Group B streptococci (GBS) have been infrequently recognized as a cause of pharyngitis. We report three cases of GBS pharyngitis in patients with underlying diseases, two of whom were treated with and responded incompletely to oral beta-lactam antibiotics. The susceptibility of 20 clinical isolates of GBS was tested by a broth dilution method to six antibiotics which could conceivably be used in the therapy of GBS pharyngitis. Penicillin G, clindamycin, and erythromycin were most active with mean minimal inhibitory concentrations (MIC) of 0.06 μg/ml or less. Rifampin and cefaclor were least active with mean MICs of 0.71 ug/ml or more. Ampicillin was intermediate in its activity. Therapy traditionally used for Group A streptococcal (GAS) pharyngitis may, at times, be suboptimal for GBS pharyngitis in compromised patients. This may be due to higher minimal bactericidal concentrations (MBC) of GBS than GAS, to inadequate penetration of penicillins into pharyngeal tissues or to host factors. It is suggested that GBS can cause pharyngitis in adults, particularly the compromised patient, and that in cases where there is a poor response to penicillin or ampicillin therapy, alternative drugs (erythromycin or clindamycin) may be used
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