Page 855 - Veterinary Immunology, 10th Edition

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FIG. 26.6 Some of the many different mechanisms by which
bacteria can avoid being killed by phagocytic cells such as
neutrophils.

Strains of Streptococcus pneumonia growing in suspension readily

activate C3 leading to deposition of C3b on their surface and
promoting their opsonization. This does not occur on pneumococci
growing within biofilms where binding of C-reactive protein and
C1q is reduced. Conversely, recruitment of FH, the regulator of the

alternative complement pathway is enhanced in biofilms. Thus
biofilm formation is an efficient method of evading both the
classical and alternative complement pathways. Staphylococcus
aureus and P. aeruginosa can also evade phagocytosis by switching

from planktonic growth to biofilm production.
Bacteria can, of course, avoid being eaten simply by killing
phagocytic cells. Thus Streptococcus canis produces streptolysin O
that lyses neutrophil cell membranes. Several Gram-negative

bacteria of veterinary importance, such as Mannheimia hemolytica
and Fusobacterium necrophorum, secrete leukotoxins that kill
leukocytes, especially granulocytes. The most important
leukotoxins are the RTX (“repeats in toxin”) proteins. M. hemolytica

secretes an RTX toxin that kills ruminant neutrophils, alveolar
macrophages, and lymphocytes. This leukotoxin binds to CD18 as
well as lipid rafts on leukocytes and induces their apoptosis.

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