Genotypic characterisation of carbapenem resistant Escherichia coli isolates from blood stream infection and fallacies associated with Modified Hodge Test to detect production of carbapenemases

Gram negative enteric bacilli are a leading cause for sepsis in children and adults. Emergence of drug resistance is a major problem which limits the options available for therapy. Carbapenemase is one of the beta lactamases conferring resistance to carbapenem group of drugs making it difficult to treat since the treatment options are lesser. Detection of carbapenem resistance plays an important role in understanding the epidemiology and spread of resistance. This pilot study was done to characterise Escherichia coli isolates from blood stream infection and identify genes responsible for carbapenemase production and correlate the results with MHT.Twenty consecutive isolates of E.coli resistant to imipenemmeropenem by disk diffusion were characterised. Multiplex PCR was performed to detect various genes responsible for carbapenemase production that are NDM, KPC, SPM, VIM, OXA-48, IMP and GES and compared with the MHT results. It was observed that NDM was the common gene conferring resistance. However, we found that among a total of 6 isolates positive for the NDM gene, 4 of them gave a false negative result with MHT. Among the 20 isolates, 3 gave a false positive result with MHT. The reasons could be excess production of ESBLs. We found that a majority of isolates (9 out of 20) were negative with PCR and MHT though they had a resistant zone of inhibition with carbapenems. The reasons for this could be due to other resistant mechanisms such as porin loss (OMP defects), efflux pumps and Amp C beta lactamase production. In conclusion conventional multiplex PCR is a sensitive technique to detect multiple genes. Phenotypic tests can serve as an inexpensive alternative. However, MHT results should be interpreted with caution as it is well known that it has a sensitivity of only 11 in detecting NDM though it picks up KPC and OXA-48 well.