Increased Pollution-Induced Bacterial Community Tolerance to Sulfadiazine in Soil Hotspots Amended with Artificial Root Exudates

Sulfadiazine (SDZ) residues constitute an important pollutant in soils that may increase environmental reservoirs of antibiotic resistance. Our primary aim was to compare the development of pollution-induced community tolerance (PICT) to SDZ concentration levels in bulk soil and nutrient amended soil hotspots. Agricultural soil microcosms were amended with different concentrations of SDZ with or without weekly additions of artificial root exudates corresponding to realistic rhizodeposition rates. Bacterial community tolerance to SDZ residues, as determined by the [<sup>3</sup>H]leucine incorporation technique, increased progressively with elevated SDZ exposure, and was significantly increased in soil hotspots (LOEC of 1 μg kg<sup>−1</sup>). An alternative PICT approach based on single-cell esterase probing by flow cytometry failed to demonstrate SDZ impacts. Bacterial growth rates ([<sup>3</sup>H]leucine incorporation) were significantly reduced in both bulk soil and hotspots 24 h after amendment with environmentally relevant concentrations of SDZ, while soil respiration remained unaffected even at 100 μg SDZ g<sup>−1</sup>. Our study for the first time demonstrates a drastically increased PICT response of a soil bacterial community due to increased carbon substrate amendment per se. Hence, hotspot soil environments such as rhizosphere and manure-soil interfaces may comprise key sites for proliferation of bacteria that are resistant or tolerant to antibiotics.