Identification of N. gonorrhoeae and Related Species
The prevalence of gonorrhea varies geographically and is beyond the scope of this page; this topic will be discussed elsewhere.
N. meningitidis strains are carried as normal flora in the oro- and nasopharynx of adults and children. The prevalence of N. meningitidis carriage varies geographically. It has been suggested that N. meningitidis may occur more frequently in adults with gonorrhea and in homosexual men. Fewer than 1% of children are colonized by N. meningitidis during the first 4 years of life; the carriage rate increases after this age.
N. lactamica colonizes the throats of children more frequently than adults. N. lactamica colonizes the throats of as many as 4% of babies of 3 months age to a peak of 21% in children 18 to 24 months; colonization by N. lactamica then declines to 2% by age 14 to 17 years. It is estimated that 59% of children have been colonized by N. lactamica at least once by the age of 4 years. This pattern of colonization may reflect the fact that young children may drink large volumes of milk. Among Neisseria species, strains of N. lactamica are unique in their ability to use lactose; this characteristic may enhance populations of N. lactamica in the throats of younger children. Unlike N. gonorrhoeae and N. meningitidis, N. lactamica has not been implicated as a primary pathogen, although it may be an opportunistic pathogen.
The commensal Neisseria species and K. denitrificans, are normal inhabitants of the oro- or nasopharynx. These species are occasionally isolated from other sites but are not considered to be normal flora of sites other than the throat.
Because strains the commensal Neisseria species rarely grow on selective media used to isolate the gonococcus, the prevalence of these species must be determined on a medium that does not contain colistin, the antibiotic incorporated in gonococcal selective medium to inhibit the growth of the commensal species.
It is impossible to accurately determine the carriage rate of commensal Neisseria species in studies performed in the early 1900s for two reasons. Firstly, most studies of the prevalence of Neisseria spp. and N. catarrhalis (M. catarrhalis) were performed with nonselective media e.g., blood agar, which neither inhibited the growth of other bacterial species nor permitted differentiation between colonies of Neisseria and related species. Thus, it is probable that the carriage rates of some species (N. cinerea, N. polysaccharea, N. lactamica) were underestimated because these species occur in relatively small numbers and were probably overgrown by either non-neisserial species or the sucrose-positive Neisseria species (N. subflava biovar perflava, N. sicca, N. mucosa). Secondly, in these studies, strains were identified with acid detection tests that were not appropriate for the detection of the relatively small amounts of acid produced by Neisseria species, and additional differential tests now used to accurately identify commensal Neisseria species were unknown.
Some studies have been performed using selective differential media that inhibited the growth of non-neisserial species and selected for commensal Neisseria species and M. catarrhalis either by differentiating between the asaccharolytic species or between several different groups of species.
Berger and Wulf determined that the commensal Neisseria species colonzied the throats of adults as follows: N. sicca, 45%; N. perflava, 40%; N. subflava–N. flava, 11%; and “N. catarrhalis,” 3%. It must be remembered that, because nitrate reduction was not used as a differential test for the identification of Neisseria species at that time, strains of N. mucosa were not recognized by these authors. Although not verifiable, it is reasonable to assume that N. mucosa strains were present in the throats of these individuals. Thus, in this study, strains of N. mucosa were identified as either N. sicca or N. perflava and the prevalences of these latter species are overestimated by the inclusion of strains of N. mucosa in their numbers. Using a selective medium at a later time, Berger found that asaccharolytic strains, i.e. strains that do not produce detectable acid from glucose, maltose, sucrose, or fructose, accounted for 15% of all neisserial isolates but, because nitrate reduction was not used to differentiate between N. cinerea and M. catarrhalis until a few years later, the relative colonization rates of these individual species were not given.
Knapp and Hook determined the prevalence and persistence of Neisseria and M. catarrhalis in the oropharynx of adults using a selective medium for commensal Neisseria species. In this study, it was not possible to differentiate between N. subflava biovar perflava (N. perflava) and N. sicca reliably because the typical colony morphology described for N. perflava (smooth, glistening, butyrous colonies) and N. sicca (dry, adherent, wrinkled colonies) was not a reproducible characteristic for the strains examined. In addition, no additional unequivocal differential tests were available to permit accurate identification of each species.
In this study, adults exhibited one of two general patterns of colonization with these species. Some adults were colonized heavily by several strains each of the sucrose-positive species, N. mucosa and N. perflava-N. sicca, whereas others were colonized sparsely by several Neisseria species. The most dramatic finding of this study not previously described was that, whereas most persons were colonized by two or three species, 5% of persons were colonized by four or five Neisseria species. These patterns of colonization were generally persistent although they were occasionally disrupted for a short period of time. For example, a person usually colonized heavily with sucrose-positive strains would occasionally exhibit an alternative pattern of colonization by many species; the original pattern of heavy colonization with sucrose-positive strains was re-established within a short period of time. It is possible that these individuals were persistently colonized by the species observed when the sucrose-positive species were suppressed, but that sucrose-positive species usually outnumbered these other species which, then, were not detected in culture. The prevalence of individual types of strains of commensal Neisseria species may vary geographically. Strains of colistin-resistant N. subflava biovar perflava have been isolated repeatedly in Chicago, yet infrequently in DeKalb County, Georgia, and Seattle, Washington. In contrast, K. denitrificans strains were isolated from 16% of patients in DeKalb County but not in Chicago or Seattle.