Medical biology, microbiology, virology, immunology department by As.Prof. O.V. Pokryshko 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 Description Filamentous hemagglutinin Causes adherence to erythrocytes FimbriaeHelp attach to solid bacteria to solid surfaces Glycocalyx or capsuleInhibits phagocytosis and aids in adherence PiliBind bacteria together for transfer of genetic material SlimeTenacious 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).

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

Action of the hemolysin on red blood cells

MICROORGANISM TOXINDISEASEACTION Clostridium botulinum Several neurotoxins BotulismParalysis; blocks neural transmission Clostridium tetani NeurotoxinTetanusSpastic paralysis; interferes with motor neurons Corynebacterium diphtheriae Cytotoxin Diphtheria Blocks protein synthesis Bordetella pertussis Pertussis toxin Whooping coughBlocks G proteins that are involved in regulation of cell pathways Streptococcus pyogenes Hemolysin Scarlet fever FoodLysis of blood cells Staphylococcus aureus EnterotoxinPoisoningIntestinal inflammation Aspergillus flavusCytotoxin AflatoxicosisBlocks transcription of DNA, thereby stopping protein synthesis Amanita phalloides Cytotoxin Mushroom food poisoning Blocks transcription of DNA,thereby stopping protein synthesis

Endotoxins 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 exotoxinsendotoxins ProteinsLipopolysaccharides Heat labile Heat stable Actively secreted by cells, diffuse into surrounding medium form part of cell wall,do not 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 % 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 LD 50 (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.