Glycan-Foraging Systems Reveal the Adaptation of Capnocytophaga canimorsus to the Dog Mouth

Capnocytophaga canimorsus is known to form two kinds of cells on blood agar plates (coccoid and bacillary), evoking phase variation. When grown in coculture with animal cells these bacteria appeared only as bacilli, but in the presence of vancomycin they were round, indicating that coccoid shapes likely result from weakening of the peptidoglycan layer. Polysaccharide utilization locus 5 (PUL5) and sialidase mutant bacteria, unable to retrieve glycans from glycoproteins, grew less than wild-type bacteria and also appeared polymorphic unless GlcNAc was added, suggesting that C. canimorsus is unable to synthesize GlcNAc, an essential component of peptidoglycan. Accordingly, a genome analysis was conducted and revealed that C. canimorsus strain 5 lacks the GlmM and GlmU enzymes, which convert glucosamine into GlcNAc. Expression of the Escherichia coli GlmM together with the acetyltransferase domain of GlmU allowed PUL5 mutant bacteria to grow normally, indicating that C. canimorsus is a natural auxotroph that relies on GlcNAc harvested from the host N-glycoproteins for peptidoglycan synthesis. Mucin, a heavily O-glycosylated protein abundant in saliva, also rescued growth and the shape of PUL5 mutant bacteria. Utilization of mucin was found to depend on Muc, a Sus-like system encoded by PUL9. Contrary to all known PUL-encoded systems, Muc cleaves peptide bonds of mucin rather than glycosidic linkages. Thus, C. canimorsus has adapted to build its peptidoglycan from the glycan-rich dog’s mouth glycoproteins.