Persistence and spread of tetracycline resistance genes and microbial community variations in the soil of animal corrals in a semi-arid planted forest

At the spring, goat and sheep herds are transferred to planted forests, in a semi-arid region in the northern Negev Desert, Israel, to reduce herbaceous biomass and, fire risk. The herds are held overnight in corrals for about 4 months, enriching the soil with organic matter and nitrogen. This research examined the effect of these enrichments on soil bacterial community structure (BCS) and the abundance of tetracycline resistance genes (TRGs) in active and abandoned corrals (1-10-years-old). Based on 16S rRNA gene sequences, the Proteobacteria and Actinobacteria phyla dominated the soil of all corrals. The Actinobacteria were less abundant in the active and 1-year-old corrals (23-26%) than in the other corrals and the control (33-38%). A principal component analysis showed that, the BCS in the active and the 1-year-old abandoned corrals was significantly different from that in the older corrals and the control. The Firmicutes phylum constituted 28% of the BCS in the active corrals, 12.5% in the 1-year-old corrals and 2% in the older corrals and the control. In contrast, the Acidobacteria phylum was hardly detected in the active and 1-year-old abandoned corrals and constituted 10% of the BCS in the older corrals. Genes conferring resistance to tetracycline were detected in high numbers. The tetG and tetW genes were detected in the active and abandoned corrals (1-10 years). The tetQ gene was detected only in the active and 1-year-old abandoned corrals. None of the genes were detected in the control soil. The three genes were detected outside an active corral, in the downstream section of an ephemeral tributary. The results prove that abandoned and unobserved periodic animal corrals are an environmental reservoir for TRGs.