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Expressed and Silent Carbapenemase Genes Detected by Multiplex PCR in both Carbapenem- Resistant and Phenotypically-Susceptible Gram Negative Bacilli

Ahmed Salah Emira
Lamiaa Abd El-Fattah Madkour
Nazmy Edward Seif
Reham Ali Dwedar



Carbapenemase-producing Gram-negative bacilli have been major culprits in hospital-associated infections (HAIs), particularly in critically ill patients suffering device-associated infections (DAIs). The current study aimed to investigate the performance of the modified Hodge test (MHT) as a phenotypic confirmatory method for the detection of carbapenemase-producing Gram-negative bacilli and to compare it to the gold standard PCR for the detection of carbapenemase production in both non-susceptible and phenotypically susceptible isolates. The latter were expected to harbor silent carbapenemase genes, as suspected from the inappropriate response to carbapenem therapy.


Ninety-five bacterial isolates from 75 critically ill patients were collected over 6 months from several ICUs at Cairo University Hospitals. The isolates were subjected to antibiotic susceptibility testing (AST) for carbapenems and were further screened by MHT, followed by genotypic analysis via multiplex PCR.


Enterobacteriaceae were the most commonly isolated pathogens (55.8% of the total isolates), followed by Acinetobacter spp. (24%). Lower respiratory tract infections were the most common HAIs (42.11%), followed by surgical site infections (27.37%). All isolates demonstrating carbapenem resistance by AST were found to harbor at least one of the following carbapenemase genes: blaKPC, blaOXA-48, blaIPM, blaVIM, and blaNDM-1. Alarmingly, 97.8% of the isolates which exhibited carbapenem-susceptible profile and negative MHT were harboring carbapenemase genes as confirmed by multiplex PCR. With the exception of one isolate (E. coli) which was not harboring any carbapenemase gene, the remaining 94 bacterial isolates were found to carry either a single or multiple carbapenemase genes.


The silent dissemination of different classes of carbapenemases even in isolates with negative MHT is a daunting challenge. It necessitates the implementation of strict antibiotic stewardship along with updated and actionable approach to detect non-expressed carbapenemase genes in phenotypically susceptible isolates.