Bacteriological safety and quality of composted products from animal, urban or sewage sludge wastes

This study investigated the presence of culturable bacterial pathogens, and antibiotic resistance and associated genes (quantitative PCR) in commercially available composted products from animal excrements or manure (n=7), urban wastes (n=1) or (sewage sludge) (n=1). Metals quantification and 16S rRNA-based bacterial community composition analyses supported the results to infer potential risks to downstream environments (e.g., soils). Bacilli and Actinomycetes were the dominant bacterial classes in seven composts, while two were dominated by different classes of Pseudomonadota or the class Bacteroidia. Salmonella spp. was not detected in all composts, meeting recommended quality criteria, while Escherichia coli and Listeria monocytogenes were only detected in the sewage sludge compost. The antibiotic resistance genes ermB and ermF were detected in most of the composts, and the antibiotic resistance gene sul1 and the intI1 gene (proxy for antibiotic resistance recombination) in all composts in the range of 6-9 log gene copy number/g dry weight. Listeria spp. and the gene bla_CTX-M were detected only in chicken/poultry composts suggesting increased risk. All composts, except the urban waste compost, presented at least one metal (zinc, copper, and/or cadmium) above the recommended value. The genes uidA, crAssphage, ermB and bla_CTX-M were negatively correlated with the abundance of total heterotrophs and moisture content, and the genes intI1 and sul1 were negatively correlated with the concentration of the metals Cr, Ni and Pb. Overall, the urban waste compost presented the best quality, exhibiting the lowest antibiotic resistance load. These findings alert for the fact that composts may contribute to the dissemination of antibiotic resistance, highlighting the need of regular assessment. It is suggested that multiple factors, including the raw materials, may influence the safety of the final compost, and the knowledge of the variables affecting compost safety need to be thoroughly investigated and understood.