Impact of compost application on the spread of antibiotic resistance: emerging risks to public and aquatic health in Sri Lanka.

Composting is becoming an increasingly popular practice in the global circular bio-economy. Municipal Solid Waste (MSW) compost specially derived from chicken manure and animal litter poses a risk of spreading antibiotic-resistant contaminants. The objectives of the present study were the isolation of Amoxicillin (AMX), Cloxacillin (CLOX), Tetracycline (TET), and Ciprofloxacin (CIP) resistant bacteria from MSW and commercially available compost samples in Western Province, the determination of Minimum Inhibitory Concentration (MIC), Multiple Antibiotic-Resistant (MAR) indexes, and the detection of Antibiotic-Resistant Genes (ARG). Among 68 Antibiotic-resistant bacterial (ARB) isolates, the highest resistance prevalence was exhibited for AMX (37%), followed by CLOX (35%), TET (15%), and CIP (13%). MIC assays revealed that 32% of AMX, 62% of CLOX, 78% of CIP, and 80% of TET-resistant bacterial isolates recorded MICs greater than 840 μg/mL. The MAR index range varied from 0.25 to 1 for the isolated ARB, with the highest proportion (45.5%) showing a MAR index of 0.25. The ARGs amplification was done using a PCR protocol. The highest percentages of the ARGs in the bacterial isolates were detected as bla TEM (40.5%) for AMX, OPR (D) (40.5%) for CLOX, gyr B (54%) for CIP, and tet (M) (56%) for TET-resistant bacteria, respectively. However, all tested compost samples contained ARB and ARGs. Detection of multidrug-resistant bacteria and ARGs from immature compost suggests that improperly controlled composting processes can facilitate resistance dissemination. Findings of this study emphasize the need for optimized composting practices and adequate maturation, with mandatory quality screening, to ensure properly processed compost serves as a safe soil amendment while reducing antibiotic resistance risks.