Long-term Application of Manure Alters Culturable Soil Microbial Populations and Leads to Occurrence of Antibiotic resistant Bacteria

The use of manure for agricultural applications is believed to be an important factor in the proliferation of antibiotic-resistant bacteria and can affect natural soil microbial communities. The goal of this study was to determine if the application of manure on cultivated fields would result in the proliferation of antibiotic-resistant bacteria and change in soil microbial population. We collected 30 soil samples from alfalfa, potato, and wheat cultivated fields with a history of about 20 years of manure application. control samples were selected from wheat cultivated fields without manure. Plate count was used to enumerate the number of microbial populations including total microorganisms, total coliforms, lactose-positive coliforms, actinomycetes, saprophytic fungi, and antibiotic (sulfadimidine and oxytetracycline)-resistant bacteria. Antibiotic-resistant bacteria were significantly higher in manure amended than control fields. Sulfadimidine-resistant bacteria were significantly higher in all 3 fields as compared to the control. Most population of the oxytetracycline-resistant bacteria was recorded in alfalfa and had a significant difference with control. The highest number of total microorganisms was found in potato and alfalfa fields, which significantly differed from that of control. Analysis of variance of total coliform and lactose-positive coliform showed a significant difference (p < .05) between fields and the highest number was in alfalfa. In the manured fields, principal component analysis showed a close association between organic carbon, oxytetracycline-resistant bacteria, sulfadimidine-resistant bacteria, total coliform, and lactose-positive coliform distributions, which turns out that the application of manure can lead to the occurrence of antibiotic-resistant bacteria and increase in coliforms in soil.