P11 Listeria monocytogenes in bulk milk from sheep farms in Tuscany and upper Lazio: what's the situation?

Purpose. Listeria monocytogenes is a ubiquitous pathogenic bacterium found in the environment and food processing facilities. It can be present in various foods, with ready-to-eat products being of particular importance in relation to public health. It can be present in raw milk and cheeses, especially those made with raw milk and short to medium aging. It is the causative agent of listeriosis, a serious disease with a high mortality rate, primarily associated with the consumption of contaminated food. The study focused on the research, quantification, and characterization (serotyping and antibiotic sensitivity) of Listeria monocytogenes in raw sheep milk samples delivered by Tuscan and Viterbo farms to a Tuscan dairy. Methods. Between February and April 2024, 42 bulk raw milk samples from the same number of farms were analyzed based on ISO standards 11920-1:2017 and ISO 11920-2:2017 for the qualitative and quantitative determination of L. monocytogenes and L. ivanovii. Subsequently, the isolated microorganisms were subjected to phenotypic identification, again according to ISO 11920-1:2017, and then to a subsequent multiplex PCR in combination with phenotypic serotyping to identify the serotype. Finally, the sensitivity spectrum to 10 antibiotics of therapeutic interest was evaluated using the agar diffusion method according to Kirby Bauer. The search and quantification of L. monocytogenes and L. ivanovii were repeated for some of the positive samples over a maximum period of one year. Results. Out of 42 samples analyzed, 10 were positive for Listeria (23.8%); L. monocytogenes was isolated from 5 of them, and L. ivanovii, a pathogenic species for sheep and, rarely, for humans, with loads up to 650 CFU/ml, was isolated from 5. All L. monocytogenes isolates were serotype 1/2a, the most frequently found in food isolates and often associated with human clinical cases. All L. monocytogenes isolates were sensitive to β-lactams and gentamicin, the active ingredients of first-choice therapy, but showed resistance to meropenem, erythromycin, and sulfamethoxazole/trimethoprim. L. ivanovii was found to be sensitive to all antibiotics. Regarding persistence over time, it was possible to follow the evolution of Listeria contamination for approximately one year after the first sampling, finding its persistence in 4 farms for L. monocytogenes and in 1 for L. ivanovii. In one case, there was an increase in L. monocytogenes from 120 to 1000 CFU/ml over time, one year after the initial control. Conclusions. The study provides useful data for risk assessment in the case of raw milk production where milk from a single farm is processed, meaning there is no heat treatment of the milk and no dilution effect due to the use of milk from many farms in the dairy. The presence of the pathogen even at levels above 100 CFU/ml in bulk milk and its sporadic persistence in the herd at increasing levels over a one-year period highlights the need for constant monitoring, also considering the difficulty in detecting L. monocytogenes mastitis in animals.