Antibiotic residues and resistance in the environment

Antibiotic usage has benefited the animal industry and helped providing affordable animal proteins to the growing human population. However, since extensive use of antibiotics results in the inhibition of susceptible organisms while selecting for the resistant ones, agricultural use is contributing substantially to the emergence and spread of antibiotic resistance in the environment. So far, scientific focus has predominantly been on the emergence and spread of resistant bacteria and genes into the environment as a result of veterinary treatment, in particular through manure but also through food products and direct animal contact. However, environmental contamination with antibiotic residues could also be an important factor in the selection and dissemination of antibiotic resistant bacteria. The persistence of antibiotics in the environment depends on factors like soil type and climate, but also on physical-chemical characteristics of the different types of antibiotics. Monitoring studies showed that substantial concentrations of antibiotic residues can occur in soil and water, in particular at locations close to intensive animal farming. Little is known about the concentrations that will exert selective pressure on environmental microorganisms and promote persistence or even enrichment of the environmental resistance gene pool. Traditionally, it was assumed that resistance is only induced at concentrations above the minimum inhibitory concentration (MIC). However, recently, evidence is accumulating that selective environments may occur at concentrations down to several hundred-folds below the MIC. However, for most of the antibiotics and environmental conditions, the minimal threshold concentrations that will induce or support propagation of antibiotic resistance in environmental microbes are still undefined. Therefore, more research is needed into the relationship between the concentrations of antibiotic residues in the environment and the prevalence and persistency of environmental antibiotic resistance. First, additional research is needed to determine what antibiotic concentrations still exert pressure on bacteria and can cause persistence or enrichment of resistant bacteria. Furthermore, the fate of antibiotics in the main reservoirs (manure, soil, water) should be studied, including antimicrobially active metabolites and their bioavailability. Finally, transmission of antibiotic compounds between reservoirs should be studied to identify the main reservoirs of interest and define intervening measures.