Detection of carbapenemase production by multidrug-resistant acinetobacter baumannii isolates from selected wards in dr george mukhari academic hospital

Abstract

Purpose: Carbapenem-resistant Acinetobacter baumannii has emerged as an important cause of nosocomial outbreaks worldwide. Detection of carbapenemase production by modified Hodge test using Mueller Hinton agar has been reported to be less sensitive. The objectives of this study were two-fold: i. to compare the performance of MacConkey agar (MA) and Mueller Hinton agar (MHA) for the detection of carbapenemases; ii. to determine the type of carbapenemases that are produced by A. baumannii isolates from Dr George Mukhari Academic Hospital (DGM). Methods & Materials: Twenty four stored carbapenem-resistant and 2 carbapenem-susceptible A. baumannii isolates were identified using Vitek 2 automated system (bioMerieux, France). The modified Hodge test (MHT) was done according to CLSI guidelines on MHA (DMP, South Africa) and MA (DMP, South Africa) using Escherichia coli ATCC 25922 and meropenem disks. All the plates were incubated at 370C overnight. Detection of carbapenem-resistance genes (OXA-23, OXA-40, OXA-58, SIM, VIM-1, VIM-2 AND IMP-like) was done according to published molecular methods. Results: Of the 24 carbapenem-resistant isolates which were screened, 87.5% (21) and 62.5% (15) were found to be carbapenemase positive by MHT on MA and MHA, respectively. The cloverleaf pattern on MHA appears to be absent for some of the isolates but it is accentuated on MA. Both MHA and MA showed 50% false positive result for the 2 susceptible isolates. Of the 24 resistant isolates which were screened for bla genes, 87.5% (21) and 25% (6) were found to have OXA-23 and OXA-40, respectively. Conclusion: MA may be used in place of MHA for the screening of carbapenem-resistant A. baumannii and thus will impact on infection control practices. OXA-23 bla gene was the most prevalent carbapenemase type at DGM Hospital. Though the numbers were small, the study confirms that phenotypic methods lack specificity and sensitivity, hence their results have to be confirmed by molecular methods.