Capnocytophaga canimorsus, a commensal bacterium of the canine oral flora, has been repeatedly isolated since 1976 from severe human infections transmitted by dog bites. Here, we show that C. canimorsus exhibits robust growth when it is in direct contact with mammalian cells, including phagocytes. This property was found to be dependent on a surface-exposed sialidase allowing C. canimorsus to utilize internal aminosugars of glycan chains from host cell glycoproteins. Although sialidase probably evolved to sustain commensalism, by releasing carbohydrates from mucosal surfaces, it also contributed to bacterial persistence in a murine infection model: the wild type, but not the sialidase-deficient mutant, grew and persisted, both when infected singly or in competition. This study reveals an example of pathogenic bacteria feeding on mammalian cells, including phagocytes by deglycosylation of host glycans, and it illustrates how the adaptation of a commensal to its ecological niche in the host, here the dog's oral cavity, contributes to being a potential pathogen.

Capnocytophaga canimorsus, a commensal bacterium from dogs' mouths, can cause septicemia or meningitis in humans through bites or scratches. Here, we describe and characterize the inflammatory response of human and mouse macrophages on C. canimorsus infection. Macrophages infected with 10 different strains failed to release tumor necrosis factor (TNF)- alpha and interleukin (IL)-1 alpha . Macrophages infected with live and heat-killed (HK) C. canimorsus 5 (Cc5), a strain isolated from a patient with fatal septicemia, did not release IL-6, IL-8, interferon- gamma , macrophage inflammatory protein-1 beta , and nitric oxide (NO). This absence of a proinflammatory response was characterized by the inability of Toll-like receptor (TLR) 4 to respond to Cc5. Moreover, live but not HK Cc5 blocked the release of TNF- alpha and NO induced by HK Yersinia enterocolitica. In addition, live Cc5 down-regulated the expression of TLR4 and dephosphorylated p38 mitogen-activated protein kinase. These results highlight passive and active mechanisms of immune evasion by C. canimorsus, which may explain its capacity to escape from the host immune system.

Capnocytophaga canimorsus is a bacterium of the canine oral flora known since 1976 to cause rare but severe septicemia and peripheral gangrene in patients that have been in contact with a dog. It was recently shown that these bacteria do not elicit an inflammatory response (H. Shin, M. Mally, M. Kuhn, C. Paroz, and G. R. Cornelis, J. Infect. Dis. 195:375-386, 2007). Here, we analyze their sensitivity to the innate immune system. Bacteria from the archetype strain Cc5 were highly resistant to killing by complement. There was little membrane attack complex (MAC) deposition in spite of C3b deposition. Cc5 bacteria were as resistant to phagocytosis by human polymorphonuclear leukocytes (PMNs) as Yersinia enterocolitica MRS40, endowed with an antiphagocytic type III secretion system. We isolated Y1C12, a transposon mutant that is hypersensitive to killing by complement via the antibody-dependent classical pathway. The mutation inactivated a putative glycosyltransferase gene, suggesting that the Y1C12 mutant was affected at the level of a capsular polysaccharide or lipopolysaccharide (LPS) structure. Cc5 appeared to have several polysaccharidic structures, one being altered in Y1C12. The structure missing in Y1C12 could be purified by classical LPS purification procedures and labeled by tritiated palmitate, indicating that it is more likely to be an LPS structure than a capsule. Y1C12 bacteria were also more sensitive to phagocytosis by PMNs than wild-type bacteria. In conclusion, a polysaccharide structure, likely an LPS, protects C. canimorsus from deposition of the complement MAC and from efficient phagocytosis by PMNs.

Bacteria from the genus Yersinia deliver a number of effectors into host cells via type III secretion (T3S). Injected Yop effectors interfere and prevent pro-inflammatory warning signals by hijacking the host's intracellular machinery. While macrophages infected by wild-type Yersinia enterocolitica did not release mature IL-1beta, macrophages infected by Y. enterocolitica deprived of all effectors released mature IL-1beta. Surprisingly, macrophages infected by Y. enterocolitica deficient for secretion of all T3S proteins, including effectors and translocators, did not release mature IL-1beta. Using different genetic constructs, we show that insertion of T3S translocation pores trigger activation of caspase-1, maturation of proIL-1beta and release of mature IL-1beta, which occurs independently of cell osmotic lysis. These data show that T3S translocation is intrinsically a pro-inflammatory phenomenon. However, in the case of Yersinia, this effect is neutralized by the action of effectors.

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