Strains of and that appeared to have similar sensitivity to ampicillin when tested by conventional techniques, behaved differently when dense cultures were exposed to ampicillin and opacity readings were taken. These differences were accompanied by distinctive morphological changes revealed by the stereoscan electron microscope. The morphological changes and the very rapid fall in opacity of on exposure to ampicillin suggest that lysis of enterobacteria by penicillins is not wholly due to destruction of osmotically sensitive spheroplasts liberated through defects in the cell wall.

The response of non-swarming strains of to the drug differed from that of swarming strains. The pattern of opacity change was intermediate between that of swarming strains and and the morphological response was distinctive.

Morphological evidence is presented of differences in the susceptibility of individual cells within a bacterial population. It is suggested that such intrapopulation differences in various lethal and non-lethal effects may account for the observed differences in response to ampicillin.


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