VBNC state and ‘regrowth’ during waste treatment: factors playing a role in loss of culturability of pathogenic bacteria seeded in digestates from agricultural biogas plants

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Maynaud, Géraldine | Pourcher, Anne-Marie | Ziebal, Christine | Wéry, Nathalie

Edité par CCSD -

International audience. During treatment of organic waste by anaerobic digestion, composting or in wastewater treatment plants, pathogenic bacteria are submitted to various biotic and abiotic stress. In treated wastes (digestates, composts…) increase of bacterial counts obtained by culture for indicators (E. coli) and pathogenic bacteria (Salmonella sp., Shigella sp., Listeria monocytogenes) are sometimes recorded, but the underlying mechanisms remain unknown. Some recent studies indicate that this may be due to the induction of Viable but Non-Culturable (VBNC) state during treatment, followed by ‘resuscitation’ during storage of treated products. Here, a microcosm approach was used to evaluate the persistence of three pathogenic bacteria (Salmonella Derby, Campylobacter coli and Listeria monocytogenes) in digestates from agricultural wastes, stored for later land spreading. Nine samples, including raw digestates, liquid fractions of digestate and composted digestates, were inoculated with each pathogen and maintained for 40 days at 24°C. Concentrations of pathogens were monitored using culture and qPCR methods. In some digestates, the concentration of the pathogens by qPCR assay was several orders of magnitude higher than the concentration of culturable cells, suggesting a potential loss of culturability and induction of VBNC state. The potential VBNC state which was generally not observed in the same digestate for the three pathogens, occurred more frequently for C. coli and L. monocytogenes than for Salmonella Derby. The effect of NH4+/NH3 on the culturability of C. coli and Salmonella Derby was also shown. Our results underline the importance of considering VBNC cells when evaluating the sanitary effect of an anaerobic digestion process and the persistence of pathogens during the storage of digestates and subsequent land spreading.

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