Flow cytometry early predicts bacterial susceptibility to antibiotics

Abstract

Objective: To evaluate a flow cytometry protocol to test bacteria for antimicrobial susceptibility. Methods: We used antimicrobial drugs, a strain of Escherichia coli (ATCC 25922) susceptible to all antimicrobials, and clinical isolates of E. coli resistant to the same drugs. Prior to testing, the susceptibility of all isolates was tested by the disc-diffusion method. For flow cytometry, standard concentrations of antimicrobial drugs were added to E. coli bacteria, incubated for 1 to 3 h at 37ºC, stained with propidium iodine and analyzed by flow cytometry. As controls, each bacterial isolate, at each indicated time, was also collected and seeded on Luria Bertani agar plate to enumerate the number of viable cells by the classical method. The growing/survival rate was evaluated in the same samples after incubating for 24h at 37ºC. Results: Antimicrobial susceptibility was estimated at 1 and 3 hours but at this time a higher number of viable cells was detected by flow cytometry compared to colonies counting on agar plates. Conclusion: Flow cytometry allows early evaluation of bacterial resistance to antibiotics and can be optimized and used as an alternative tool to detect multi-resistant bacterial isolates.