Microbial Food Safety Aspects Along the Supply Chain of African Indigenous Vegetables. A Case Study of Leaf Amaranth (Amaranthus spp.) in Kenya

Microbial food safety aspects along the supply chain of fresh leaf amaranth (Amaranthus spp.) were evaluated from farms to urban supermarkets in Kenya. A total of 150 leaf samples were collected from amaranth farms, transport, wholesale, peri-urban retail markets, and supermarkets. Microbiological indicators of sanitary conditions were analyzed in terms of the prevalence and load of aerobic mesophilic bacteria, Enterobacteriaceae, total coliforms, yeasts & molds, Escherichia coli, Salmonella, and Staphylococcus aureus. Foodborne pathogens, E. coli, Salmonella, and S. aureus, were characterized through molecular screening for selected virulence genes. The prevalence of microbial contamination was highest in peri-urban retail markets, followed by wholesale markets, supermarkets, and transportation, and least on farms. Throughout the supply chain, coliforms were the most prevalent, detected in 98.7% of the samples, followed by Enterobacteriaceae (97.3%), aerobic mesophiles (96.7%), and yeasts and molds (92%). However, S. aureus was the most prevalent pathogen (87.3%), followed by Salmonella (79.3%) and E. coli the least (67.3%). Microbial load increased from the farm to peri-urban retail markets and was significantly higher in the peri-urban retail markets, i.e., 5.08–6.22 Log CFU/g (aerobic mesophiles), 5.28–6.32 Log CFU/g (Enterobacteriaceae), 5.56–6.47 Log CFU/g (coliforms), 2.02–3.59 Log CFU/g (E. coli), 3.87–4.64 Log CFU/g (S. aureus), and 3.36–4.23 Log CFU/g (Salmonella), respectively. Shiga toxin-producing E. coli virulence genes (stx1 and stx2), enterotoxin (stn), and plasmid (pCT) Salmonella virulence genes, and S. aureus virulence genes (sea) were detected in isolates from vegetables at different nodes of the supply chain. Microbial contamination levels were above the international and national thresholds, and foodborne microorganisms had pathogenic attributes. Initiatives to improve sanitation measures at each stage of the supply chain are recommended to guarantee food safety.