Rapid and cost-efficient screening of bacterial antibiotic susceptibility in Enterobacteriaceae – an assay measured by absorbance

Abstract Objectives Due to the rise in antimicrobial resistance worldwide, it is becoming increasingly important to rapidly receive information on the antimicrobial susceptibility of isolated bacteria. Traditional phenotypic methods are slow but remain useful and have advantages such as low cost, as well as being easy to perform and having established analysis criteria. The objective of this study was to develop and evaluate a rapid assay that can detect antibiotic resistant bacteria phenotypically in less than 2 hours. Methods The microplate assay is based on absorbance measurements of bacterial strains grown in liquid media with and without antibiotics (cefotaxime, meropenem and ciprofloxacin). The absorbance was measured at the beginning of the assay, and after 90 minutes of incubation. If a bacterial isolate in absence of antibiotic had more than a 50% growth increase after 90 minutes, then the susceptibility/resistance was calculated by comparing the absorbance value of the culture in the presence of antibiotic at 90 minutes with the initial absorbance value. Results Here we show that it is possible to phenotypically screen antibiotic susceptibility of both clinical and environmental Enterobacteriaceae isolates in 90 minutes for three different antibiotics. The test showed an accuracy of 98,5%, a sensitivity of 97,4%, and a specificity of 99,3%. False susceptibility rates were 0,4% and false resistance rates were 1,2%. Conclusion This rapid and simple absorbance test has proven suitable for screening of antibiotic susceptibility for a large number of Enterobacteriaceae strains with high accuracy and sensitivity. The method can be performed without specialized and costly materials and/or equipment, which makes it highly suitable for laboratories with limited resources and might be especially important in laboratories situated in low income countries.