Fate of Horizontal-Gene-Transfer Markers and Beta-Lactamase Genes during Thermophilic Composting of Human Excreta

Thermophilic composting is a suitable treatment for the recycling of organic wastes for agriculture. However, using human excreta as feedstock for composting raises concerns about antibiotic resistances. We analyzed samples from the start and end of a thermophilic composting trial of human excreta, together with green cuttings and straw, with and without biochar. Beta-lactamase genes , , and conferring resistance to broad-spectrum beta-lactam antibiotics, as well as horizontal gene transfer marker genes, and , were quantified using qPCR. We found low concentrations of the beta-lactamase genes in all samples, with non-significant mean decreases in and copy numbers and a mean increase in copy numbers. The decrease in both and genes from start to end of composting indicated that thermophilic composting can decrease the horizontal spread of resistance genes. Thus, thermophilic composting can be a suitable treatment for the recycling of human excreta.