Independent and combined effects of oxytetracycline and antibiotic-resistant Escherichia coli O157:H7 on soil microbial activity and partial nitrification processes

The common practice of field-spreading animal manure on agricultural land contributes to the dissemination of antibiotics and antibiotic-resistant pathogens, which may disrupt important soil microbial functions. In this study, the potential independent and combined effects of oxytetracycline (OTC) and antibiotic-resistant Escherichia coli O157:H7 (E. coli O157:H7) on soil microbial activity and partial nitrification processes were investigated by testing the abundance of 16S rRNA and 18S rRNA, ammonia-oxidizing bacteria (AOB) and archaea (AOA). Treatments included manure-amended soil inoculated vs. not inoculated with E. coli O157:H7, treated or not treated with OTC at environmentally releant concentration. Results indicated that OTC did not affect soil bacterial diversity or abundance but increased the abundance of 18S rRNA, the AOB amoA gene, and the activity of urease. The invasion of E. coli O157:H7 significantly decreased the abundance of 16S rRNA, the AOA amoA gene, and soil microbial diversity from 1 to 14 days, while there was no significant impact of E. coli O157:H7 on soil microbial activity and function from 14 to 28 days. The dual treatment with OTC and E. coli O157:H7 significantly increased the abundance of AOB at day 14 and 28, which resulted in higher concentrations of NO3−N in the soil. The interaction between OTC and E. coli O157:H7 on decreasing the abundance of 16S rRNA and microbial diversity was statistically significant after 1 day of incubation. Additionally, OTC and E. coli O157:H7 had significant interactive effects on urease activity, which may be also attributed to the impact on the partial nitrification process.