Principles of Microbial Pathogenicity

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Introduction:

Pathogenicity :

Is the ability to cause disease by overcoming the defences of

a host

Virulence : Is the degree of pathogenicity.


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How Microorganisms Enter a HostThe specific rout by which a particular pathogen gains access

to the body is called its portal of entry.

Portal of Entry:Many of microorganisms can penetrate Mucous

membranes of the Conjunctiva and the Respiratory(Microorganisms that are inhaled with droplets of moisture and

dust particles gain access to the respiratory tract. And It is the

most common portal of entry).

Gastrointestinal (Microorganisms enter the gastrointestinal

tract via food, water, and contaminated fingers).

Genitourinary tracts (Microorganisms that gain access via

the genitourinary tract can enter the body through mucous

membranes).


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Most microorganisms

cannot penetrate intact

skin; they enter hair

follicles and sweatducts.

Some microorganisms

can gain access to

tissues by inoculation

through the skin and

mucous membranes in

bites , injections , andother wounds. This

rout of penetration is

called the parenteral

route.


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The Underlying Mechanisms of BacterialPathogenicity

Two broad qualities of pathogenic bacteria underlie the means

by which they cause disease:

1. Invasiveness is the ability to invade tissues. It

encompasses mechanisms forcolonization (adherence and

initial multiplication), production of extracellular substances

which facilitate invasion (invasions) and ability to bypass

or overcome host defence mechanisms.

2.Toxigenesis is the ability to produce toxins. Bacteria may

produce two types of toxins called exotoxins and endotoxins.


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Colonization:The first stage ofmicrobial infection is

colonization: the

establishment of thepathogen at the

appropriate portal of

entry. Pathogens usually

colonize host tissues thatare in contact with the

external environment

Sites of entry in human

hosts include the

urogenital tract, thedigestive tract, the

respiratory tract and the

conjunctiva. Organisms

that infect these regionshave usually developed

tissue adherence

mechanisms and some

ability to overcome orwithstand the constant

pressure of the host

defenses at the surface.


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Mechanisms of

Adherence to Cell or

Tissue Surfaces

The mechanisms for

adherence may involve two

steps:1. nonspecific adherence:

reversible attachment of the

bacterium to the eucaryotic

surface (sometimes called"docking") .

2. specific adherence: reversible

permanent attachment of the

microorganism to the surface

(sometimes called "anchoring").

Specific adherence


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How bacterial pathogens penetrate HostDefenses :

CapsulesSome pathogens have capsules that prevent them from beingphagocytized.


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Cell wall component

Proteins in the cell wall can facilitate adherence orprevent a pathogen from being phagocytized

Enzymes-Local infections can be protected in fibrin clot caused by the

bacterial enzyme coagulase.

-Bacterial can spread from a focal infection by means of

kinases

(which destroy the blood clots) , hyaluronidase (whichdestroys a mucopolysaccharide that holds cellstogether) , and collagenase (which hydrolyzes connectivetissue collagen).-IgA proteases destroy IgA antibodies.


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Antigenic Variation:Some microbes vary expression

of antigens, thus avoiding the

hosts antibodies.

Penetration into the HostCell Cytoskeleton:

Bacterial may produce proteinsthat alter the action of the host

cells cytoskeleton allowing

bacteria into the cell.


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Killing of phagocytes:

An alterative strategy is for the microorganism to kill

the phagocyte, This can be achieved by the

production of leucocidins (e.g. Staphylococci,streptococci ) wich promote the discharge of

lysosomal substances into the cytoplasm of the

phagocyte rather than into the vacuole, thus directing

the phagocytes lethal activity towards itself


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How Bacterial Pathogens DamageHost Cell:

Using the Hosts Nutrients: SiderophoresBacteria get iron from the host using siderophores.

Direct DamageHost cells can be destroyed when pathogens

metabolize and multiply inside the host cells.

The production of Toxins


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References:

Tortora & Funke & Case (2010) Microbiology: AnIntroduction,10th ed., Benjamin Cummings.

Hugo & Russell`s (2004) Pharmaceutical microbiology, 7th

ed., Blackwell Publishing

Todar's Online Textbook of Bacteriology

(http://www.textbookofbacteriology.net/pathogenesis.html).


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Principles of Microbial Pathogenicity

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