Determination of effects of selected veterinary antibiotics on biogas production in anaerobic digestion systems and analysis of resistance gene promotion

Abstract

Animal husbandry developed into a gigantic business sector to meet demands of modern society. Manure produced in animal husbandry is a suitable substrate for anaerobic digestion since it brings solution to the problem of manure accumulation. Although anaerobic digestion is an old and well known process; it is still far from understood completely. Microbial communities and its importance have been revealed recently. A better understanding of operational and microbiological parameters should be reached to modify and reshape anaerobic digestion systems according to demands. In recent years, antibiotic build up has been detected in receiving environments due to common use in treating both men and animals. Due to their persistence, they begin to accumulate in environment. Antibiotics may create a selective pressure on microbial communities. Microorganisms can become resistant to antibiotics when antibiotics are in non-therapeutic concentrations. This study presents results of investigation of many laboratory scale anaerobic digesters operated with cattle manure of OTC vaccinated animal. Excretion pattern of oxytetracycline after treatment was observed and excreted amount of antibiotic was calculated. Anaerobic digestion of cattle manure was monitored and performances were recorded in presence of antibiotic and changing operational parameters. Manure amendment to soil was performed to monitor fate and effect of oxytetracycline and digester microbial communities on soil microbial communities Within the scope of the dissertation, it was found that OTC inhibits digester performance significantly. Effect of OTC was higher on bacterial population rather than archaea. OTC excreted from animal body as at least 18% of initial amount within solid manure. This amount will further increase because of repetitive injections and liquid manure addition. These manures were used as feedstock in anaerobic digesters operated under common operational parameters for farm type anaerobic digesters. 50% inhibition in biogas production was observed. Temperature was most influential parameter of digester performance followed by solid content and mixing rate. Temperature has been also most effective parameter on degradation of OTC. Mixing rate also helped degradation of OTC slightly. Also solid content was in negative relation with OTC inhibition. It has been found that half-life of OTC was ranging from 22-28 days in mesophilic and 16-18 days in thermophilic digesters, respectively. Molecular analyses showed that bacterial community of digesters was dominated by fermentative and hydrolytic bacteria belonging groups of Firmicutes and acidogenic Proteobacteria. Methanobacteriales, Methanomicrobiales and Methanosarcina spp. were found as methanogenic community. FISH analyses indicated an increase in microbial activity after 20 days; after that activity dropped significantly. Manure and digestate were amended to soil to monitor the elimination of antibiotic and its effects on microbial structures. It has been found that the half-life of OTC is far longer (135 days) in soil than in digesters (18-24 days) and microbial structures were changed after amendment. Tetracycline resistance gene analyses showed presence of 104-106 resistance gene copies in digester studies and 103-105 resistance gene copies in soil studies Analyses also showed that ratio of antibiotic resistance genes to bacterial 16S rRNA genes were increased 50 fold in mesophilic digesters and 100 fold in thermophilic digesters. In soil studies ratio don’t showed a significant change.