In vitro susceptibility of resistant Escherichia coli field isolates to antimicrobial combinations

Avian colibacillosis is an infectious disease of chickens caused by avian pathogenic Escherichia coli (APEC). Respiratory viruses and Mycoplasma infections cause damage to the respiratory system of chickens and predispose them to APEC. The APEC field isolates have begun showing increased antimicrobial resistance, leading to failure of treatment and increasing treatment costs and production losses. The objective of this study was to investigate the susceptibility of APEC to different antimicrobial combinations by using minimum inhibitory concentration and checkerboard tests. All 18 APEC isolates were resistant to amoxicillin, clavulanic acid, doxycycline, oxytetracycline, and erythromycin, whereas 71 and 76% of these isolates were resistant to ciprofloxacin and enrofloxacin, respectively. Fosfomycin had the best activity against the resistant isolates, followed by gentamicin, spectinomycin, and florfenicol, with resistant percentages of 35, 41, 47, and 53%, respectively. A total of 46 antibiotic combinations were used to test each E. coli field isolate using the checkerboard technique. On the basis of checkerboard results for synergistic and partial synergistic activities, the combinations of amoxicillin-clavulanic acid, ciprofloxacin-fosfomycin, oxytetracycline-erythromycin, oxytetracycline-florfenicol, amoxicillin-gentamicin, oxytetracycline-spectinomycin, and spectinomycin-erythromycin were the best in vitro antimicrobials, with synergistic and partial synergistic activities in more than 80% of the E. coli field isolates. Further in vivo studies are needed to correlate the in vitro susceptibility results with field efficacy studies to reduce economic losses in the poultry industry.