Bacterial interference. IV. Epidemiological determinants of the antagonistic activity of the normal throat flora against group A streptococci.

A study was performed to identify epidemiological factors such as age, race, sex, and time of culture that might influence the ability of the normal pharyngeal flora to interfere with growth of group A streptococci. From March 1974 through February 1975, throat swabs were obtained from 952 individuals. Cultures were assayed by an agar overlay procedure for the presence of bacteria capable of inhibiting growth of group A streptococci. The observed inhibition was then determined to be bacteriostatic or bactericidal by use of a broth filtrate technique. Regardless of age, race, or sex, subjects were more likely to harbor interfering flora if cultured during the months of March and April, which coincided with the highest prevalence of group A streptococci in the community. Race and sex of subjects appeared not to influence the inhibitory activity of throat flora either quantitatively or qualitatively. However, among individuals with interfering flora, the prevalence of bactericidal organisms increased and bacteriostatic organisms decreased with advancing age. Since the presence of bactericidal, and not bacteriostatic, organisms has been associated with resistance to colonization of the throat by group A streptococci, this higher prevalence of bactericidal organisms in older individuals suggests that bacterial interference may be one of the mechanisms that account for the greater resistance of adults than children to streptococcal throat infection.