Listeria is Frequently Present in Biofilms in Albanian Dairies

Biofilms in food processing environments are a major concern for hygiene and food safety as they can harbor spoilage bacteria and pathogens. The aim of this study was to determine the prevalence of biofilms and Listeria (L.) species (spp.) in five Albanian dairies. We analyzed the presence of biofilms on 80 visibly clean sites by enumeration of the bacterial load (>100 bacterial cell equivalents /cm²) and quantification of extracellular DNA, carbohydrates, and proteins of the biofilm matrix. We furthermore isolated and characterized Listeria spp. from 200 samples, including 80 biofilm and 120 additional samples. The biofilm prevalence (50% of 80 biofilm samples) and Listeria contamination (20% of 200 samples, including biofilm and additional samples), were very high, indicating a low hygiene status of the five companies. We detected 40 biofilms in total, including food contact surfaces such as storage tanks. Furthermore, 40 of the 200 sampling sites were positive for Listeria. We isolated 46 strains, including 38 L. innocua, 7 L. monocytogenes, and 1 L. grayi isolates, resulting in a prevalence of 19% for L. innocua, 3.5% for L. monocytogenes, and 0.5% for L. grayi. Listeria were frequently present in biofilms (n = 11), 27.5% of 40 biofilms harbored Listeria. The genetic diversity of the L. innocua strains was low, as the isolates mainly belonged to sequence type (ST) 1489 (n = 21) and 1085 (n = 6). The L. monocytogenes strains, assigned to ST3, ST5, and ST619, harbored the full set of essential virulence genes, indicating a virulent genotype. Gene profiling confirmed the presence of many stress resistance genes in all Listeria spp. strains and the presence of plasmids in 54.3% strains. In conclusion, we successfully identified biofilm and Listeria spp. contamination hotspots in Albanian dairies, which should intensify cleaning and disinfection to improve food safety.