Stucture of Bacterium

Part 3
• OBJECTIVES
• To describe the various shapes of bacteria
• To describe the structure of a bacterial cell
• To explain the functions of a various parts of a
bacterial cell
Shapes of Bacteria
• Coccus
➢Chain = Streptococcus
➢Set of two=diplococcus
➢Cluster =
Staphylococcus
• Bacillus
➢Chain = Streptobacillus
• Coccobacillus
• Vibrio = curved
• Spirillum
• Spirochete
THE STRUCTURE OF A BACTERIUM
Chapter 4
BASIC CELL STRUCTURES:
CELL WALL
Properties: Rigid and Porous
Function: Defines Shape of the cell ,Protection from differences in
the internal and external osmotic pressure
Composition:
 Composed of Peptidoglycan (also called murein) .
 Murein composed of glucose, muramic acid, and polypeptides
 Variation in composition is detected by doing Gram stain and
Acid Fast stain.
 If the cell is Gram positive it is composed of 80% murein, which
form multiple layers .
 Gram negative cell have 40% murein & 40% lipids.
 Acid Fast cells have 20% murein & 60% lipids (waxy)
PRINCIPLE OF THE GRAM STAINING TECHNIQUE
• Reagents for gram staining-crystal violet,iodine,alcohol,safranin
ACTION EFFECT ON GRAM EFFECT ON GRAM
POSITIVE NEGATIVE
Crystal violet + iodine Forms a crystal violet + Forms a crystal violet +
iodine complex iodine complex
Alcohol • Dehydrates the cell wall • dissolves the lipid layer

• closes pores on cell wall • causes the crystal violet
• retain blue colour +iodine to leach in to
solvent
Counter stain safranin No effect Stain the cells pink
caution Prolonged exposure to Some cell poorly stained by

alcohol may remove crystal safranin e.g. Haemophilus
violet +iodine complex- spp., Legionella spp.
false results(mixture of Such counter stained by
G+ve and G-ve basic fuschin
CELL MEMBRANE
Properties: flexible and Semi permeable
Function: Controls the movement of nutrients & wastes across
the membrane and is the Site of some metabolic reactions
Movement across the membrane
• Passive transport - osmosis, diffusion
• Active transport e.g. sodium ion pump
Composition: Composed of a bilayer of Phospholipids &
proteins.
❖Phospholipids have a Phosphate “head” - hydrophilic and
Fatty acid “tails” – hydrophobic
CYTOPLASM
Properties:
 It’s a viscous fluid with the consistency of egg whites
Function:
 It forms the Filling of cell (“cell sap”).
 it’s a Suspension medium.
 It’s also the site of most metabolic processes
Composition:
 It composed of 80% water and 20% soluble substances -
nutrients, wastes
Bacterial chromosome
Properties: it is a single ,Long, highly folded ,double
stranded molecule that is suspended in cytoplasm in a
region called the nucleoid region. It is not enclosed in a
nucleus.
Composition: It’s a Single molecule composed of DNA .It
lacks histones
Function : Control centre of the cell.
✓ It Controls reproduction and metabolism
✓it Contains genetic code (inherited characteristics)
RIBOSOMES
Properties : Large, dense granules which appear scattered
through out cytoplasm
Composition :composed of RNA and Proteins. It is a 70S
composed of a 30S smaller subunit and 50S larger subunit
Function :Sites of protein synthesis
INCLUSIONS
These are the nutrient (storage) granules
Properties : are Very small, numerous and appear scattered
through out cytoplasm
Function : Storage of nutrients
Composition :Its composed of Insoluble nutrients namely
glycogen, phosphate (volutin),and inorganic ions
(minerals
SPECIAL CELL STRUCTURES: GLYCOCALYX :CAPSULE, SLIME
LAYER
Properties
They are Jelly-like layers secreted by cell ; they adhere
to cell wall
 Capsule is a thick , close fitting regular layer
 Slime layer is a thin , loose fitting and irregular layer
 When found together the capsule and slime layer make
the glycocalyx
Functions :
 Capsule -Protects the bacterium from phagocytes
,Enhances bacterium’s ability to produce disease
 Slime layer- Protects the cells against drying , Allows
Bacteria with capsule: E. bacteria to adhere to surfaces , or to each other
coli, Salmonella (colonize)
Bacteria with slime Composition:
mould: Bacillus  Capsule and slime layer are composed of Polypeptides
and polysaccharides. The Exact composition varies in
anthracis, Pseudomonas each individual species
aeruginosa  These are only Found only on some cocci, bacilli and
such microorganisms do not absorb dyes hence need
special staining techniques to observe
FLAGELLUM (FLAGELLA)
Properties: its a Long, thin, thread-like
appendage. attached to the cell
membrane from a basal granule
Function : Responsible for mobility i.e.
Rotates clockwise or counter
clockwise. It aids during chemotaxis (
movement toward nutrients, away
from harmful chemicals)
Vibrio cholerae Composition : composed of a Protein
called flagellin
Number and Location:
 Number maybe single, two, or more.
 Located at one end, both ends, all
over cell
 They are Found only on some bacilli,
spirals and Require special stain to
demonstrate
Bacillus cereus
PILUS (PILI)
Properties: are Thin, shorter, hair-like
appendages.
• They Vary in length and Extend from entire
surface of cell.
• They are of Two types: Attachment pili
and Conjugation (sex) pili
Composition :They are composed of Protein -
pilin
Function :
 Attachment pili
 Allow bacteria to adhere to the surface of
mucus membranes and to each other,
 Enables the cells to colonize and Enhances
the bacteria’s ability to cause disease
 The Conjugation (sex) pili are longer,
 Allows two bacteria to adhere to each
other hence providing a pathway for a
segment of DNA to be transferred from one
cell to the other (conjugation)
PLASMIDS
Properties :These are Small circles
of DNA which are not connected
to the nucleoid
Function:
• They Contains genetic
information that is not essential
to the cell’s survival
• May contain genes for R factors
(drug resistance), toxins.
• They may be involved in
conjugation.
• they are used in genetic
engineering as vectors
ENDOSPORES
Properties :
 they are Dense, round to oval structure
occurring in cytoplasm.
 they are Extremely resistant to
environmental conditions (heat, drying,
lack of nutrients, chemicals, radiation).
 They remain viable but dormant after
vegetative cell dies and only germinate
when introduced into a favourable
environment producing a cell identical
to the parent cell
Endospores appear bluish-green; Composition :
other structures appear pink to  Endospore consist of DNA surrounded
red. by an Endospore cortex formed of
repeated layers of cell wall and cell
membrane
Bacteria with endospores: Function : Survival. Endospore are Formed
Bacillus subtilis, Clostridium inside cell to protect the cell from
botulinum adverse environmental conditions
Class exercise
• Make short notes on the following special bacteria structures .You
should follow the outline followed in describing and explaining
previous structures
1. Chromatophores
2. mesosomes

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Part 3

 Gram negative cell have 40% murein & 40% lipids.

Alcohol • Dehydrates the cell wall • dissolves the lipid layer

caution Prolonged exposure to Some cell poorly stained by

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