Soil microbial response in terms of antibiotic resistance mechanisms when exposed to increasing concentrations of sulfamethoxazole

International audience

English. In the context of scarce water resources, the reuse of treated wastewater (TWW) appears to be a fair and sustainable solution for irrigation of agro-ecosystems. However, TWW is known to contain pathogens & pharmaceutical residues including antibiotics and resistant bacteria. To assess whether or not TWW participates in the dissemination of antibiotic resistance, we have explored the response of soil microbial communities when exposed to TWW spiked with increasing sulfamethoxazole antibiotic concentrations (0, 10, 100, 500 and 1000 g/kgdry soil). Using microcosms and controlled conditions, it was possible to identify a sulfamethoxazole concentration threshold (100g/kgdry soil) above which the degradation rate increased by a factor of 5, from 0.08 day-1 to 0.44 day-1. In each case sulfamethoxazole was no longer detected after 28 days of incubation and several transformation products were identified instead. Bacterial selection and inhibition effect were observed for sulfamethoxazole concentration above 10g/kgdry soil, and specifically during the early stage (0-7 days of incubation). After 1.5 months, the fingerprint of sulfamethoxazole addition was still noticeable, with the inhibition of some low abundant bacteria (< 1%). This study also highlighted a gradient in the abundance of the sulfonamide resistance gene sul1 and the mobile genetic element intl1, correlated with the sulfamethoxazole concentration gradient. At the end of the experiment (1.5 months) these two selected genes still exhibited higher number of copies for sulfamethoxazole concentration above 10g/kgdry soil (+0.5 to 1 log) suggesting resistance transfer. Finally, it was shown that the concentration of sulfamethoxazole had a direct effect on the rate of antibiotic degradation, soil microbial communities and the soil antibiotic resistance pool, even after 1.5 months of incubation. It now raises the question of the effect of repeated and long term TWW irrigation and the influence of the antibiotic nature.