Virulence

Microbiology ; Dr. younis Alkhafaji Lecture 2
Virulence
Virulence in laboratory may be measured by using laboratory or test animals or its cell in tissue culture , in the procedure using test animals involves injection of graduation doses of a microorganism or toxin into selected laboratory animal (for example mice), The dosage which contain the smallest number of microorganisms or the minimum concentration of toxin that will infect or kill 50 % of the animals injected is referred to as the end- point ..,an infective dosage 50 % (LD50), or the lethal dosage 50 % (LD50), for example, LD50 for anthrax bacilli …..5 spores is reported.
Another method for measuring virulence employed the dosage of microorganisms or the concentration of toxin necessary to kill all the test animals and this is referred to as the minimal lethal dosage (MLD). The LD50 determination is consider more accurate in measurement of the virulence using the laboratory animals or cell culture than MLD.

Adhesion, or adherence or attachment
Adherence may be the determining factor in the location of organism in areas especially subjected to fluid flow. The ability of microorganism to adhere is related to such surface receptors (structures) of that microorganism as pili, fussy coat, and capsule slime, which complement with surface structures of the host cell receptors .
The lipoteichoic acid (LTA) in the form of surface fibrils of bacterial surface (cell wall) appear to be involved in the adherence to host structure by electrostatic interaction between complementing chemical structures. LTA rather than M –protein involved in binding of Streptococcus pyogenes to oral epithelial cells . It has been proposed that LTA in cariogenic plaque containing Streptococcus mutans , in addition to increasing adherence to teeth appear to function as a diffusion barrier and fosters the accumulation of acids within the plaque .
Streptococcus sanguis has the ability to synthesize surface slime that associated with its ability to stick to teeth. Adhesion alone does not mean that an organism is pathogenic . The presence of indigenous microflora at specific location is based in part on their ability to adhere to a specific cells of the host and not to their potential for pathogenicity.
The pathogenicity of most of most microorganisms is related to the presence of : 1- ability to adhere, 2- ability to penetrate and grow in and on mucosal cells of respiratory, gastrointestinal, and genitourinary tract, 3 – to binding and bring about pathologic changes that result in disease.
Adheion molecules : for example the integrins and selectins ; these are molecules that mediate the bind of cells to other cells or to other extracellular matrix molicules such as fibronectin. For some microorganisms, example corynibacterium diphtheria , the combination of adherence, multiplication and toxin production on the mucosal epithelium without penetration determines disease.
It has been stated that most disease affecting human come from endogenous sources ,…. These disease are caused by members of the indigenous microflora that are carried in the human oral cavity, on the skin , in gastrointestinal tract and in other area of the body.
Oral ulcerative lesions that frequently follow abrasion of the mucous membrane are caused by the indigenous microflora. Dental caries and periodontitis are examples of diseases of the oral flora.
The ability of a microorganism to adhere , penetrate the normal mechanical barrier and resist the chemical barrier of the skin and mucous membranes of the respiratory , gastrointestinal, and genitourinary tracts ,surviving and grow is a prerequist for both the indigenous (opportunistic) parasite and the true pathogen to initiate infectious disease.
The attributes or factors of microorganism that may influence their virulence to induce infectious disease are presented in the following table under (1) surface components , ( 2) toxin, and (3) enzymatic and related factors.
Some microbial factors that may influence virulence:

Factors characteristics
I- surface components
A- Pili ; appear to function in adherence of microorganisms to host epithelial cells and other structures, also act as Prerequisite to infection for certain microorganism.

B- Capsule
a - Macrocapsule
* A mucoid surface component of the bacterial cell that may protect the cell from phagocytosis and other harmful agents.
* Ides in the adherence of certain microorganism to host cell and other host structures such as the teeth and also other non capsular microbial cell to form clump of cells.
* Is antigenic and stimulate antibodies * macrocapsule formed by Pneumococci, Klebsiella , Meningococci, many oral Streptococci , the yeast creptococci and other microorganism.


b- Microcapsule
* Envelop type antigen-K and V are closely associated with the outer surface of the cell wall, certain Gram s negative microorganism……They are carbohydrate.
The envelop K – antigens are believe to be associated with virulence of certain strain of Escherichia coli
* Those strains that produce large amount of K – antigen are less susceptible to killing following Phagocytosis .
* Certain strain of Salmonella typhi and certain strain of Escherichia coli contain Vi – antigen
* The Vi- antigen is though to interferes with serum bactericidal activity and also fagocytosis.

