of Bacteriology 181: 1703–1712.
underlying these effects is not known. Conversely, the Eisenbach M (1991) Signal transduction in bacterial chemotaxis.
demethylation reaction is enhanced by chemorepellents Modern Cell Biology 10: 137–208.
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consequence of modulation of the phosphorylation level of
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CheB. It turns out that there is a remarkable sequence System. New York: Raven Press.
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CheY is phosphorylated by CheA, but also CheB, with a American Society for Microbiology.

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Macnab RM (1996) Flagella and motility. In: Neidhardt FC, Ingraham Stock JB and Surette MG (1996) Chemotaxis. In: Neidhardt FC,
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