Sialidase and N-acetylneuraminate catabolism in nutrition of Mycoplasma alligatoris.

The contribution of N-acetylneuraminate scavenging to the nutrition of Mycoplasma alligatoris was examined. The wild-type grew substantially faster (P<0.01) than the mutant strains that were unable either to liberate (extracellular NanI- mutants) or to catabolize (NanA- mutants) N-acetylneuraminate from glycoconjugates in minimal SP-4 medium supplemented only with serum, but the growth of sialidase-negative mutants could not be restored to wild-type rate simply by adding unconjugated sialic acid to the culture medium. In 1 : 1 growth competition assays the wild-type was recovered in >99-fold excess of a sialidase-negative mutant after co-culture on pulmonary fibroblasts in serum-free RPMI 1640 medium, even with supplemental glucose. The advantage of nutrient scavenging via this mechanism in a complex glycan-rich environment may help to balance the expected selective disadvantage conferred by the pathogenic effects of mycoplasmal sialidase in an infected host.

Cellular Microbiology of Mycoplasma canis.

Mycoplasma canis can infect many mammalian hosts but is best known as a commensal or opportunistic pathogen of dogs. The unexpected presence of M. canis in brains of dogs with idiopathic meningoencephalitis prompted new in vitro studies to help fill the void of basic knowledge about the organism's candidate virulence factors, the host responses that it elicits, and its potential roles in pathogenesis. Secretion of reactive oxygen species and sialidase varied quantitatively (P < 0.01) among strains of M. canis isolated from canine brain tissue or mucosal surfaces. All strains colonized the surface of canine MDCK epithelial and DH82 histiocyte cells and murine C8-D1A astrocytes. Transit through MDCK and DH82 cells was demonstrated by gentamicin protection assays and three-dimensional immunofluorescence imaging. Strains further varied (P < 0.01) in the extents to which they influenced the secretion of tumor necrosis factor alpha (TNF-α) and the neuroendocrine regulatory peptide endothelin-1 by DH82 cells. Inoculation with M. canis also decreased major histocompatibility complex class II (MHC-II) antigen expression by DH82 cells (P < 0.01), while secretion of gamma interferon (IFN-γ), interleukin-6 (IL-6), interleukin-10 (IL-10), and complement factor H was unaffected. The basis for differences in the responses elicited by these strains was not obvious in their genome sequences. No acute cytopathic effects on any homogeneous cell line, or consistent patterns of M. canis polyvalent antigen distribution in canine meningoencephalitis case brain tissues, were apparent. Thus, while it is not likely a primary neuropathogen, M. canis has the capacity to influence meningoencephalitis through complex interactions within the multicellular and neurochemical in vivo milieu.