Carbapenem susceptible metallo beta lactamase producing gram negative bacilli: beware of wolf in sheep dress.

Context: MBL producing gram negative bacilli are associated with serious infections and failure of therapy. Many isolates carrying the MBL gene have been found susceptible to imipenem. These so called “hidden MBLs” create a significant problem with respect to their detection. Such organisms may pose a significant risk, particularly due to their role in unnoticed spread within institution and their ability to participate in horizontal transfer with other pathogen related organisms Aims: This study was undertaken to determine the prevalence, risk factors and treatment outcome of carbapenem susceptible metallo beta lactamase producing gram negative bacteria. Methods and Material: Gram negative bacteria resistant to ceftazidime, ticarcillin and ceftazidime clavulanate / ticarcillin clavulanate but susceptible to imipenem were included in the study. Phenotypic detection of metallo beta lactamases involved combined disk test (CDT), double disk synergy test (DDST) and susceptibility to aztreonem. MIC was determined by agar dilution method (for imipenem and imipenem-EDTA) as well as Etest. Results: Out of 119 isolates included in the study, a total of 30 (25.21%) isolates were found to be MBL producers. A fourfold reduction in MIC by imipenem-EDTA combination compared to imipenem was also found to be a sensitive indicator for the detection of MBL carrying isolates. Prolonged hospitalization and indwelling urinary catheterization were the possible risk factors. Conclusion: This study suggests the use of a simple and highly sensitive method for the detection of MBL production in carbapenem susceptible isolates of gram negative bacteria in resource poor setting. Key messages: Carbapenem susceptible metallo beta lactamase producing gram negative bacilli are a threat to infection control programs. Appropriate and sensitive phenotypic methods are available for their detection.