Citrobacter diversus NF85 produced a chromosomal β-lactamase that was induced by a variety of β-lactam antibiotics. Two major forms of the enzyme, with isoelectric points (pI's) of 5·7 and 6·2, were found in crude cell extracts. Derepressed mutants of NF85, generated by nitrosoguanidine treatment, displayed different levels of β-lactamase expression to the parent strain and had different patterns of resistance to a range of β-lactam antibiotics. Those mutants of NF85 that were totally derepressed, expressing high, constitutive levels of enzyme, were found to have an additional β-lactamase activity with a pI of 6·8. © 1990 by The British Society for Antimicrobial Chemotherapy.
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