Amino acid substitutions causing inhibitor resistance in TEM beta-lactamases compromise the extended-spectrum phenotype in SHV extended-spectrum beta-lactamases

Three amino acid substitutions, Met-69 --> IIe, Arg-244 --> Ser and/or Asn- 276 --> Asp, mediate inhibitor resistance (IR) in TEM beta -lactamases, The y were introduced in all possible combinations at homologous positions into either SHV-1 or the respective extended-spectrum beta -lactamases (ESBLs), SHV-2 or SHV-5, Susceptibility testing of the resulting set of seven varia nts of each parental strain, all in an isogenic background, was performed. The phenotypes of the constructions revealed that most substitutions result ed in reduced resistance to most tested single beta -ractam formulations. T his decrease over-compensated for the expected increase in inhibitor resist ance, so that most mutants showed no rise in resistance to inhibitor/beta - lactam combinations, although increases of MICs from one- to 43-fold compar ed with the respective parental strains were also measured. Combination of several IR-determining substitutions impaired both phenotypes in the carrie r strains even more. None of the 14 mutants derived from the ESBLs, SHV-2 a nd SHV-5, showed a clinically relevant combined ESBL-IR phenotype, These fi ndings indicate that the SHV beta -lactamase does not benefit proportionall y from simultaneous substitution of residues relevant for the ESBL and the IR phenotype.