Medical Bacteriology Unit 1

NORMAL MICROBIALFLORA OF
HUMANBODY

SYNOPSIS
*ORIGIN OF THE NORMAL FLORA

*NORMAL FLORA AND THE HUMAN HOST

*DISTRIBUTION AND OCCURRENCE OF THE NORMAL FLORA

SKIN

EYE

RESPIRATORY TRACT

MOUTH

INTESTINAL TRACT

GENITOURINARY TRACT

INTRODUCTION
We are constantly in contact with a myriad of microorganisms in the environment. However,
we are in even more intimate contact with an enormous number of microorganisms that
inhabit our bodies. It is estimated that the adult human body is composed of
approximately1013eukaryotic cells; what may be less apparent is that the human body also
serves as the natural habitat for 10 times that number of microbial cells! These
microorganisms, most of which are bacteria, comprise the normal flora, also termed the
normal microbiota.

They inhabit mainly the skin and the inner surfaces of the body such as the mucous
membranes of the oral cavity, upper respiratory tract , intestinal tract and genitourinary tract .

It is useful to know the normal flora of the healthy human body for the following reasons;

1.The term normal flora implies that these microbes are harmless, and for the most part they
do not cause disease and are even beneficial, most are commensals : they benefit from the

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association with the host : they benefit the host in some fashion while thriving in the host’s
body. It is of interest to learn what these beneficial effects are and how they may be lost due
to changes in the normal flora caused by the use of antibiotics or other means.

2. Some normal flora organisms can be opportunistic pathogens; I.e., they may cause
infections if tissue injury occurs at specific sites or if the resistance of the body to infection is
decreased. This is especially important because in recent years there has been a rising
incidence of infections from these microorganisms.

ORGIN OF THE NORMAL FLORA

Before birth a healthy human fetus is bacteria. Under natural circumstances, the fetus first
acquires microorganisms while passing down the birth canal. It acquires them bysurface
contact, swallowing, or inhaling. These microbes are soon joined by other microbes from
many sources in the newborn’s immediate surroundings. Microorganism which find suitable
environments , either on the outer or inner body surfaces, quickly multiply and establish
themselves.

NORMAL FLORA AND THE HUMAN HOST

What effect does the establishment of a normal flora (colonization )have on the body? Three
approaches have been used to answer the questions:

1.Use of germ free animals .If the colonizationof experimental animals by microorganisms
can be prevented,one can compare the properties of such germfree animals with those of
normal animals.The result can be helpful in understanding the functions of the normal flora
of humanbeings.

2.Use of antimicrobial agents. If the balance that occurs between the normal flora and the
human host is altered by the use of such agents,various effect may occur that are useful for
indicating the role of the normal flora of the humanbody

3.Knowledge of certain characteristics of normal flora of organisms.The nature of these

characteristics suggests that normal flora organism may help to discourage the growth of
microorganism that are not part of the indigenous.

DISTRIBUTION AND OCCURRENCE OF THE NORMAL FLORA

Bacteria make up most of the normal flora of the human body, and , therefore ,deals mainly
with the distribution and numbers of various bacterial genera and species. Although various

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fungi(mainly yeast) and protozoa may also inhabit the body, their numbers are usually very
low compared to the bacterial flora. As viruses , it is not clear whether any can be considered
as true normal flora even though some may be harboured for long periods in the absence of
disease symptoms.

Skin

Skin has a slightly acidic pH (4-6) due to organic acids produced by staphylococci and
secretions from skin oil and sweet glands. The acidic pH discourages colonization of many
bacteria.

Lack of moisture drives many resident microbiota into a dormant state. However in certain
parts of the body (scalp, ears andaxillary areas) moisture is sufficiently high to support a
resident microbiota.

Most of these bacteria species of staphylococcus (mainly staphylococcus epidermidis) and aerobic
corynebacteria ,or diptheriods.In the deep sebaceous glands are found lipophlic anaerobic bacteria
such as Propionibacterium acnes.

Eye

Lining the eyelids and covering the eyeball is a delicate membranes called the conjunctiva.
This membrane is continually being washed by a flow of fluid (tears),which tends to remove
microorganisms. Moreover , lysozyme is secreted in tears.

The main organisms found are: Staphylococcus epidermis, Staphylococcus aureus

,Corynebacteriumxerosis , Streptococcus pneumonia, Neisseria species, Moraxella species,
Haemophilusparainfluenzae;other organisms may be isolatedoccasionally.

