PHASE VARIATION IN PNEUMOCOCCAL OPACITY - RELATIONSHIP BETWEEN COLONIAL MORPHOLOGY AND NASOPHARYNGEAL COLONIZATION

When colonies of encapsulated isolates of Streptococcus pneumoniae are viewed with oblique, transmitted light on a transparent surface, they are heterogeneous in appearance because of variation in opacity. Ther e is spontaneous phase variation among at least three discernible phen otypes at frequencies from 10(-3) to 10(-6). The ability to detect dif ferences in opacity varies according to serotype, but variation is ind ependent of capsule expression. Electron microscopy shows no differenc e in chain length but suggests that autolysis occurs earlier in the gr owth of the transparent variant. There was no identifiable difference in membrane protein profiles of opaque and transparent variants of the same strain. In an infant rat model of nasopharyngeal carriage, there was no significant colonization by opaque variants. Efficient and sta ble colonization by the transparent variants was observed, suggesting a selective advantage for this phenotype in the nasopharynx. In contra st, there was no difference in the incidence of bacteremia or in the 5 0% lethal dose among the variants following their intraperitoneal inoc ulation. These results suggest that phase variation which is marked by differences in colonial morphology may provide insight into the inter action of the pneumococcus with its host.