Evaluation of solid (disc diffusion)- and liquid (turbidity)-phase antibiogram methods for clinical isolates of diarrheagenic E. coli and correlation with efflux

Multiple drug resistance (MDR) in bacteria causes higher mortality and morbidity, complicates treatment and increases health-care outlay. With no new-generation antibiotics in sight, its rapid spread through the environment poses grave danger. Therefore, rapid detection to identify effective antibiotics and to prevent their indiscriminate use is imperative. However, the widely used clinical method for antibiogram, the Kirby–Bauer disc-diffusion method (DDM), requires 2–3 days, has inherent shortcomings of solid-phase assays and is not suitable for high-throughput operations. In our research on MDR associated with childhood diarrhea, we determined the antibiogram of 73 clinical diarrheagenic Escherichia coli strains using both the DDM and the more reliable liquid turbidity method (LTM) performed in 96-microwell plates. The results were further correlated with a dye-exclusion efflux assay using fluorescein diacetate. Although LTM is apparently superior in saving critical time, suitability to high-throughput operations and reliability, we found that the serious shortcomings of DDM could be effectively countered by just doubling the dosage of antibiotics currently used in discs or by using two discs in place of one. With 48 of the 49 MDR strains being positive for efflux and the 12 strains ‘susceptible’ to all the antibiotics being negative, the efflux assay could be useful as an integral component of the antibiogram test or for additional confirmation. The presence of 65% of MDR strains among diarrheagenic E. coli is a matter of serious concern, although most of them could be treated with either Gentamycin or Amikacin, as is the practice by experience.