Degradation of the sulfamethoxazole antibiotic in an agricultural soil

Livestock raising in most countries involves the use of antibiotics in veterinary practices and to increase animal productivity ("production-purpose uses") (1. These compounds are designed to be quickly excreted from the threated body, through animal feces, either unaltered or as metabolites. The use of livestock manure or of the digestate (anaerobic digestion of the manure) as amending and fertilizer in agricultural practices, implicates the introduction of antibiotics, antibiotic resistant bacteria (ARB) and antibiotic resistant genes (ARGs) in terrestrial ecosystem (2. Due to their continual input into the environment and antimicrobial nature, antibiotics are classified as emerging pseudo-persistent organic pollutants (pseudo-POPs) (3. Unlike the other conventional POPs, antibiotics not only can kill or inhibit natural bacterial population involved in ecosystem functioning, but also cause the appearance and rapid spread of resistant bacteria in the environment (4. Indeed antibiotics, ARB and ARGs are recently included in the contaminant list of emerging concern (5. The synthetic compound sulfamethoxazole (SMX) is one of the most popularly prescribed and consumed antibiotics in human and veterinary medicines leading to its presence in aquatic and terrestrial ecosystems. SMX is bacteriostatic, inhibiting the synthesis of folic acid metabolism and causing bone marrow suppression. This compound is among the top ten drugs with high priority identified in the European assessment of pharmaceuticals and personal care compounds (6. The current knowledge about the persistence and the potential effects of SMX addition on the natural microbial community of an agricultural soil are still scarce. The aim of the present study was to evaluate the possible degradation of SMX and the response of the microbial community of a cattle manure amended and SMX-treated soil. Microcosm biodegradation experiments were performed by using microbiologically active (containing the natural microbial community) or sterilized cattle manure amended soil. The role of the natural microbial community on the degradation of the antibiotic and the effects of the latter on the microbial structure and functioning were investigated. Furthermore, a molecular marker for antibiotic resistance (7, the intI 1 gene was also analyzed.