Impact of long term input of antibiotics in soil plots.. Impactance

In a context where pathogens acquire resistance to more and more antibiotic treatments in hospital settings a number of questions are raised regarding mobilization, acquisition and dissemination of antibiotic-resistance traits between and within bacterial populations. Since antibiotic resistance genes (ARG) are common features of many saprophytic soil bacteria [1] a significant ARG reservoir is readily accessible to previously sensitive pathogens by means of horizontal gene transfer (HGT) [2, 3]. It is known since decades [4] that environmental bacteria and pathogens share resistance genes hence the importance to clearly establish transmission routes of such traits in order to prevent their dissemination. Crop fertilization using manure from animals previously treated with antibiotics is a major anthropogenic entry point of these pharmaceuticals in soils. A number of risk-assessment studies focused on the effect of manure fertilization on soil bacterial community and showed that such practice can indeed increase ARG abundance. Such studies are however most often depicting short- to mid-term impact and cannot distinguish what is leading increase in resistance genes abundance. Our objective was therefore to assess if long-term (1999-present) impact of repeated antibiotic molecules input would have a similar impact on soil bacterial communities, in absence of manure. Can similar concentrations of antibiotic select for resistance traits and increase their dissemination potential in absence of organic matter and intrinsic bacterial community added to soil via manure? To test this hypothesis we analyzed environmental DNA extracted from soil samples under crop rotation and treated yearly with veterinary antibiotics (tylosin, chlortetracycline and sulfonamides) for 15 years (1999-present). Antibiotic molecule concentration added to soil each year represented similar amount to what is conventionally found in antibiotic-treated animal manure. Contaminated soil plots examined in this analysis represents a 12 years time frame (2001 - 2012) of this long-term experimental setup.