Tolerance to penicillin was investigated in 40 isolates of optochin-resistant, alpha-haemolytic streptococci. Thirteen strains exhibited tolerance to penicillin (MBC:MIC ratio greater than or equal to 32) when stationary phase inocula were used, but only seven strains retained the tolerance phenotype in experiments with logarithmic phase inocula. There was a striking association between tolerance and Eagle's optimum dosage effect, particularly among strains that displayed tolerance in both the stationary and the logarithmic growth phases. Sequential viable counts on representative strains showed that reliance on the arbitrary criterion of bactericidal activity of 99.9% reduction of the original inoculum after 24 hours' exposure may occasionally lead to difficulties in the recognition of penicillin tolerance. In general, however, the 99.9% killing criterion provided a useful discriminator between strains that were rapidly killed by penicillin and those (tolerant strains) in which the bactericidal activity was much reduced.
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