Quantification of Aspergillus fumigatus and enteric bacteria in European compost and biochar

Although most potential human pathogens (PHPs) can be inactivated during composting, the risk that such substrates represent for human health remains largely unknown due to the shortage of information on presence and abundance of PHPs in finished composts. This study focused on the assessment of Salmonella spp., Listeria monocytogenes, Shiga toxin-producing Escherichia coli (STEC), and the opportunistic fungal pathogen Aspergillus fumigatus in different compost commodities. A total of fifteen European composts, made from different waste types and processes, were evaluated for the occurrence of the selected PHPs using molecular and traditional techniques. The analyses were extended to five biochar because of their growing application in agriculture, horticulture, floriculture, and private gardening. Enteric bacteria were detected by molecular methods in eight out of fifteen composts; however, viable propagules were confirmed for L. monocytogenes only in two composts, and for STEC in three more composts. No bacterial pathogens were found in biochar. Living A. fumigatus was present in eleven composts and two biochars. None of the eighteen isolates contained single nucleotide polymorphisms (SNPs) relevant for resistance to azole fungicides. The role of compost and biochar as a source of PHPs in the environment and the risk for human health is discussed.