Analyses of microbial community in nutrient solution with biofertilizer and risk assessment of establishment of pathogens

Abstract: The societal interest for a more sustainable and circular food production is increasing. Hydroponic farming is an alternative way of growing vegetables that can bring farming into urban environments, reducing long transports of food. In parallel, the focus in waste treatment is being more directed toward resource recovery. Food waste is used as a resource for biogas production, producing a liquid residue that is rich in the nutrients needed for cultivation of crops that can be certified for use as a biofertilizer. The application of this biofertilizer in hydroponic cultivation systems could pave way for a circular urban food production. Striving for this, the project “Food waste to new food in an urban context - production, risk assessment and consumer acceptance”, that is a collaboration between SLU (Swedish University of Agricultural Sciences) in Alnarp and Ulltuna (Uppsala) and Lund University, led by SLU. The project´s overall scope is to investigate the possibilities of hydroponic cultivation of different vegetables such as pak choi with a biofertilizer produced from anaerobically digested food waste (a residue from biogas production) as a nutrient source, replacing the customary inorganic fertilizer. Since the biofertilizer in hydroponic setups is in direct contact with the crops, ensuring microbiological safety of the biofertilizer in a food safety perspective is paramount. This master thesis project aims to perform an in-depth microbiological risk assessment of the biofertilizer, utilizing 16S rRNA gene amplicon sequencing of samples collected from greenhouse experiments of hydroponic setups with the biofertilizer. Additional investigations of the microbial community was performed utilizing MALDI-TOF MS and calorimetry. As a simulation of a contamination, challenge tests of the biofertilizer with the food-borne pathogens Bacillus cereus, Salmonella enterica and Listeria monocytogenes was performed to investigate their establishment in the biofertilizer. The 16S rRNA gene amplicon sequencing showed that Mycobacterium is the most abundant genus of bacteria in the biofertilizer used in the greenhouse experiments. The challenge testing experiments revealed that low levels of Bacillus cereus (~10 CFU/ml) is naturally present in the biofertilizer. The inoculation of 10^5 CFU/ml of the three food-borne pathogens resulted in S. enterica and L. monocytogenes no longer being detectable in the biofertilizer with selective plating after 48 hours of incubation, and four log10 reductions of B. cereus within 24 hours of incubation. Additionally, results from the investigation of the biofertilizer using calorimetry indicate that the biofertilizer does not seem to support microbial proliferation without the addition of substrates containing a carbon source.