The use of antimicrobials in livestock can lead to antimicrobial resistant bacteria, which can spread more widely in the environment when manure or slurry are used as organic fertiliser. Use of sewage sludge pellets (biosolids) could be a lower-risk alternative. The aim of this ongoing study is to determine whether spreading biosolids has any effect on the abundance of antimicrobial resistance genes in the soil. Initial data analyses suggested there was, as yet, no statistically significant evidence of any differences in resistant gene abundance.

The prevalence of antimicrobial-resistant bacteria in the environment is driven by the use of antimicrobials in human and veterinary medicine. Livestock farms may provide an avenue for drug-resistant bacteria to transfer to humans. The aim of an ongoing study run by MRI scientists is to determine whether spreading sewage sludge pellets (biosolids), which are byproducts of sewage treatment, could be a viable alternative to current practices of spreading manure or slurry. Two fields were used to investigate if there was an effect on the abundance of antimicrobial resistance genes in the soil, where in one field biosolids were spread, and in the other control field, ‘traditional’ farmyard manure and slurry practices were followed. Soil samples were collected at 25 spatial locations per field in the spring and autumn of 2019 and 2020 before and approximately 2 weeks after spreading. At each collection time, three pools of six samples were run at Resitomap Oy (Helsinki) to identify the abundance of various genes (144 in 2019 and 216 in 2020), which belonged to 13 antibiotic classes. 

BioSS analysed the data from 2019 and 2020 separately to identify if there was any evidence of the biosolid treatment affecting the abundance of antimicrobial resistance genes. The abundance was explored using principal components analysis biplots to identify if there was any difference in the soil samples before and after spreading. The analysis showed no evidence of a difference before and after spreading in either spring or autumn in either year. Linear mixed models were fitted for each detected gene to investigate the effect of treatment on abundance and to allow for correlation between samples from similar spatial locations within each field. Since a large number of models were being fitted, distributions of p-values were examined to identify if the p-values associates with treatment were more likely to be statistically significant than would be expected by chance, or if genes belonging to a specific class were statistically significant to a greater extent than expected by chance. The interaction of treatment (spreading biosolids versus control) with timing (before and after spreading) was 19 not found to be statistically significant and there was no evidence that genes belonging to a particular class were more likely to be statistically significant than any other class. Overall, when comparing soil samples before and after spreading, the analyses suggested that there was no evidence of any difference in resistant gene abundance associated with the choice of material used for spreading.

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This work was done in collaboration with Nuno Silva at the Department of Disease Control, Moredun Research Institute, Dr Michelle Bellingham, School of Biodiversity, One Health, and Veterinary Medicine, College of Medical, Veterinary and Life Sciences, University of Glasgow and Prof. Karen Scott, Microbial Ecology Group, Rowett Institute. The work was funded under the Underpinning National Capacity element of the Scottish Government's Strategic Research Programme for environment, agriculture and food.