IWRA Proceedings

The incidence of antibiotic resistant infections is a critical public health issue in the U.S. and globally. The misuse and overuse of antibiotics in agriculture have resulted in the release of antibiotics to the environment. There are a number of studies that link non-therapeutic uses of antibiotics in livestock production with the emergence of antibiotic resistant bacteria in humans and the environment. The risk to human health associated with contaminant antibiotics in the soil is not well understood, in part because the persistence of contaminant antibiotics in soils, which lead to selection of resistance, has not been established.

 

Oxytetracycline (OTC) is one of the most commonly administered antibiotics in livestock.  It is a broad-spectrum antibiotic used to treat human infections and is categorized by the World Health Organization (WHO) as “critically important”. When introduced into the environment, OTC sorbs strongly to soils, which may prolong its bioavailability to microorganisms.  A wide range of partitioning coefficients from soil to water (Kd) have been reported (from 13.5 to 269,097 L/kg). Such variability of Kd values complicate predictions of OTC availability to microorganisms and thus its potential for resistance development.  

 

This paper presents a review of reported OTC concentrations in different environmental media, including: waste water treatment plants (influent and effluent), treatment lagoons, runoff, surface water, groundwater, sediments, cattle, swine and poultry manure, and soil. A mathematical equation will be presented to calculate partitioning from soil to water, using linear regression analysis of reported correlations between soil characteristics and sorption processes. This equation is used to convert concentrations in sediments, manure and soils to equivalent liquid concentrations. The potential of environmental OTC concentrations to promote resistance in S. typhimurium  will be evaluated by comparing liquid OTC concentrations with reported values of the lowest antibiotic concentration at which Salmonella enterica select for resistance.  

 

This analysis emphasizes the importance of monitoring environmental antibiotic concentrations in different media to determine the potential for selecting antibiotic resistant bacteria. The OTC occurrence in different media along with the soil-water partitioning equation will enable us to assess environmental conditions likely to promote the development of antibiotic resistance.