Removal of beta-lactam antibiotics during growth of Escherichia coli strains hosting pUR and pEX vectors

Beta-lactam antibiotics supplied at 200 μg/mL were rapidly removed from culture fluids of E. coli strains hosting pUR and pEX vectors, indicating that a substantial period of growth occurred in the absence of positive selection. The retention period of the drugs depended on the specific activity and type of the beta-lactamase, the nature of the antibiotic and growth rate of the host. © 1987 Chapman & Hall.