Diversity of spore-forming bacteria in cattle manure, slaughterhouse waste and samples from biogas plants

As biowaste intended for biogas production can contain pathogenic micro-organisms, the recommended treatment is pasteurization at 70°C for 60min. This reduces pathogens such as Salmonella spp., whereas spore-forming bacteria (Bacillus spp. and Clostridium spp.) survive. Most spore-forming bacteria are harmless, but some can cause diseases such as blackleg, botulism and anthrax. In this study, the effect of the biogas process on Bacillus spp. and Clostridium spp. was investigated.

We analysed 97 faecal samples, 20 slaughterhouse waste samples and 60 samples collected at different stages in the biogas process. Bacillus spp. and Clostridium spp. were quantified and subcultured. The isolates were identified by biochemical methods and by 16S rRNA gene sequencing. Phylogenetic trees were constructed from the sequences obtained from isolates from the samples. Clostridium botulinum/Clostridium spp. and Clostridium sordellii were found both before and after pasteurization, but not after digestion (AD). Some of the isolated strains probably represented new members of the genera Clostridium and Bacillus.

After digestion, the numbers of clostridia decreased, but none of the pathogenic bacteria did, whereas Bacillus spp. remained constant during the process.

Biogas is gaining in importance as an energy source and because the residues are used as fertilizers, we needed to study the prevalence of pathogenic bacteria in such material.