Risk Assessment of Human Listeriosis from Semisoft Cheeses Made from Raw Sheep's Milk in Lazio and Tuscany (Italy)
Roberto CondoleoZiad MezherS. MarozziAntonella GuzzonR. FischettiMatteo SeneseStefania SetteLuca Bucchini
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Abstract:
Semisoft cheese made from raw sheep's milk is traditionally and economically important in southern Europe. However, raw milk cheese is also a known vehicle of human listeriosis and contamination of sheep cheese with Listeria monocytogenes has been reported. In the present study, we have developed and applied a quantitative risk assessment model, based on available evidence and challenge testing, to estimate risk of invasive listeriosis due to consumption of an artisanal sheep cheese made with raw milk collected from a single flock in central Italy. In the model, contamination of milk may originate from the farm environment or from mastitic animals, with potential growth of the pathogen in bulk milk and during cheese ripening. Based on the 48‐day challenge test of a local semisoft raw sheep's milk cheese we found limited growth only during the initial phase of ripening (24 hours) and no growth or limited decline during the following ripening period. In our simulation, in the baseline scenario, 2.2% of cheese servings are estimated to have at least 1 colony forming unit (CFU) per gram. Of these, 15.1% would be above the current E.U. limit of 100 CFU/g (5.2% would exceed 1,000 CFU/g). Risk of invasive listeriosis per random serving is estimated in the 10 −12 range (mean) for healthy adults, and in the 10 −10 range (mean) for vulnerable populations. When small flocks (10–36 animals) are combined with the presence of a sheep with undetected subclinical mastitis, risk of listeriosis increases and such flocks may represent a public health risk.Keywords:
Flock
Subclinical infection
Sheep milk
Cheesemaking
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The purpose of this study was to determine the viability span of listeria monocytogenes during the ripening of brined savak white cheese which is produced by and has a significant role in the nutrition of people living in Elazig, Tunceli, Bingol and Erzincan. Ewes milk was inoculated with various strains (4b, 1/2c and 1/2a) of listeria monocytogenes at the rate of 1.00x103 - 1.35x106 cells/ml and then savak cheese was produced from the milk. The cheese samples were analysed after 0, 15, 30, 60, 90 and 120 days of ripening for the numbers of listeria monocytogenes with the direct plating method and enrichment methods (at 30oC for 48 hours or 7 days). The numbers of listeria monocytogenes were 3.70x105 - 6.88x105 cfu/g in the curd, which fell to 0.50x10-7.08x10 cfu/g in the cheese after 120 days of ripening and the trends were almost identical. The numbers of listeria monocytogenes determined with the enrichment methods, followed different paths during the ripening. The pH of the cheese until day 60, in all series of the samples. Later in the experiment (days 90 and 120) the values increased. It was conluded that it might be possible to isolate listeria monocytogenes from savak cheese made from raw ewes milk containing a minimum of 1.00x103 cells/ml; using direct isolation rate in enrichment at 30oC for 48 hours is higher than in enrichment at 30oC for 7 days. In addition, it was evident that listeria monocytogenes survive until the end of the ripening process and pose a danger to public health.
Cheese ripening
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Physicochemical, microbiological and sensory characteristics, evolution of lipolysis monitored by measuring acid degree value (ADV) and the main mineral elements were studied during ripening and storage of artisanal Xinotyri cheese from raw goat's milk. Cheeses were characterized by a high content in total solids (TS; 83%) and fat (59% of TS), and very low pH (4.0), water activity (0.87) and moisture (17%). Protein and salt contents at the end of storage were 31% and 2.8%, respectively. Lipolysis increased, while variations in mineral (Ca, P, Mg and Zn) content were found during ripening and storage. Cheeses were free of Salmonella and Listeria in 25 g, while enterobacteria, pseudomonads, and coagulase‐positive staphylococci were < 100 cfu/g. Mesophilic lactic acid bacteria increased above 8 log cfu/g by day 6, but declined by 2–3 logs in the ripened cheese (45 days) and by 3–4 logs during cheese storage at 4°C for up to 180 days.
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Fate of Listeria monocytogenes during Ripening of Iranian Traditional Koozeh Cheese Made from Raw Ewe's Milk
Cheese ripening
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To identify the source of contamination of raw bulk milk with Listeria, we attempted to isolate the bacteria from various samples such as cattle-related samples, bulk storage tanks and the environments on three farms. On farms A and B, Listeria monocytogenes was repeatedly isolated from raw milk, while on farm C, it was scarcely isolated from it. On the former farms, Listeria was detected in cattle-related samples and the environments. On the other hand, only one fecal sample on the latter farm was Listeria-positive. Especially, we demonstrated that the bulk tank on farm A was contaminated with L. monocytogenes. Then, L. monocytogenes was controlled by continuously washing the bulk tank on farm A with alkaline detergent.
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Food contaminant
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Ewe's milk farm production is permanently associated with the risk of contamination by pathogenic bacteria, including Listeria monocytogenes. In the present study, the prevalence and diversity of L. monocytogenes strains repeatedly isolated from tank ewe's milk and the milking environment on a farm in Slovakia during a prolonged period were investigated to identify the source of potentially persistent contamination. A total of 140 samples along the milk production chain were collected during an 18-month period. From all these samples, 45 samples were found L. monocytogenes positive with 90.3% positivity of tank milk samples (28 positive samples from 31 analysed). Pulsed-field gel electrophoresis profiling resulted in strain discrimination into six profiles with one pulsotype (NS1) corresponding to MLST-ST14 being predominant. A total of 17 proportionally selected L. monocytogenes isolates, including 11 NS1/ST14 isolates, were subjected to whole genome sequencing. Resulted data were used to compare the genomes diversity and to confirm the persistent contamination when <10 allelic differences threshold in cgMLST analysis was applied. The source of persistent contamination was localized inside the milking apparatus, probably in shelters that were very difficult to clean. Despite great efforts, the ewe's milk contamination could not be eliminated during the reporting period.
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Bulk tank
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Food contaminant
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