Summary: The effect of penicillins and cephalosporins on the rate of growth of individual cells of on agar was determined by measurement of cell length using photomicrographs taken at frequent intervals. When exposed to cephalexin or carbenicillin at twice the minimum inhibitory concentration cell division was inhibited but growth in length proceeded at a uniform exponential rate until abruptly terminated by lysis. The rate of growth of such filamentous cells did not differ significantly from that of normal cells grown in the absence of antibiotic. In contrast, exposure to cephaloridine and amoxycillin at twice the minimum inhibitory concentration resulted in a marked diminution in the rate of cell growth, and lysis occurred very much sooner. At concentrations greatly in excess of the minimum inhibitory concentration (> 100-fold) cephalexin also resulted in a diminution in the rate of cell growth and this was accompanied by earlier lysis. The significance of these findings is discussed in relation to the mode of action of β-lactam antibiotics.


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