Microbial Mechanisms of Pathogenicity

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Pathogenicity. This is the potential capacity of certain species of microbes to cause an infectious process.
Virulence signifies the degree of pathogenicity of the given culture (strain). Virulence, therefore, is an index of the qualitative individual nature of the pathogenic microorganism. Virulence in pathogenic microbes changes under the influence of natural conditions.

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Microbial Mechanisms 

of Pathogenicity

Pathogenicity. This is the potential capacity of certain species of microbes to cause an infectious process.

 Virulence signifies the degree of pathogenicity of the given culture (strain). Virulence, therefore, is an index of the qualitative individual nature of the pathogenic microorganism. Virulence in pathogenic microbes changes under the influence of natural conditions.


Main Features 

of Pathogenic Microorganisms.

The virulence of pathogenic microorganisms is associated with

  • adherence,
  • invasiveness,
  • capsule production,
  • toxin production,
  • aggressiveness
  • and other factors.

The adherence

Adherence factor




Filamentous hemagglutinin 


Causes adherence to erythrocytes 




Help attach to solid  bacteria  to solid surfaces


Glycocalyx or capsule


Inhibits phagocytosis and aids  in adherence




Bind bacteria together for transfer  of genetic material




Tenacious bacterial film that  is less compact than a capsule 


Teichoic and lipoteichoic acid


Cell wall components in gram  positive bacteria that aid in adhesion

Adherence bacteria to cell surfaces

Adherence of vibrio cholera on  the mucose

Capsule production


Capsule production makes the microbes resistant to phagocytosis and antibodies, and increases their invasive properties.

Thus, for example, capsular anthrax bacilli are not subject to phagocytosis, while noncapsular variants are easily phagocytized.

The role of capsular material  
in bacterial virulence.


Some pathogenic microorganisms  
(B. anthracis, C. perfringens, S. pneumo-niae, causative agents of plague and tularaemia) are capable of producing a capsule in animal and human bodies. Certain microorganisms produce capsules in the organism as well as in nutrient media (causative agents of rhinoscleroma, ozaena, pneumonia).

Virulent microbes are characterized  by the ability to penetrate tissues of  the infected organism (invasive properties).


  • collagenase and hyaluronidase 
  • immunoglobulin A protease
  • leukocidins
  • M-protein
  • protein A


Invasive properties of pathogenic  bacteria

Collagenase and hyaluronidase degrade collagen and hyaluronic acid, respectively, thereby allowing the bacteria to spread through subcutaneus tissue (Streptococci, Staphylococci, Clostridium ).


Immunoglobulin A protease degrades IgA, allowing the organism to adhere to mucous membranes, and is produseed chiefly by N. gonorrhoeae, Haemophilus influenzae, and S. pneumoniae.

Leukocidins can destroy both neutrophilic leukocytes and macrophages.


M-protein of S. pyogenes is antiphagocytic.


Protein A of S. aureus binds to IgG and prevents the activation of complement.

Coagulase, which is produced by S. aureus and accelerate the formation of a fibrin clot from its precursor, fibrinogen (this clot may protect the bacteria from phagocytosis by walling off the infected area and by coating the organisms with a layer of fibrin)

The invasion of cells by  bacteria

According to the nature of  production, microbial toxins are subdivided  into exotoxins and endotoxins.


More than 50 protein exotoxins of bacteria are known to date.


Toxin production

  • Exotoxins easily diffuse from the cell into the surrounding nutrient medium. 
  • They are characterized by a markedly distinct toxicity, and act on the susceptible organism in very small doses.
  • Exotoxins have the properties of enzymes hydrolysing vitally important components of the cells of tissues and organs.

Exotoxins exert their effects in a variety of ways – by inhibition of protein synthesis, inhibition of nerve synapse function, disruption of membrane trans-port, damage to plasma membranes.

Exotoxins may be devided into fifth categories on the basis of the site affected:


  • neurotoxins (tetanotoxin, botulotoxin)  
    C. tetani, C. botulinum, B. cereus, S. aureus;
  • cytotoxins (enterotoxins, dermatonecrotoxin) 
    E. coli, Salmonella spp., Klebsiella spp., V. cholerae, C. perfringens;
  • functional blocators (cholerogen),  
    V. cholerae;
  • membranotoxins (hemolysins, leucocidin),  
    S. aureus;
  • exfoliatin  
    S. aureus.

Action of the hemolysin on  red blood cells













Clostridium botulinum



Several neurotoxins      






Paralysis; blocks neural transmission



Clostridium tetani









Spastic paralysis; interferes with  motor neurons



Corynebacterium diphtheriae










Blocks protein synthesis



Bordetella pertussis



Pertussis toxin



Whooping cough



Blocks G proteins that are  involved in regulation of cell pathways



Streptococcus pyogenes







Scarlet fever Food



Lysis of blood cells 



Staphylococcus aureus 









Intestinal inflammation



Aspergillus flavus









Blocks transcription of DNA, thereby  stopping protein synthesis



Amanita phalloides






Mushroom food poisoning



Blocks transcription of DNA,thereby  stopping protein synthesis




  • are more firmly 
  • bound with the body of the bacterial cell,
  • are less toxic and act on the organism in large doses;
  • their latent period is usually estimated in hours,
  • the selective action is poorly expressed.

According to chemical structure,  endotoxins are related to glucoside-lipid and  polysaccharide compounds or phospholipid-protein  complexes. 

They are thermostable. Some endotoxins  withstand boiling and autoclaving at 120°C  for 30 minutes. 


Action of the endotoxin


Endotoxin in the bloodstream

Differences between exotoxins and  endotoxins










Heat labile


Heat stable


Actively secreted by cells, diffuse  into surrounding medium


 form part of cell wall,do not diffuse into surrounding medium


Readily separable from cultures  by physical means  such as filtration


Obtained only by cell lysing


Action often enzymic


No enzymic action


Specific pharmacological effect for  each exotoxin


Non-specific action of all endotoxins

Specific tissue affinities


No specific tissue affinity


Active in very minute doses


Active only in very large  doses


Highly antigenic


Weakly antigenic


Stimulate formation of antitoxin  which neutralizes toxin


Do not stimulate formation of  antitoxin


Converted into toxoid by formaldehyde


Can not be toxoided


Produced by both gram-positive  bacteria and gram-negative bacteria


Produced by gram-negative bacteria  only


Frequently controlled by extrachromosomal  genes (e.g. plasmids)


Synthesis directed by chromosomal  genes genes

In characterizing pathogenic microbes  a unit of virulence has been established.

  • Dlm (Dosis letalis minima), representing the minimum amount of live microbes which in a certain period of time bring about 95-97 % death of the corresponding laboratory animals.
  • the absolute lethal dose of pathogenic microbe Dcl (Dosis certa letalis) which will kill 100 % of the experimental animals has been established.
  • At present LD50 (the dose which is lethal to one half of the infected animals) is considered to be the most suitable, and may serve as an objective criterion for comparison with other units of virulence.




Produced by both gram-positive bacteria and  gram-negative bacteria

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