Inactivation of pathogenic bacteria in digestates by competition with indigenous bacteria

Mesophilic anaerobic digestion (AD) is less efficient than thermophilic AD for inactivating bacterial pathogens. Decreases in fecal bacteria indicators (Escherichia coli, Enterococcus spp.) and pathogenic bacteria range from 1 to 3 Log. However, during the storage of digestates, natural "disinfection" occurs due to diverse microorganisms present in the digestate. Further investigation of the biotic mechanisms at work would make it possible to assess the value of digestate recovery in terms of pathogen elimination. The objective of the PRObiotic project was to study how the activity of digestate microorganisms, which is related to competition for available nutrients, influences the inactivation of pathogenic bacteria. A combination of physicochemical and biochemical analyses (fractionation of organic matter, 3D spectrofluorimetry), microbiology (culture, qPCR, diversity analysis by NGS sequencing) and biodegradability tests under anaerobic and aerobic conditions has shown that the survival of Salmonella enterica serotype Derby inoculated into digestates decreases as availability of organic matter and microbial activity increases. When the organic matter of the digestate is strongly stabilized, the 'barrier effect' of the indigenous microorganisms of the digestate decreases. In the case of Listeria monocytogenes, the nature of the post-treatment is more important: this bacterium does not often survive when inoculated into composted digestates, whereas it can be maintained for 40 days after inoculation into non-post-treated digestates. This study shows how understanding the biotic processes involved can help improve both the microbial controls dynamics and microbiological risk management.