Soil amendment with sewage sludge affects soil prokaryotic community composition, mobilome and resistome

Municipal sewage sludge (MSS) is often directly applied to fields despite concerns regarding the spread of harmful microbes and associated resistance genes (RGs). In this four month microcosm study, the dynamics of prokaryotic communities in agricultural soil and changes in mobile genetic elements and RGs following amendment with stabilized MSS were investigated. TaqMan-based quantitative real-time (q)PCR showed that RG prevalence was high when compared to untreated soil and genes for class 1 integrons (intI1), streptomycin RGs (aadA, strA) and tetracycline RG (tet(W)) were detectable for the duration of the four month study. High-throughput qPCR revealed an enhanced prevalence of aminoglycoside RGs (aacC, aadE), macrolide lincosamide-streptogramin B RGs (ermB, ermF) and tetracycline RGs (tet(L), tet(M), tet(X)). Illumina MiSeq sequencing of 16S rRNA gene fragments amplified from total community DNA revealed significant changes in the prokaryotic community composition both at phylum and genus levels, with lower richness and evenness after MSS amendment followed by gradual recovery after 119 days. Conjugative plasmids captured from MSS using exogenous isolation belonged predominantly to the IncP-1 plasmid group. Our results provide new insights into short- and medium-term effects of MSS amendment on soil prokaryotic communities, including the mobilome and resistome.