[The performance of two different chromogenic media for the diagnosis of extended spectrum beta-lactamase producing strains]

Abstract

The aim of this study was to evaluate the performances of 2 different chromogenic media for the detection of extended spectrum beta-lactamase (ESBL) producing Escherichia coli and Klebsiella spp. isolated from different clinical samples of patients who were admitted to Baskent University Faculty of Medicine, Adana Application and Research Hospital between September to November 2007. A total of 365 strains [251 were ESBL positive and 114 were negative by double disc synergy (DDS) test] of which 255 were E. coli and 110 were Klebsiella spp. were included to the study. All the isolates have been inoculated onto Drigalski agar (prepared following the formula described by Stürenburg et al.) and chromID ESBL agar (bioMérieux, France) and the production of ESBL were evaluated at 4th and 24th hours for Drigalski agar, and at 24th hours for chromID ESBL agar. The strains which yielded contradictory results by DDS test and chromogenic media, were tested for the presence of TEM, SHV and CTX-M genes by polymerase chain reaction (PCR). The number of ESBL positive strains on Drigalski agar at 4th and 24th hours were 238 and 235, respectively, while chromID ESBL agar detected 259 ESBL positive strains at 24th hours. All (100%) of the 159 ESBL positive E. coli strains by DDS test were also found positive on chromID ESBL agar, and 150 (94.3%) were found positive on Drigalski agar. These rates were detected as 100% (92/92) and 92.3% (85/92) for Klebsiella spp. isolates. Eight of the strains (2 E. coli, 6 Klebsiella spp.) which yielded negative results by DDS test but positive on chromlD ESBL agar, harboured SHV (n = 1), CTX-M (n = 6) and TEM + CTX-M (n = 1) genes detected by PCR. As a result, the consistency of the results obtained by Drigalski agar at 4th and 24th hours has indicated that this medium may provide advantages in rapid diagnosis of ESBL producing bacteria in routine laboratories. The data obtained for chromogenic media seemed to be favourable for the rapid diagnosis of ESBL production, however comparative studies with the use of standard reference methods are needed in order to determine diagnostic sensitivity and specificity of these media.