R.K. Manojkumar Singh, Nishith Kumar Pal, Mandira Banerjee, Soma Sarkar, Puranjoy Saha, Manideepa SenGupta
Background: AmpC beta-lactamases are cephalosporinases that confer resistance to a wide variety of beta-lactam antibiotics. Detecting AmpC-mediated Â resistance in gram negative organisms is a challenge for laboratories due to misleading results in phenotypic tests. There is always a search for newer and reliable methods which will be more user-friendly to detect these enzymes in routine diagnostic laboratories. Objective: To design a Â simplified three-dimensional test for detecting the occurrence of AmpC Î²-lactamases. Material and Methods: A total of 201 consecutive, non-duplicate, gram-negative clinical isolates ofÂ E.coli (n=65), Klebsiella spp.(n=70), Proteus spp.(n=4), citrobacter spp. (n=9), Enterobacter spp.( n=8), P.aeruginosa (n=27) and Acinetobacter spp.(n=18),Â obtained over a period ofÂ six months (June to December, 2011) from patients withÂ nosocomial infections were screened for AmpC producers by Kirby Bauer disk diffusion method using cefoxitin (30Î¼g). Isolates with zone diameter less than 18 mm were tested for inducible AmpC beta-lactamases by disk antagonism test, and plasmid mediated AmpC activity by indigenously developed simplified three dimensional and AmpC disk tests. Results: Amongst the 201 gram-negative clinical isolates tested, the simplified three dimensional test identified 60(29.8%) and AmpC disk test Â yielded 51(25.3%) as plasmid AmpC producers from 69 screening positive isolates. Production of inducible AmpC beta-lactamase was shown in 3(1.5%) isolates by disk antagonism test. Co-productions of extended spectrum beta-lactamases and metallo-beta-lactamases with AmpC beta-lactamases were observed in 3.9% and 4.9% isolates respectively. Conclusion: This simplified three dimensional technique is simple, reproducible, convenient, reliable and accurate means of detection of AmpC beta-lactamases. It can serve as a potentially useful diagnostic tool in those health care settings where polymerase chain reaction and inhibitor based methods for the detection of AmpC enzymes are costly and not readily available.