Spread of biosolids-borne antibiotic resistance is a growing public and environmental health concern. Herein we conducted incubation experiments involving biosolids, derived from sewage treatment plants, and biosolids-amended soil. Quantitative reverse transcription PCR (RT-qPCR) was employed 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 antibiotic concentrations 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.
IMPORTANCE
Although previous studies have indicated that biosolids may be important spreaders of antibiotics and antibiotic resistance genes (ARGs) in environments, the potential activities of ARGs or their responses to environmental parameters have been understudied. This study highlights that certain biosolids-borne antibiotics can induce transcriptional activities of ARGs and mobile genetic elements in biosolids and biosolids-amended soil, even when present at environmentally relevant concentrations. Furthermore, these antibiotics can alter the structure of microbial population expressing ARGs. Our findings indicate the bioavailability of the antibiotics in biosolids and provide evidence that biosolids can promote the activities and dissemination of ARGs and mobile genes in biosolids and soils that receive contaminated biosolids; thus, underscoring the importance of investigating anthropogenically-induced antibiotic resistance in the environment under real-world scenarios.
Distribution of antibiotic resistance genes and their association with bacteria and viruses in decentralized sewage treatment facilities
