False susceptibility of antibitoics to carbapenemase producers and means to overcome

The aim of this study was to identify the reason for frequent failure of carbapenems and β-lactam and β-lactamase inhibitor combination drugs in clinical settings despite susceptibility. For this different isolates collected from clinical specimens were tested for susceptibility and identification of carbapenemase producers was done by phenotypic method (Modified Hodge Test). Further, phenotypic results were compared with the Gold standard PCR results to assess accuracy of the results. A total of 541 isolates collected from various centres across India including Escherichia coli (n=154), Klebsiella pneumoniae (n=161), Acinetobacter baumannii (n=103) and Pseudomonas aeruginosa (n=123) were included in the study. These isolates were tested for suceptibility according to Clinical and Laboratory Standards Institute (CLSI) methods and then screened for carbapenemase following the same guidelines. The isolates were then subjected to Modified Hodge Test with and without zinc sulfate with subsequent confirmation with the genotypic assay. Out of 541 clinical isolates, 464 isolates were identified as probabale carbapenemase producers. Further phenotypic screening of these isolates, only 316 (68.1 %) isolates were found to be positive to MHT. After addition of zinc into MHT sensitivity of the test was improved and now 431 (92.9 %) isolates were found to be positive to MHT indicating it is 24.8 % more effective compared to MHT without zinc sulfate. Subsequently, when 316 MHT positive isolates were screened through PCR, 30 % (95/316) isolates showed false positive and 19.6 % (29/316) isolates showed false negative, results indicating higher degree of false positive and negative results are associated with MHT. However, with the addition of zinc sulfate to MHT, false positive and negative results are minimized significantly. Among the tested drugs, Elores appeared to be the most efficacious with 90.9 % to 95.4 % susceptibility to carbapenemase producing organisms. Intrestingly, penems (meropenem and imipenem plus cilastatin) exhibited higher resistance varing from 90 % to 96.1 %. For the proper treatment of patients, it is important to use simple and reliable tests for identification of carbapenemase producing clinical isolates among gram negative organisms. This study showed that the MHT technique is highly sensitive for detecting carbapenemases after addition of zinc. When susceptibility of various drugs were tested against carbapenemase producing isolates, Elores was found as the most efficacious drugs.