Characterisation and Antibiotic Resistance of Selected Bacterial Pathogens Recovered from Dairy Cattle Manure during Anaerobic Mono-Digestion in a Balloon-Type Digester

Abstract

Dairy cattle manure serves as a potential source of contamination and infection of animals, humans and the environment. Manure samples withdrawn from a balloon-type digester during anaerobic digestion were evaluated for the presence of antibiotic-resistant bacterial pathogens. The bacterial load of the samples was determined via a viable plate count method and the recovered isolates were subjected to characterisation and identification. These isolates were employed in antibiotic susceptibility testing using a disc diffusion method against a suite of 10 conventional antibiotics. The multiple antibiotic resistance (MAR) index was calculated and MAR phenotypes were generated. Although all the bacterial pathogens showed a certain degree of resistance to the studied antibiotics, a marked resistance was demonstrated by Campylobacter sp. to co-trimoxazole (87.5%) and nalidixic acid (81.5%). Remarkably, a high resistance (82.42%) was demonstrated against the antibiotic class, macrolide, followed by beta-lactams (40.44%), suggesting that bacterial resistance depended on the chemical structure of the antibiotics. However, individual bacterial isolates varied in resistance to particular antibiotics. Of the 83 bacterial isolates, 40(48.19%) observed MAR > 0.2 and, thus, were described as multidrug-resistant isolates. A total of 28 MAR phenotypes were revealed with the highest frequency of MAR phenotypes (37.5%) expressed against 3 antibiotics. Results indicated a high risk of exposure to various antibiotics and wide diversity of antibiotic resistance.