Use of temocillin for identification of class D carbapenemases

Objectives. Currently, the main difficulty in laboratory confirmation of carbapenemases production concerns the identification of class D enzymes (OXA-48, OXA-181 and OXA-181-like), harbored in Enterobacteriaceae. Nevertheless, determination of susceptibility to temocillin provides an important indication of their possible presence. Aims of the study are to evaluate the susceptibility to temocillin of Enterobacteriaceae strains with reduced susceptibility to carbapenems, and to correlate it with the mechanism of resistance defined by genotypic tests, in order to assess the performance of the temocillin-susceptibility test. Methods. 231 strains of Enterobacteriaceae with reduced susceptibility to carbapenems at routine analysis (Vitek2 - BioMérieux), were tested, selected on the basis of the results to disk diffusion synergy test for detection and typing of carbapenemases (DDST, Rosco). These strains, belonging to different genus and species (Klebsiella, Escherichia, Enterobacter, Citrobacter, Proteus), have also been molecularly characterized with Hyplex® PCR (Amplex), a multiplex-PCR for blaKPC, blaVIM, blaIMP, blaNDM and blaOXA-48; their susceptibility to temocillin has been evaluated by disk diffusion test. Results. The 231 tested strains, divided according to DDST, gave the following results: - 33 class A carbapenemase-producers: 33 resulted positive for blaKPC, 31 subsceptible to temocillin, 2 resistant; - 41 class B (MBL) producers: 34 resulted positive for blaVIM (12 susceptible to temocillin, 22 resistant), and 7 for blaNDM (6 susceptible to temocillin, 1 susceptible); - 147 negative to DDST: 2 resulted positive for blaOXA-48, both resistant to temocillin, while the other 145 were negative to multiplex PCR and temocillin-susceptible; - 10 AmpC-producers: all resulted negative to multiplex PCR and susceptible to temocillin. Conclusion. Susceptibility to temocillin proved to be the only phenotypic test that allows the identification of class D carbapenemases-producers strains if combined to DDST, even if this study suggest that temocillin resistance, not performed with DDST, does not provide a result specific for class D carbapenemases. However its introduction in the laboratory routine could allow the clinical microbiologist to provide a reliable result about presence/absence of all main known carbapenemases in strains with reduced susceptibility to carbapenems; particularly, a negative result to DDST, associated with susceptibility totemocillin, allows to exclude with very high probability the presence of any carbapenemases (negative predictive value of 100%).