Description of two morphotypes of Peptostreptococcus micros.

Peptostreptococcus micros is often isolated from abscesses in several parts of the human body. The oral cavity is considered the natural habitat for the species, which has been implicated as a periodontal pathogen. In plaque samples from periodontitis patients we observed the presence of a rough morphotype of P. micros in addition to the previously recognized smooth morphotype. The rough morphotype has not been described previously. Both morphotypes are frequently isolated simultaneously from the same patient. In this paper strains of both morphotypes are described. The smooth morphotype, represented by the type strain, grew as small, dome-shaped, bright white, nonhemolytic colonies. The rough morphotype grew as equally white dry colonies which were hemolytic and had wrinkled edges. DNA-DNA reassociation studies revealed homology at the species level between the two morphotypes; in addition, no differences in physiological characteristics were observed when the organisms were tested with API-32A and API-ZYM kits. The rough cells had long, thin fibrillar structures outside the cell envelope when they were stained negatively for electron microscopy. In the smooth morphotype these structures were not present. The sodium dodecyl sulfate-polyacrylamide gel electrophoresis profiles of whole-cell extracts were different for the two morphotypes. In xylene-water phase partition studies, the smooth morphotype was found to be hydrophobic, whereas the rough morphotype was found to be relatively hydrophilic. The distinct morphotypes were stable on blood agar; however, the rough morphotype changed to a nonfibrillar type with a smooth colony morphology after repeated subculturing in broth.