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Some bacteria employ strategies to avoid engulfment (ingestion) if phagocytes do make contact with them. Many important pathogenic bacteria bear on their surfaces substances that inhibit phagocytic adsorption or engulfment.
Clearly it is the bacterial surface that matters. Resistance to phagocytic ingestion is usually due to a component of the bacterial cell surface (
cell wall, or
fimbriae, or a
capsule). Classical examples of antiphagocytic substances on bacterial surfaces include:
1. Polysaccharide capsules of
S. pneumoniae,
Haemophilus influenzae,
Treponema pallidum, and
Klebsiella pneumoniae.
2. M protein and fimbriae of Group A streptococci.
3. Surface slime (polysaccharide) produced as a biofilm by
Pseudomonas aeruginosa.
4. O polysaccharide associated with LPS of
E. coli.
5. K antigen (acidic polysaccharides) of
E. coli or the analogous Vi antigen of
Salmonella typhi.
6. Cell-bound or soluble Protein A produced by
Staphylococcus aureus. Protein A attaches to the Fc region of IgG and blocks the cytophilic (cell-binding) domain of the Ab. Thus, the ability of IgG to act as an opsonic factor is inhibited, and opsonin-mediated ingestion of the bacteria is blocked.
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Most antibacterial drugs disrupt or destroy bacterial cellular characteristics t
