Relationship of pili to colonial morphology among pathogenic and nonpathogenic species of Neisseria

Abstract

Growth in colonies with type 1 morphology and the presence of pili are characteristics that are associated with virulence of gonococci for humans. To determine whether the presence of pili per se might be responsible for colony type 1 morphology, the relationship of pili to colony type was examined in various Neisseria spp. Short pili (175-210 nm in length) were seen only on nonpathogenic Neisseria, long pili (up to 4300 nm) were seen on nonpathogenic and pathogenic species. Although long pili, similar to those found on organisms from high-domed, type 1 colonies of gonococci, were observed on organisms from high-domed, type 1 colonies of nonpathogenic Neisseria spp., they were also observed on low-convex, type 4 colonies of meningococci [N. meningitidis] and nonpathogenic Neisseria. Among meningococci there was no difference in the morphology of colonies consisting of organisms with many long pili and colonies consisting of organisms that completely lacked pili. Thus, there was no consistent relationship of pili to colonial morphology. Unless the pili of N. gonorrhoeae are unique among Neisseria spp. in their influence on colonial morphology, it is likely that factors other than pili determine colony type 1 morphology of gonococci. Whether these same factors, either alone or in conjunction with pili, are also responsible for gonococcal virulence warrants further investigation.