Evaluation of temocillin for phenotypic carbapenemase screening of Escherichia coli and Salmonella enterica isolates in relation to the presence of genes encoding ESBLs and carbapenemase production.

BACKGROUND: The expression of enzymes of the OXA-48 carbapenemase group is difficult to detect by phenotypic methods owing to frequent low levels of carbapenem resistance and negative results with some screening methods. Temocillin has been shown to be a good option for phenotypic screening as it is hydrolysed by the OXA-48-group enzymes, whereas ESBLs, AmpC and some other carbapenemases have a lower hydrolytic effect on this antimicrobial. However, no epidemiological cut-off for temocillin is available.

OBJECTIVES: To evaluate temocillin MICs in relation to the presence or absence of genes encoding ESBLs and carbapenemases in Escherichia coli and Salmonella enterica.

METHODS: In this study, 111 E. coli and 102 S. enterica isolates, including WT and well-characterized ESBL-, AmpC- or carbapenemase-producing isolates, were tested by three independent laboratories. MICs were determined according to the CLSI guidelines by agar dilution with the test range from 0.5 to 512 mg/L temocillin and WGS was performed and analysed with ResFinder.

RESULTS: Some overlap was detected between temocillin MICs for WT and ESBL- or AmpC-producing isolates. However, isolates carrying genes encoding carbapenemases showed a broader range of MICs for both E. coli and S. enterica. Higher MICs were observed for the OXA-48 group, VIM and some NDM-producing isolates, whereas isolates harbouring KPC enzymes showed low MICs.

CONCLUSIONS: The results indicate that temocillin MICs enable phenotypic distinction between strains producing OXA-48-group enzymes and both WT susceptible and ESBL/AmpC-carrying isolates, whereas the distinction from other carbapenemases likely requires genotypic testing.