Identification of N. gonorrhoeae and Related Species
Of the Neisseria and related species, only N. gonorrhoeae is considered always to be pathogenic i.e., cause disease. N. gonorrhoeae is not considered to be normal flora under any circumstances.
N. gonorrhoeae strains may infect the mucosal surfaces of urogenital sites (cervix, urethra, rectum) and the oro- and nasopharynx (throat), causing symptomatic or asymptomatic infections. Gonococcal infections of the urogenital sites are more frequently symptomatic than asymptomatic; however, asymptomatic infections may occur. Gonococcal infections of the oro- and nasopharynx and the rectum may be asymptomatic more frequently than symptomatic. Some specific strains of N. gonorrhoeae have been associated with asymptomatic infections of urogenital sites; these include strains that require arginine, hypoxanthine, and uracil (AHU) or proline, arginine (citrulline), and uracil (PAU) to grow on chemically defined media on which gonococcal strains are characterized.
Strains of N. meningitidis may be carried as normal flora in the throat. Under some circumstances, N. meningitidis may also be pathogenic, causing sporadic cases or epidemics of meningitis. Between 3% and 30% of healthy persons in nonepidemic geographic areas may be asymptomatic carriers of N. meningitidis i.e., meningococci have colonized their throats without causing disease. The carrier state may persist for many months.
N. meningitidis causes epidemic meningitis in many parts of the world such as Sub-Saharan Africa. Certain types of N. meningitidis are usually associated with meningitis. Of a total of 13 serogroups of N. meningitidis, strains belonging to the serogroups A, B, C, and W-135 have most frequently been associated with epidemics. Group A strains have been associated with most epidemics, whereas group B, C, and W-135 strains have caused sporadic epidemics. Epidemics may occur in military recruits; in these groups, more than 95% of recruits may be asymptomatic carries of the epidemic strain while only 1% develop meningitis.
Occasionally, N. meningitidis has been isolated from the cervix or urethra of men or women. N. meningitidis is not normal flora in genitourinary sites.
Other Neisseria species are considered to be commensals i.e., they colonize the host without causing disease. Strains of these species are normal flora in the throat.
Commensal Neisseria species appear to be opportunistic pathogens i.e., these species may cause infections although are not routinely associated with specific types of infections or infections of specific sites. Most Neisseria species have been isolated occasionally from blood, cerebrospinal fluid, abscesses etc. but no consistent association between between any individual species and syndrome has been established that would warrant designating any of these species as pathogens. Some infections caused by commensal Neisseria species have occurred in persons who have deficient immune systems and who, thus, may be predisposed to infections with organisms that would not normally cause disease.
For decades, M. catarrhalis was thought to be normal flora of the oro- and nasopharynx. M. catarrhalis is now recognized to be a pathogen which causes pneumonia, systemic disease, sinusitis, otitis media and ophthalmia neonatorum. M. catarrhalis is thought to be normal flora of the nasopharynx although this species is not isolated frequently from the throats of healthy adults. It is possible that this species may colonize certain individuals in a carrier state similar to that observed with N. meningitidis.