Summary Strains of Escherichia coli showed two types of response to erythromycin in conventional doubling-dilution titrations. For 11 of 16 strains, the original inoculum could be wholly or largely recovered from tubes in which there was no growth after overnight incubation. For the remaining five strains, no survivors could be recovered from tubes in which growth was inhibited. Two strains selected as typifying the bacteristatic and bactericidal responses behaved differently when exposed to erythromycin and continuously diluted with fresh medium as in the urinary bladder. Growth, and also lysis by ampicillin, of a strain showing the bactericidal response were inhibited by lower concentrations of erythromycin than those required to produce comparable effects in a strain showing the bacteristatic effect. On the other hand, the inhibitory effects on growth and ampicillin lysis disappeared as continuing dilution reduced the concentration of erythromycin below the MIC for a strain showing the bactericidal effect but not for the strain showing the bacteristatic effect.
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