Repiratory tract:

The upper respiratory tract is heavily colonized by normal flora. But the lower respiratory
tract.(trachea, bronchi, bronchioles and alveoli) do not have a normal microbiota (it is sterile)
as the removed by :

 The continuous stream of mucus generated by the ciliated epithelial cells.

 Phagocytic action of alveolar macrophages.
 Lysozyme, which is present in nasal mucus has bactericidal effect.

Mouth

The normal flora of the oral cavity contains bacteria able to resist mechanical removal by
adhering to surface like the gum and teeth.Those that can not attach are removed by the
mechanical flushing from the oral cavity to the stomach where they are destroyed by
hydrochloric acid.

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The predominant bacterial species belong to the genera Streptococci, Neisseria, Veillonella
,Actinomycetes , Lactobacillus ;Yeast also present.

INTESTINAL TRACT

Stomach :Although the stomach constantly receives numerous bacteria from the oral cavity,
the fluid contents of the healthy stomach generally contain less than 10 bacteria per millilitre
because of the bactericidal effect of the hydrochloric acid in the gastric acid secretion. The
few organisms found are mainly lactobacilli and Yeast such as Candida species.

Small Intestine : The upper portion (or duodenum) of the small intestine has few bacteria. Of
those present ,the majority are Gram positive cocci and bacilli. In the jejunum (second part
of small intestine) there are occasionally found species of enterococci,lactobacilli, and
diphtheroids.

In the distal portion (ileum) of the small intestine, the flora begins to resemble that of the
large intestine. Anaerobic bacteria and members of the family Enterobacteriaceae grow
extensively here and appear in large numbers.

Large Intestine :In thehuman body, the colon,or large intestine,has the largest microbial
population.The anaerobic Gram-negative bacilli present include species of Bacteriods and
Fusobacterium.

The Gram-positive bacilli are represented mainly by species of Bifidobacterium, Eubacterium

, and Lactobacillus.The facultatively anaerobic species found in the intestine belong to the
genera. Escherichia,Proteus,Klebsiella,and Enterobacter.Peptosteptococci (anerobic
streptococci) are common.The Yeast Candida albicans is also present.

Genitourinary tract

In a healthy person,thekidney,urinary bladder, and ureters (the tube that convey the urine
from the kidney to the bladder)are free of microorganisms.

A few bacteria such as :

Staphylococcus epidermis, Enterococcus faecalis,Corynebacterium species aer usually

present distal portion of urethera.

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GENERAL ATTRIBUTIONS OF
VIRULENCE FACTORS OF
BACTERIA CAUSING INFECTIONS
VIRULENCE FACTORS

 The term virulence is derived from the Latin word Virulentia,meaning poison.
 Virulence is described as an ability of an organism to infect the host and cause a
disease.
 Virulence factors are bacteria - associated molecules that are required for a
bacterium to cause disease while infecting eukaryotic hosts such as humans.
 Virulence factors are extracellular products released by bacterial cells and are
involved in pathogenesis.
 Bacterial virulence factors enable a host to replicate and disseminate within a host
in part by subverting or eluding host defences.

FACTORS PREDISPOSING TO MICROBIAL

PATHOGENECITY
OR VIRULENCE
I) ADHESION

 Adhesion or Attachment of microorganisms is the first step of microbial pathogenicity

 Adhesion mainly takes place with the help of Surface appendages in bacteria like
Flagella, Pili and Fimbriae.
 The protein Adhesin present in the Surface appendages of bacteria help for adhesion.
 Adhesin can serve as a Virulence factor and Antigenic in nature.

II) INVASIVENESS

 The ability of the pathogen to spread in the host tissue after establishing infection is
called as Invasiveness.
 Highly invasive pathogens can cause Generalized or Systemic infection.
 Less invasive pathogens can cause Localized or Local infection.

III) EXTRACELLULAR ENZYMES

 Many pathogens secrete enzymes that enable them to dissolve structuralchemicals in

the body and thereby maintain an infection, invade further, andavoid body defenses.

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  Extracellular enzymes of bacteria are Virulent in nature.
 Hyaluronidase (digests Hyaluronic acid which acts as a Glue in animalcell).
 Collagenase (breakdown the Collage which acts as a body’s chiefstructure protein).
 Coagulase (causes Blood proteins to clot).

 Staphylokinase and Streptokinase (digests Blood clot).

