TEM-1 β-lactamase as a source of resistance to sulbactam in clinical strains of Acinetobacter baumannii

Objectives: Sulbactam is well known to have clinically relevant intrinsic activity against Acinetobacter baumannii. Although secondary resistance to this drug has long been reported in acinetobacters, virtually nothing is known about its molecular basis. The aim of this study was to test the hypothesis that beta-lactamase TEM-1 is responsible for sulbactam resistance in A. baumannii. Methods: Seventeen clinical strains of A. baumannii were selected to represent different combinations of quantitative susceptibilities to sulbactam and molecular typing characteristics. The strains were screened by PCR for the presence of the bla(TEM-1) gene and its variants. Amplicons encompassing the bla(TEM) genes, including their promoters, were sequenced. The expression and copy number of the bla(TEM) genes were assessed using semi-quantitative real-time PCR. Transfer of the bla(TEM-1) gene into a susceptible A. baumannii strain was achieved by electroporation. Results: Six strains were negative for the bla(TEM) gene and had sulbactam MICs of 0.5-1.0 mg/L, 10 strains harboured bla(TEM-1) and showed MICs >= 8.0 mg/L, except for one strain with an MIC of 2 mg/L, while the remaining strain carried bla(TEM-19) and had an MIC of 1 mg/L. The level of bla(TEM-1) expression positively correlated with the MICs of sulbactam (r=0.92). Promoter P4 was linked to the bla(TEM) gene in all strains except for a P3-carrying strain (an MIC of 2 mg/L). Transformation of the susceptible A. baumannii strain with bla(TEM-1) resulted in a 64-fold increase in sulbactam MIC and in resistance to ticarcillin and piperacillin, but no change in susceptibility to broad-spectrum generation cephalosporins, aztreonam or carbapenems. Conclusions: The results presented suggest that TEM-1 represents a clinically relevant mechanism of sulbactam resistance in A. baumannii.