Responses of the Soil Bacterial Community, Resistome, and Mobilome to a Decade of Annual Exposure to Macrolide Antibiotics.

Biosolids that are applied to agricultural soil as an organic fertilizer are frequently contaminated with pharmaceutical residues that have persisted during wastewater treatment and partitioned into the organic phase. Macrolide antibiotics, which serve as a critically important human medicine, have been detected within biosolids. To determine the impacts of macrolide antibiotics on soil bacteria, every year for a decade, a series of replicated field plots received an application of a mixture of erythromycin, clarithromycin, and azithromycin at a realistic (0.1 mg kg soil-1) or an unrealistically high (10 mg kg soil-1) dose or were left untreated. The effects of repeated antibiotic exposure on the soil bacterial community, resistome, mobilome, and integron gene cassette content were evaluated by 16S rRNA and integron gene cassette amplicon sequencing, as well as whole-metagenome sequencing. At the unrealistically high dose, the overall diversity of the resistome and mobilome was altered, as 21 clinically important antibiotic resistance genes predicted to encode resistance to 10 different antibiotic drug classes were increased and 20 mobile genetic element variants (tnpA, intI1, tnpAN, and IS91) were increased. In contrast, at the realistic dose, no effect was observed on the overall diversity of the soil bacterial community, resistome, mobilome, or integron gene cassette-carrying genes. Overall, these results suggest that macrolide antibiotics entrained into soil at concentrations anticipated with biosolid applications would not result in major changes to these endpoints. IMPORTANCE Biosolids, produced from the treatment of sewage sludge, are rich in plant nutrients and are a valuable alternative to inorganic fertilizer when applied to agricultural soil. However, the use of biosolids in agriculture, which are frequently contaminated with pharmaceuticals, such as macrolide antibiotics, may pose a risk to human health by selecting for antibiotic resistance genes that could be transferred to plant-based food destined for human consumption. The consequences of long-term, repeated macrolide antibiotic exposure on the diversity of the soil bacterial community, resistome, and mobilome were evaluated. At unrealistically high concentrations, macrolide antibiotics alter the overall diversity of the resistome and mobilome, enriching for antibiotic resistance genes and mobile genetic elements of concern to human health. However, at realistic antibiotic concentrations, no effect on these endpoints was observed, suggesting that current biosolids land management practices are unlikely to pose a risk to human health due to macrolide antibiotic contamination alone.

Abnormal Social Interactions in a Drosophila Mutant of an Autism Candidate Gene: Neuroligin 3.

Social interactions are typically impaired in neuropsychiatric disorders such as autism, for which the genetic underpinnings are very complex. Social interactions can be modeled by analysis of behaviors, including social spacing, sociability, and aggression, in simpler organisms such as Drosophila melanogaster. Here, we examined the effects of mutants of the autism-related gene neuroligin 3 (nlg3) on fly social and non-social behaviors. Startled-induced negative geotaxis is affected by a loss of function nlg3 mutation. Social space and aggression are also altered in a sex- and social-experience-specific manner in nlg3 mutant flies. In light of the conserved roles that neuroligins play in social behavior, our results offer insight into the regulation of social behavior in other organisms, including humans.