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The possible role of β-lactamase-producing bacteria in protecting group A streptococci from the activity of penicillin is not precisely clear. As Dr Brook indicates in his letter, the reasons for high failure rates in second and third courses of penicillin are "still unknown and speculative." I believe further confirmation of the cited studies is necessary to place all of the issues in perspective.
Ferrieri P. Treatment of Group A Streptococcal Pharyngotonsillitis-Reply. JAMA. 1982;247(18):2496. doi:10.1001/jama.1982.03320430020008