Antibiotic resistance in biogas processes

Anaerobic digestion (AD) is a well-established technology that can play a key role in development of a sustainable society. In AD, organic wastes such as animal manure, food waste and crop residues are used as substrate and converted to biogas and digestate, which represent green energy and a biofertiliser. Due to intensive use of veterinary antibiotics, antibiotic residues, antibiotic-resistant bacteria (ARB), antibiotic resistance genes (ARGs) and mobile genetic elements (MGEs) enter the AD process via the substrates and end up in the digestate. Thus, digestate may represent a source of spread of antibiotic resistance. Antibiotic resistance is one of the greatest global public health challenges of our time and is predicted to cause around 300 million premature deaths by 2050, so countering its spread is critically important. However, research on the antibiotic resistance level in AD is still quite limited. This thesis contributed essential new knowledge by a) identifying ARB communities in digestates originating from food waste, crops and dairy manure; b) assessing antibiotic resistance in plant-based substrates; c) investigating phenotypic and genotypic resistance pattern and resistance transferability of isolated ARB; and d) comparing molecular and culture-dependent methods in evaluation of antibiotic resistance. Bacillus and closely-related genera such as Paenibacillus and Lysinibacillus were found to dominate the ARB community isolated from digestate, irrespective of substrate type. Most ARGs identified for these ARB were located on chromosomes, although several ARB strains had extra-chromosomal genomes. Only one was identified as a plasmid (pAMαl), which proved to be nontransferable in plasmid conjugation testing. Thus, the dominant ARB community from the digestates studied likely poses a limited risk of antibiotic resistance spread, although even plant-based substrates were found to contain variant antibiotic resistance components. Combined use of molecular and culturedependent methods was required to reveal the true antibiotic resistance situation in the AD process