The generation curves of Escherichia coli B/r and E. coli NIHJ JC‐2 in the presence of several β‐lactam antibiotics were studied from the kinetic point of view. Apparent first‐order regrowth of resistant organisms was observed ∼6 h after addition of these antibiotics. The time courses of apparent viable counts could be interpreted in terms of the sum of the viable counts of sensitive and resistant organisms. To clarify the nature of the regrowth, experiments involving a second addition of antibiotic, single colonization by subculture, and synchronous cell culture were carried out. Several possible explanations for the results are discussed, including β‐lactamase production, selection in terms of membrane permeability, and mutation to acquire drug resistance. A selection process or a modification of membrane permeability caused by contact with the drug seems to be the most probable reason for the regrowth of the organisms.
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