Heated scallop-shell powder and lime nitrogen effectively decrease the abundance of antimicrobial-resistant bacteria in aerobic compost

The emergence and dissemination of antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs) are global public health concerns. Hence, measures should be implemented to reduce the abundance of ARB and prevent the spread of ARGs from livestock to the environment. In this study, to clarify the potential of reducing ARB abundance in livestock waste during aerobic composting, the effects of adding heated scallop-shell powder (HSSP) and lime nitrogen (LN) to manure compost on the abundance of several bacterial species were investigated. Several bacterial species (Escherichia coli, Pseudomonas aeruginosa, Salmonella enterica, Staphylococcus aureus, and Enterococcus faecalis), spores of Clostridioides difficile, and various concentrations of HSSP/LN were added to livestock manure. After 24 h, the tested bacteria and spores were eliminated using 2% (w/w) and 4% (w/w) HSSP/LN, respectively. Subsequently, field trials were conducted on a pig farm to investigate the effect of adding HSSP/LN to manure composts. Swine manure and rice husk were mixed with or without 4% (w/w) HSSP/LN, and aerobic composting was continued for 13 days. The results showed that all the tested bacteria were eliminated after adding HSSP/LN, but the abundance of antibiotic resistance genes was not significantly altered. Adding HSSP/LN significantly affected the bacterial diversity in the compost. In conclusion, using HSSP/LN as an additive in aerobic composting effectively reduced the abundance of ARB, including sporeforming bacteria.