Evaluation of a Chromogenic Medium for the Detection of ESBL with Comparison to Double Disk Synergy Test

Background: Extended Spectrum Beta Lactamase (ESBL) producing bacterial strains are the major causes of nosocomial and community-acquired infections worldwide. The aim of the study was to evaluate the effectiveness of Brilliance ESBL Agar (BEA) (a chromogenic culture medium) for the detection of ESBL in comparison with Double Disc Synergy Test (DDST) and confirm results from both methods by Single-plex Polymerase Chain Reaction (PCR) as gold standard.

Materials and Methods: A total of 75 clinical isolates of Escherichia coli were screened for ESBL production using BEA & DDST from various clinical specimens. The antibiotic susceptibility testing was done by the Kirby-Bauer disc diffusion method using Cefotaxime (30 µg) and Ceftazidime (30 µg) discs on Mueller Hinton agar. ESBL producing strains were detected phenotypically by DDST and BEA at 24 h and 48 h, respectively. Isolates screened by both methods were confirmed using PCR for the detection of blaSHV, blaTEM, blaCTX-M genes.

Results: The prevalence of ESBL was 61%. The sensitivity and specificity of DDST at 24 h and 48hours incubation time was 91.3% and 89.5%, respectively. BEA showed an increase in sensitivity and specificity at 48 h with 97.8% and 98.0%, respectively. All ESBL producing strains detected by phenotypic tests were also found harboring ESBL genes (blaSHV, blaTEM, blaCTX-M) by PCR.

Conclusion: The use of BEA in the screening of ESBL production was found to give much better results than DDST and can be used where PCR cannot be performed.