Bacterial interference: effects of oral antibiotics on the normal throat flora and its ability to interfere with group A streptococci

Abstract

The effects of orally administered penicillin and tetracycline on the composition of the normal throat flora and its interference with the growth of group A streptococci were evaluated by throat culture and an agar overlay technique. Tetracycline caused only a slight, transient quantitative decrease in the composition of the flora and interference activity. Penicillin caused significant quantitative and qualitative decreases in both the composition of the flora and interference activity. The diminution in interference activity persisted up to 3 weeks after therapy. The differences observed between the antibiotic regimens correlated with differences in initial susceptibility of the flora to the antibiotic used and emergence of the resistance during therapy. Results indicate that although effects of antibiotics on the composition of the flora are transient, effects on its ability to interfere with group A streptococci may persist long after therapy is discontinued. It is thus possible that penicillin therapy may enhance susceptibility of certain individuals to subsequent infection with group A streptococci.It has long been recognized that the administration of antibiotics may suppress man's indigenous microflora (6-9). Although this may result in superinfection (12, 13), it is usually without obvious clinical consequence and is soon reversed after cessation of therapy. In a previous prospective study, throat cultures were obtained from children and tested for the presence of organisms that were capable of inhibiting the growth of group A streptococci in vitro (3). During two sequential epidemics of asymptomatic group A streptococcal infections, it was observed that cultures from children who did not become infected more frequently contained inhibitory flora than cultures from children who became infected. Since the presence of inhibitory organisms was shown to be associated with resistance to infection, antibiotic suppression might be expected to diminish any protection afforded by the normal flora. The purpose of the present study was to evaluate the effects of orally administered antibiotics on the composition of the normal throat flora and its ability to inhibit the growth of group A streptococci. MATERIALS AND METHODS Throat cultures. Throat cultures were obtained with a dry, sterile cotton-tipped swab pressed firmly around Waldeyer's ring and in a crisscross pattern over the posterior pharyngeal wall. Swabs were immediately placed in 2 ml of brain heart infusion broth (Difco) and shaken vigorously for 3 min. Then 0.01 ml of the broth was placed on the surface of a 5% sheep blood agar plate and streaked for isolation of colonies in the four-quadrant fashion. Cultures were read after incubation for 24 h at 37 C in 10% CO2 in air. Therefore only aerobic and facultative constituents of the throat flora were considered in this study.Identification and quantitation of normal throat flora. Neisseria sp., Micrococcus sp., diphtheroids, and alpha-hemolytic and nonhemolytic streptococci were identified by colonial morph...

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