Tracing of false negative results in phenotypic methods for identification of carbapenemase by Real-time PCR

Carbapenemase production causes multi antibiotics resistant in Gram-negative bacteria. A simple rapid and accurate phenotypic test for detection of Gram-negative carbapenemase-producing bacteria is useful for the treatment of infections. The aim of this study was to track the negative results in carbapenemase phenotypic test by Real-time PCR.

In this study, 161 imipenem resistant Gram-negative bacteria were surveyed. Modified Hodge Test (MHT), boronic acid (BA), EDTA and dipicolinic acid were used for detection of Klebsiella pneumoniae carbapenemase (KPC) and metallo-beta-lactamase (MBLs). Different phenotypic methods and PCR confirmation followed by Real-time PCR for determination of phenotypic false-negative results were used.

Our results indicated that 85, 51 and 112 strains were MHT, BA and dipicolinic acid positive, respectively. No synergistic effect was observed between imipenem and EDTA. Sixty-nine strains were confirmed as carbapenemase-producers according to the results of molecular tests. All of the isolates carried the gene and expressed carbapenemase.

Comparison between the results of phenotypic and genotypic methods showed that the phenotypic methods could be used as the primary screening and the PCR remains as the gold standard for detection of carbapenemase positive strains.