Effects of administration mode of antibiotics on antibiotic resistance of Enterococcus faecalis in aquatic ecosystems

In this study we investigated the effects of antibiotic administration mode on development of antibiotic resistance in a freshwater model ecosystem. Mesocosms were established in 2.25 m3 cement pools containing tap water and yellow soil free of antibiotic contamination. Oxytetracycline (5 mg L−1) and ciprofloxacin (2 mg L−1) were used as experimental antibiotics and Enterococcus faecalis was used as indicator bacteria. Antibiotics were either administered directly to the pools, or were administered in a feed-antibiotic mixture or feces-antibiotic mixture. Surface sediment samples were collected and analyzed at 0, 3, 7, 14, 21, 30, 60 and 120 d after inoculation. E. faecalis was isolated and identified by polymerase chain reaction (PCR) and a drug sensitivity test was performed using the micro-broth-dilution method. Maximum antibiotic resistance was reached on day 3 with all methods of administration. The effect was significantly higher in the feed-administration group compared with the direct administration and feces-administration groups, and between the latter two groups the effect appeared to be slightly higher in the direct administration group. The minimum inhibitory concentration (MIC) values showed bacterial resistance percent decreased in a time-dependent manner in all groups; however, the decreasing trend was less dramatic in bacteria isolated from the feed-antibiotics group. Overall, the percent and level of resistance to oxytetracycline was higher than to ciprofloxacin, and the sensitivity recovery time was longer for bacteria exposed to oxytetracycline than to ciprofloxacin. Our findings suggest that antibiotics introduced to water via feed can induce high percent of antibiotic-resistant bacteria in aquatic ecosystems.