Antibiotics and Resistant Genes in Paddy Soil

Abstract

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. 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 hundredfolds 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. Finally, transmission of antibiotic compounds between reservoirs should be studied to identify the main reservoirs of interest and define intervening measure.