Microbial respiration-based detection of enrofloxacin in milk using capillary-tube indicators

A simple method was developed for detecting enrofloxacin in milk using microbial respiration and capillary-tube indicators. A glass vial containing E. coli ATCC 11303 in LB was spiked with enrofloxacin and sealed with a screw cap supporting a silicon septum. A capillary tube with a drop of ink was inserted through the septum, and the ink-level change was measured. The growth of E. coli produced CO₂gas, which increased the pressure inside the vial and raised the ink level in the capillary tube; in this manner, small changes in the gas volume were translated into large changes in the ink level. The increase in the ink level was inversely proportional to the concentration of enrofloxacin, which suppressed the growth of E. coli. The detection limit of enrofloxacin was found to be 10 ng/mL using the naked eye after the microbial culture had been permitted to grow over 2 h.