Existence of Metallo beta lactamases in carbapenem susceptible gram negative bacilli: a cause for concern
2679-2684
Correspondence
Dr Renu Goyal, Department of Microbiology, Institute of Human Behaviour and Allied Sciences, Dilshad Garden, Delhi 110095, INDIA, Phone numbers: 9868396833, E-mail address renugoyal_123@yahoo.co.in
Background: Metallo-ß-lactamases (MBLs) have been increasingly recognized among imipenem resistant isolates, but they also appear to exist in imipenem susceptible isolates. These undetected hidden MBLs in sensitive isolates can spread unnoticed in hospitals if such isolates are reported susceptible to carbepenem without screening for the presence of MBLs. The laboratory detection of such isolates is crucial as they pose a serious therapeutic challenge.
Aims: The aim of our study was to detect MBLs in both the imipenem resistant and sensitive isolates by using combination of available phenotypic methods.
Settings and design: This was a hospital based prospective study which was carried out in a tertiary neuropsychiatric centre from April 2008 to April 2009.
Methods and Material: A total of 130 gram negative isolates (50 imipenem sensitive, 30 imipenem intermediate and 50 imipenem resistant by the Kirby Bauer disc diffusion method) were tested for the presence of MBLs by the double disk synergy test (imipenem, EDTA 750µg/ml), the combined disk test (imipenem, imipenem + EDTA 750µg/ml) and the MBL E test strip.
Statistical analysis: Descriptive statistics was used and the percentages of MBLs carrying imipenem resistant and sensitive isolates were calculated.
Results: MBLs were detected in 11 (20%) imipenem sensitive isolates. Out of these 11 imipenem sensitive MBL producing strains, 8 were from the imipenem sensitive category and 3 were from the intermediate category. These 3 isolates in the intermediate category had MIC for imipenem in the sensitive range (MIC ≤4 ug/ml) by E test method. All the MBL carrying imipenem sensitive isolates had zone diameters in between 16 to 22 mm by the Kirby Baeur disc diffusion method. Among the imipenem resistant isolates, 38 isolates were MBL producers (32 from the imipenem resistant category and 6 from the intermediate category).
Conclusions: This study reports the existence of MBLs in carbapenem susceptible organisms and proposes that gram negative bacterial isolates having an imipenem zone diameter ≤22mm by the disk diffusion method should be routinely screened for presence of MBLs.