Use of penicillin-gradient and replicate plates for the demonstration of tolerance to penicillin in streptococci

A new method was developed for the demonstration of tolerance to penicillin in streptococci of groups B and D. The sequential use of penicillin-gradient and replicate plates was simple, reliable, and less time-consuming than conventional methods for the measurement of ratios of minimal bactericidal to minimal inhibitory concentrations and rates of bacterial killing. This method promises to be of value in the further characterization and understanding of penicillin tolerance in vitro and in vivo.