Journal of Antimicrobial Chemotherapy, Vol 40, 525-532, Copyright © 1997 by The British Society for Antimicrobial Chemotherapy
X Xiang, K Shannon and G French
Some isolates of SHV-5 beta-lactamase-producing Klebsiella pneumoniae K2
from a single-strain outbreak of cross-infection produced approximately
five-fold more beta-lactamase than others. We investigated three possible
genetic mechanisms of this hyperproduction: the presence of a more powerful
promoter, an increase in plasmid copy number or an amplification of the
gene on a plasmid. No differences between low and high beta-lactamase
producers were detected in the promoter region of the SHV-5 beta-lactamase
gene, which closely resembled that of SHV-2. SHV-5 beta-lactamase
production was encoded on a low copy number plasmid, but DNA-DNA
hybridization with an SHV- specific probe detected a higher gene dose in
hyperproducers. The beta- lactamase hyperproduction was unstable on
repeated subculture, with a reduction of about 75% after 100 generations.
Hyperproducing mutants of a low-producing Klebsiella and its Escherichia
coli K-12 transconjugants could be selected in vitro at a frequency of
10(-5) to 10(-6) and these variants had an increased SHV-5 beta-lactamase
gene copy number on low copy number plasmids. We conclude that
hyperproduction of the extended-spectrum beta-lactamase was caused by gene
amplification that could be easily lost or gained in vitro. Since the
change to hyperproduction occurred at a high frequency and hyperproducers
showed increased resistance to many beta-lactams and
beta-lactam/beta-lactamase inhibitor combinations, we suspect that these
variants may readily be selected in patients during antibiotic therapy.
ORIGINAL ARTICLES
Mechanism and stability of hyperproduction of the extended-spectrum beta-lactamase SHV-5 in Klebsiella pneumoniae
Department of Microbiology, UMDS, London, UK.
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