Penicillin tolerance among oral streptococci

Penicillin tolerance was elicited in 18 of 46 strains of viridans streptococci isolated from the mouths of 19 of 20 healthy subjects and in 31 of 54 consecutive blood-culture isolates of streptococci. Enterococci and Streptococcus sanguis were the organisms most frequently tolerant but the property was also common among isolates of S. mutans, S. mitior and Lancefield Group G streptococci. Pneumococci and S. salivarius were rarely tolerant. When incubated with penicillin at 5 x MIC in batch or continuous culture, both tolerant and sensitive strains of oral streptococci declined in number less rapidly than S. pyogenes. However, combinations of penicillin and gentamicin killed tolerant and sensitive oral streptococci.