Hyperproduction of inhibitor-susceptible TEM beta-lactamase is responsible for resistance of Serratia marcescens to beta-lactam-beta-lactamase inhibitor combinations.
BACKGROUND
Some clinical isolates of Serratia marcescens showed high-level resistance to expanded-spectrum cephalosporins alone and in combination with beta-lactamase inhibitor, but the beta-lactamase extracted from these strains was sensitive to the inhibitors. This study examines the possible mechanisms responsible for the discrepancy.


RESULTS
Three clinical isolates of S. marcescens bore the bla(TEM) gene coded on a plasmid. This was confirmed by detection of the bla gene using PCR analysis and by transferring resistance determinants to Escherichia coli via conjugation and transformation. All of the E. coli transconjugants and transformants acquired a similar level of resistance to penicillins and narrow-spectrum cephalosporins, and showed the reduced susceptibility to expanded-spectrum cephalosporins alone and in combination with clavulanate. As a result of the highly constitutive expression of the TEM gene, up to 247-690 U of beta-lactamase were produced by 10(10) cells of wild-type S. marcescens and the transconjugants and transformants. In the presence of 0.24 U/ml of TEM enzyme, the minimum inhibition concentrations of cefotaxime against E. coli ATCC 25922 increased from 0.125 to 512 microg/ml. The TEM-type beta-lactamase extracted from these strains was sensitive to clavulanate, and 62.2-92.1% of its activity was inhibited after preincubation with 0.1 mM clavulanate.


CONCLUSIONS
The TEM-type beta-lactamase plays a critical role in the resistance of S. marcescens to beta-lactams, and the hyperproduction of inhibitor-susceptible TEM beta-lactamase is responsible for the resistance to beta-lactam-beta-lactamase inhibitor combinations.
Zhao
Wei-Hua
WH
Department of Microbiology and Immunology, Showa University Hospital, Showa University School of Medicine, Tokyo, Japan. whzhao@med.showa-u.ac.jp
Hu
Zhi-Qing
ZQ
Chen
Gelin
G
Ito
Ribu
R
Shimamura
Tadakatsu
T
D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D002511:Cephalosporins / Q000494:pharmacology*; D019818:Clavulanic Acid / Q000494:pharmacology; D003227:Conjugation, Genetic; D004926:Escherichia coli; D015964:Gene Expression Regulation, Bacterial; D008826:Microbial Sensitivity Tests; D010406:Penicillins / Q000494:pharmacology; D010957:Plasmids; D016133:Polymerase Chain Reaction; D012706:Serratia marcescens / Q000187:drug effects*; D018440:beta-Lactam Resistance / Q000235:genetics*; D065093:beta-Lactamase Inhibitors; D001618:beta-Lactamases / Q000235:genetics* / Q000378:metabolism
D000900:Anti-Bacterial Agents; D002511:Cephalosporins; D010406:Penicillins; D065093:beta-Lactamase Inhibitors; D019818:Clavulanic Acid; D001618:beta-Lactamases