Isolation of the smooth (Sm) morphotype of , a suspected oral pathogen, is sometimes accompanied by isolation of a rough (Rg) morphotype of . The Rg type readily changes to a Sm-like variant (Rg) in broth culture. Sm and Rg isolates and Rg variants were compared to determine whether these three types are the result of phase variation. The Rg variants resembled the Sm morphotype in colony morphology; furthermore, the Sm type and the Rg type did not have the fibrillar surface structures characteristic of the Rg type, and the Sm and Rg types were more hydrophobic than the Rg type. However, when we compared the sodium dodecyl sulfate-polyacrylamide gel electrophoresis patterns of whole-cell proteins, serotyping data, pyrolysis mass spectrometry data, 16S ribosomal DNA sequences, and hemolytic activities, the Rg variants and the Rg isolates were very similar and were clearly distinct from the Sm isolates. These results suggest that the Rg and Rg types form a cluster distinct from the Sm type and thus provide evidence that can be differentiated into two groups, one consisting of the Sm type and the other consisting of the Rg and Rg types.


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