Cattle manure application triggers short-term dominance of Acinetobacter in soil microbial communities

Abstract

Manure application improves soil productivity but also spreads microorganisms, some of which can be of clinical relevance. The ability of manure to spread common human pathogens has been widely studied but we lack understanding on whether it also disseminates opportunistic pathogens like Acinetobacter and other non-fermenting Gram-negative bacteria (NFGNB). We designed a microcosm experiment simulating the application of fresh manure to soil to analyse the effects on soil microbial communities (and vice versa), focusing on Acinetobacter and other NFGNB. We conducted two independent experiments with fresh cattle manure from a dairy farm and two pasture soils from different organic farms. We sampled the microcosms on days 2, 7, 14, 28 and 84, and characterized the microbial communities through sequencing of 16S rRNA amplicons from i) total communities and ii) those cultured on CHROMagar Acinetobacter (i.e., selective for NFGNB) after 24-h growth. Manure altered the community composition of soil microorganisms whereas the reverse effects were weaker, showing a transition to an environmentally structured community. Acinetobacter species increased their relative abundance in manure and soil under manure on day 2, especially in soils previously exposed to γ-irradiation to reduce the load of native microorganisms. Although manure spread most Acinetobacter phylotypes in the soil, it also stimulated a few from the soil that became occasionally abundant in manure. This study demonstrates that Acinetobacter species may dominate in soil and manure for a short time after deposition, and highlights their high responsiveness and competitiveness to changes likely associated with an increase in labile resources.