Azithromycin and Ciprofloxacin can Facilitate Antibiotic Resistance in Biosolids and Biosolids-amended Soils

Spread of biosolids-borne antibiotic resistance is a growing public and environmental health concern. Herein we conducted incubation experiments involving biosolids and biosolids-amended soil, wherein we utilized quantitative reverse transcription PCR (RT-PCR) to assess responses of select antibiotic resistance genes (ARGs) and mobile elements to environmentally relevant concentrations of two biosolids-borne antibiotics, azithromycin (AZ) and ciprofloxacin (CIP). Additionally, we examined sequence distribution of gyrA (encoding DNA gyrase; site of action of CIP) to assess potential shifts in genotype. Increasing concentrations of the two antibiotics generally increased the transcriptional activities of qnrS (encoding CIP resistance) and ermB and mefE (encoding AZ resistance). The transcriptional activity of intl1, a marker of Class 1 integrons, was unaffected by CIP or AZ concentrations, but biosolids amendment increased intl1 activity in the soil by 4 to 5 times which persisted throughout incubation. While the dominant gyrA sequences found herein were unrelated to known CIP-resistant genotypes, the increasing CIP concentrations significantly decreased the diversity of genes encoding gyrA, suggesting changes in microbial community structures. This study suggests that biosolids harbor transcriptionally active ARGs and mobile elements that could survive and spread in biosolids-amended soils. However, more research is warranted to investigate these trends under field conditions.