II- Toxin
a-Exotoxins
*Highly poisonous substance, generally protein , formed and lebiratedby many growing organisms, including Cl0stridium botulinum, Clostridium tetani , Clostridium perfringens, certain strain of Siaphylococcus aurous, Bordetella pertusis, Streptococcuspyogenes, Vibrio cholerae,Shigella dysenteriae, and Corynibacterium diphtherium.
* Exotoxins show preference in their effect on tissue such as nerve, heart, kidney, intestine and muscles.
*some exotoxin inhibit prtein synthesis , other exotoxins injure perephral nerve ending or effect the central nerve system, increase fluid loss from intestinal cells causing diarrhea and vomiting, or cause skin rash.
*Exotoxin are heat labile, boiling 10 -15 minutes destroy the botulinum toxin, while the enterotoxin of S. Aureus resist boiling for 30 minutes also the botulinum toxin and Staphylococcus enterotoxin resist Protiolytic enzyme .
* The exotoxin of diphtheria , tetanus, botulinum, and other microorganism, form toxoid by treatment with formalin and are render non toxic.
b- Endotoxins
III- Enzymatic And related factors:
a-Hyaluronidase
* Breakdown the intracellular substance, hyaluronic acid and facilitate spread of infection through tissue
* this enzyme formed by many organisms including Staphylococci, Streptococci , Pneumococci, Diphtheroid bacilli And tetani
* Hyaluronidase is antigenic , immunologically specific for groups streptococci; Staphylococcus hyaluronidase is antigenically homologous.
b-coagulase
*Causing clotting of human and rabbit plasma resulting in a fibrous covering about the microorganism or the lesion.
* formed by many Staphylococci, coliform bacilli, Pseudomonas, Serratia marcescens and other organism
* May function in localizing infection as boils and pimples.
* Might be involved in the formation of clots in the blood stream.
* Activation of this enzyme required an accessory factor or ((RF)); when activated it converts fibrinogen to fibrin , which result in the clot.
* cell bound coagulase has been reported and can be demonstrated in the test tube, dose not need the reactive factor.
* This antigen is antigenic and antigenic study indicate seven different varieties . Although coagulase may protect the organism from phagocytosis and the ability to form clot coagulase reported to be closely associated with virulence of Staphylococcus. Coagulase negative mutant Have been shown to be just as virulent as the parent strain that produce the Enzyme, it is though that fibrin clot ( the result of coagulation of plasma)on the surface of individual
Staphylococci, which may help protect them from phagocytosis or from destruction within phagocytic cells.

c- Streptokinase
* A substance that activates a protiolytic enzyme of plasma this enzyme is then able to dissolve coagulated (or fibrinolysin) plasma and probably aids in the rapid spread of streptococci through tissues .Streptokinase has been used in
treatment of acute myocardial infection to dissolve fibrin clot.

d- Hemolysin
* Many bacteria produce substance that are cytolysins ,ie, they dissolve red blood cell or kill tissue cells. For example streptolysin O is produce by group A streptococci and is lethal for mice and hemolytic for red blood cells from many animal
*Streptolysin O is oxygen - labile and can be oxidized and inactivated and can be reactivatedby reducing agents. It is antigenic. The same Streptococci also produce oxygen stable , serum –inducible
*streptolysin –S which is not antigenic .
*Clostridia produce various hemolysin, including ?, B, and ? , hemolysins , including the lecithinase.

e- Leukocidines
* Destroy polymorphonuclear leukocytes formed by many organisms, usually Streptococciand Staphylococci * In some cases leukocidines appear to be the same as certain hemolysins, the leukocidin of Staphylococc Is the same as an alpha hemolysin .

f- Diphosphorpyridin nucleiotidase
* associated with toxicity for leukocytes , formed by strains of S, pyogenes.

g- Necrotoxin
* Formed by Staphylococcus , kill tissue cells.

h- Hypothermic factor
* lower body temperature , formed by Shigella dysenteriae.
i- Edema producing factor
* Formed by Pneumococci cause edema.
j- Catalase
*An enzyme that may be associated with pathogenicity of certain organisms such as Mycobacterium tuberculosis and Brucella abortus. Brucella abortus , has high catalase contant that might protect it againsthydrogen peroxide (H2O2)in phagocytosis.
* Virulent strain of Mycobacterium tuberclosis are catalase positive , avirulent strain are catalase negative And also lost their acid-fast property.
k- Deoxyribonuclease (streptodornase)
* Liquifies purulent exudate , formed by certain Streptococci and might contribute to invassivenessof the organism by liquifing the host barrier which localize infection.

l- IgA1- proteases
* Some bacteria such as Neisseria meningitides Neisseria gonorrhoeae Hemophilus influenziae and Streptococcus pneumoniae produce IgA1 prot(immunoglobulin-1) proteases , that split IgA-1 at specific Prolin – threonineb or prolin- serine bounds in the hinge region And inactivate its antibody activity
* IgA-1 protease is an important virulence factor of the pathogens of oral cavityb such as some strain of Prevotella melaninogenica , some streptococci associated with dental disease.
Some bacteria ( Few ) example , Capnocytophaga and Bordetella produce soluble factor or toxin that inhibit chemotaxis by leukocytes and thus evade phagocytosis by different mechanism.