 Fungi that cause “Ring worm” produce an enzyme Keratinase that digests
Keratin (the main component of skin, hair, and nails).
 Entamoebahistolytica secretes Mucinase to digest the mucus lining the Intestinaltract,
allowing the Amoeba entry to the underlying cells, where it causesAmoebic
dysentery.

IV) ANTIPHAGOCYTIC FACTORS

a) Capsules

 The capsules of many pathogenic bacteria are effective virulence factorsbecause many
capsules are composed of Polysaccharides and they do notstimulate a host’s immune
response.
 Capsules are often slippery, making it difficult for phagocytes to surroundand
phagocytize.

b) Antiphagocytic chemicals

 Some bacteria produce chemicals that prevent the fusion of Lysosomeswith

phagocytic vesicles, allowing the bacteria to survive insidephagocytes.
 Streptococcus pyogenes produces a protein on its Cell wall and Fimbriaecalled M -
protein that inhibits Phagocytosis and thus increases virulence.
 Other bacteria produce Leukocidins, which are chemicals capable ofdestroying
phagocytic white blood cells outright.

V) TOXINS

Toxins are the poison of plant or animal or microbial origin that can act as anantigen of the
body.

o Toxins are of two types. They are

a) Exotoxins

(i) Cytotoxin

(ii) Neurotoxin

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(iii) Enterotoxin

b) Endotoxins.

Some characteristics of exotoxins and endotoxins

Feature Exotoxins Endotoxins

Bacterial source Secreted by living Released from cell walls of
microorganism lysed gram-negative bacteria

Chemical nature Protein Lipopolysaccharide

Heat tolerance Inactivated easily by boiling; Will withstand autoclaving;

heat-labile heat-stable

Immunology Can be converted to toxoids Cannot form toxoids;

and readily neutralized by neutralization with anti-toxin
anti- toxin not possible or only with
difficulty

Pharmacology Each has a highly All act similarly to cause their

characteristic mechanism of effects; action characterized
action by pyrogenicity, blood
changes, and shock

Lethal dose Much larger

small

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HOST PARASITE RELATIONSHIP
Introduction
Host –Parasite relationship means a reaction between the virulence factors of microbes and
the resistance mechanisms of the host.

HOST: a larger organism that supports the survival and growth of a smaller organism is
called the host.

PARASITES: those organism that live on or within a host organism and are metabolically
dependent on the host (any organism that cause disease of a parasite).

The aim of a microbe is to penetrate the host defence and to colonize it. On the other hand ,
the host employs several physical and chemical attributes to prevent infection. These
defences include the antibacterial factors in secretions covering mucosal surfaces; rapid rate
of replacement of skin and mucosal epithelial cells; engulfment by phagocytic cells; action of
interleukins, eicosanoids, and complement system; inflammatory response; etc., pathogens
try to either evade or overcome these mechanisms to the help of virulence factors.

Similarly, susceptibility to infections depends on the physiologic and immunologic condition

of the host. Thus, it can be said the host. Parasite interaction is a competition between the
virulence factors of a pathogens and the defence mechanisms of the host. If the defence
mechanisms are stronger, then the host is said to be resistant and the pathogens is said to be
non-virulent.

Contrary to this, if the virulence factor of the pathogen overcome the defence mechanisms
of the host, then the host is said to be susceptible and the pathogen highly virulent. Several
microbes live in our body as a part of the normal flora, which do not harm the host.

Types of symbiosis

1.Mutualism;

Both members of association benefit. Thus, the relationship between host and microbes at
times can beneficial. For example, E.coli in the intestine produces vitamin k & other
nutrients; lactobacilli maintain urogenital pH around 4 in vagina. This type of relationship is
termed as mutualism.

2.commensalism:

In commensalism the microbial flora benefits from the interaction, but the host has no
advantage.

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3.opportunism:

Opportunism is the interaction in which the organism that are a part of the normal flora
becomes pathogens because of circumstance. Only parasitism and opportunism result in
infection as the microbe benefits at the expense of the host.

References

1. pelczarJr. M.J.chan. E.C.S. and kreig.N.R.(2006).”Microbiology” 5 th edition McGraw Hill

Inc. New york.

2.Ananthanarayanan R. and JayaramPanickerc.k. (1994). Text book of Microbiology.Orient

Longman.

3.Salle, a.j. (1992). Fundamental principles of Bacteriology. 7th edition, mc. Graw hill
publishing co. Ltd., New york.

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