Hyperproduction of Inhibitor-Susceptible TEM β-Lactamase Is Responsible for Resistance of Serratia marcescens to β-Lactam-β-Lactamase Inhibitor Combinations

Abstract

Background: Some clinical isolates of Serratia marcescens showed high-level resistance to expanded-spectrum cephalosporins alone and in combination with β-lactamase inhibitor, but the β-lactamase extracted from these strains was sensitive to the inhibitors. This study examines the possible mechanisms responsible for the discrepancy. Methods and 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 β-lactamase were produced by 10¹⁰ 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 µg/ml. The TEM-type β-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. Conclusion:The TEM-type β-lactamase plays a critical role in the resistance of S. marcescens to β-lactams, and the hyperproduction of inhibitor-susceptible TEM β-lactamase is responsible for the resistance to β-lactam-β-lactamase inhibitor combinations.