Description
Increasing use of antibiotics in agriculture and human and veterinary medicine has led to increased concentrations of these substances in both engineered systems and the environment. Where antibiotics are not eliminated by engineering controls, they pass into the environment, where their effects are difficult to anticipate. The two main concerns associated with this contamination are ecological effects related to toxicity and selection and the implications for human medicine and agriculture resulting from the development of antibiotic resistance. The concentrations of antibiotic substances detected thus far in the environment fall into the range of nanogram-microgram/liter. These concentrations fall well below the minimum inhibitory concentrations that would be associated with dominant ecological changes, so the development of resistance becomes a dominant concern. This research develops a method for and investigates the potential for residual antibiotic contamination to result in increased levels of antibiotic resistance. Kinetic growth studies of E. coli in serial dilutions of CIP were used to determine appropriate exposure concentrations. E. coli was then cultured in the presence of these concentrations (125 μg/l -- 7.63 ng/l) in batch reactors with samples taken at 12, 24, 48 and 72 hr. These samples were then analyzed for minimum inhibitory concentration (MIC) using microdilution assay. A single exposure concentration of CIP that showed significant MIC development during the study period was then used to test the effect of variations in solution pH (5.0, 7.0, 9.0), Ionic Strength (0.008, 0.05, 0.1, 0.5 N as NaCl) and Temperature (15, 25, 37°C) on MIC development. Results illustrate a strong dependence of MIC development on exposure concentration, even at low levels, with MIC increasing over the course of the study by factors ranging from 2 to 27. The variation of environmental conditions considered in this study did not have a significant effect on MIC, with the exception of incubation at 15°C showing a short lived, but significant increase in MIC relative to a baseline
