Bacterial Virulence Mechanisms

Bacterial virulence mechanisms are organised here by mechanism rather than toxin type. Toxin production is a distinct mechanism. The remaining mechanisms are developed with specific, named organism examples.

Adhesins are surface structures or molecules mediating attachment to host cells or tissue. This is the essential first step in colonisation for almost every organism.

Pili and fimbriae examples:

• The type 1 and P pili of uropathogenic E. coli mediate bladder and kidney uroepithelial attachment respectively.
• The type IV pili of Neisseria gonorrhoeae are subject to extensive antigenic and phase variation.
• The toxin-coregulated pili of V. cholerae also serve as the receptor for the cholera toxin-encoding bacteriophage.

Surface protein adhesins include filamentous haemagglutinin and pertactin of Bordetella pertussis, which mediate attachment to ciliated respiratory epithelium.

M protein of Streptococcus pyogenes is simultaneously an adhesin, an antiphagocytic factor and, via molecular mimicry, the trigger for rheumatic fever. A single virulence factor can serve multiple mechanistic roles at once.

Capsules are antiphagocytic polysaccharide, occasionally polypeptide, surface layers. They are the dominant virulence mechanism for a recurring group of encapsulated pathogens:

• Streptococcus pneumoniae
• Neisseria meningitidis
• Haemophilus influenzae type b
• Klebsiella pneumoniae, notable for its markedly mucoid colony phenotype
• group B Streptococcus
• Bacillus anthracis, atypical among Gram positive spore-formers and unusual in having an antiphagocytic polypeptide rather than polysaccharide capsule

The shared mechanism masks underlying antigens and impedes complement deposition and phagocytic engulfment.

This explains the vulnerability of asplenic patients to overwhelming infection with encapsulated organisms, since the spleen is central to clearing poorly opsonised, encapsulated bacteria. It also underlies the shared strategy of conjugate vaccination against the three most clinically dominant encapsulated pathogens: S. pneumoniae, N. meningitidis and H. influenzae type b.

Invasins are molecules or systems that actively promote bacterial entry into host cells or deeper tissue penetration, distinct from simple surface adhesion.

Type III secretion system-mediated invasion occurs when Salmonella and Shigella both inject effector proteins directly into host intestinal epithelial cells via a type III secretion system. This triggers cytoskeletal rearrangement that promotes bacterial uptake.

The two organisms otherwise cause clinically distinct disease patterns. Typical nontyphoidal Salmonella is systemic and noninvasive-lumen-confined, whereas Shigella causes locally invasive dysentery.

Enzymatic tissue-spreading factors include hyaluronidase and streptokinase of S. pyogenes. They degrade connective tissue matrix and fibrin respectively to facilitate spread through tissue.

Intracellular survival is the capacity to survive and replicate within host cells, typically macrophages, evading extracellular antibody and complement surveillance entirely.

This mechanism recurs across a genuinely diverse range of organisms:

• Listeria monocytogenes: phagosomal escape via listeriolysin O, followed by actin-based cell-to-cell spread
• the mycobacteria: inhibition of phagosome-lysosome fusion
• Brucella and Francisella: both inhibit phagolysosome fusion
• Legionella pneumophila: survival within alveolar macrophages via a type IV secretion system
• Salmonella, particularly the typhoidal serovars: these disseminate systemically via intracellular survival within macrophages of the reticuloendothelial system

The shared consequence across this entire group is that cell-mediated, T cell, immunity, not antibody-mediated immunity, is the essential host defence, directly explaining why impaired cell-mediated immunity, HIV infection being the paradigm example, disproportionately increases susceptibility to this specific cluster of organisms.

Secretion systems are specialised molecular machinery injecting effector proteins directly into host cells. They are also relevant to invasins and intracellular survival, and are a distinct mechanistic category in their own right.

Type III secretion systems are used by Salmonella, Shigella, Pseudomonas aeruginosa and Yersinia species. They inject effector proteins that manipulate host cell cytoskeleton, signalling or immune response. They function essentially as a molecular syringe directly bridging the bacterial and host cell cytoplasm.

Type IV secretion systems are used by Legionella pneumophila and Bordetella pertussis. Bordetella pertussis uses this system, in addition to its A-B pertussis toxin, to translocate effector molecules.

Iron acquisition systems scavenge iron from host iron-binding proteins, transferrin, lactoferrin, since the host actively restricts free iron availability as a nonspecific defence strategy termed nutritional immunity.

Siderophores, high-affinity iron-chelating molecules, are the principal mechanism, produced by a wide range of organisms.

They have particular clinical relevance for organisms causing systemic, iron-dependent infection such as E. coli and Pseudomonas aeruginosa, both possessing well-characterised siderophore systems contributing to their capacity for invasive disease.

Biofilms recur as a clinical theme across bacteriology:

• Staphylococcus epidermidis on prosthetic devices and catheters
• Pseudomonas aeruginosa in chronic cystic fibrosis airway colonisation
• Enterococcus species contributing to endocarditis on damaged valves
• dental plaque, a naturally occurring biofilm relevant to Actinomyces and other oral flora

Secreted enzymes degrade host tissue barriers or defence molecules, facilitating spread and immune evasion.

Coagulase, from S. aureus, converts fibrinogen to fibrin, forming a protective fibrin wall around the organism and contributing to abscess formation.

IgA proteases cleave secretory IgA, the dominant mucosal antibody class. They are produced by Neisseria gonorrhoeae, Neisseria meningitidis, Streptococcus pneumoniae and Haemophilus influenzae, a shared mechanism across the classic encapsulated and mucosal respiratory pathogens.

Urease is produced by Proteus species and by Helicobacter pylori. In Proteus it alkalinises urine and promotes struvite stone formation. In Helicobacter pylori it locally neutralises gastric acid, enabling gastric survival. The same enzyme serves entirely different ecological purposes in two unrelated organisms and clinical contexts.

Hyaluronidase is a further tissue-spreading enzyme, produced by S. pyogenes.

Antigenic variation is alteration of surface antigens over time, allowing an organism to repeatedly evade an immune response generated against a previous antigenic form, a mechanism recurring across several organisms with otherwise unrelated biology.

Pilin and Opa protein phase and antigenic variation in Neisseria gonorrhoeae explains the organism's capacity for repeated reinfection and has directly frustrated vaccine development.

Relapsing fever Borrelia shows sequential antigenic variation of surface proteins, producing the organism's defining pattern of recurring febrile episodes, each representing a new antigenic variant temporarily evading the existing antibody response.

M protein variation in S. pyogenes shows different M protein serotypes explaining the capacity for repeated streptococcal pharyngitis or skin infection across a lifetime despite prior exposure.