Determination of minimum inhibitory concentration

The minimum inhibitory concentration (MIC) of each E. coli isolate to enrofloxacin was determined using broth microdilution according to Clinical and Laboratory Standards Institute guidelines (Clinical Laboratory Standards Institute, 2013). Each isolate was grown overnight on blood agar. A single colony was inocluated into Mueller Hinton broth and brought to a 0.5 McFarland Standard. Fifty μl of bacterial suspension was inoculated into 50 μl of 2-fold serial dilutions of enrofloxacin ranging in concentration from 0.03 to 32 mg/L (USP, Rockville, MD). Plates were incubated overnight (18 h) at 37°C. The first well with no visible growth within a given isolate was determined to be the MIC.