P06 | Isolation and characterization of <i>Salmonella spp.</i> in swine mesenteric lymph nodes: implications for food safety

Purpose. The study aims to evaluate the presence and characteristics of Salmonella spp. in the mesenteric lymph nodes of pigs isolated in a slaughterhouse in northern Italy. Analysis of this matrix provides a reliable diagnosis of the presence of such microorganisms and represents a sensitive indicator of the animal's infection status. Among the characteristics analyzed are the pathogens' ability to form biofilms, a crucial factor for their persistence in processing environments, and the presence of antibiotic-resistant strains. One of the main resistance mechanisms in Enterobacteriaceae is the ability to synthesize β-lactamases, enzymes that inhibit the action of β-lactam antibiotics, which are widely used in the treatment of Salmonella infections. Methods. Between May 2022 and February 2024, 103 samples of mesenteric lymph nodes were collected from 42 batches of pigs slaughtered at a facility located in northern Italy. After external decontamination of the mesenteric lymph nodes with ethyl alcohol, the isolation, biochemical tests for identification, and serotyping of Salmonella spp. were conducted according to ISO 6579-1:2017 at the Istituto Zooprofilattico Sperimentale della Lombardia ed Emilia Romagna, Brescia branch. Antimicrobial resistance to cephalosporins (cefotaxime, CTX) and carbapenems (meropenem, MEM) was evaluated by determining the minimum inhibitory concentration (MIC). According to the EUCAST 2024 guidelines, isolates are considered resistant with CTX MIC values > 2 µg/mL and MEM > 8 µg/mL. The isolates' ability to form biofilms was also evaluated. An optical density (OD) value lower than the non-adherent control (ODc) indicates the absence of biofilm production; A value between ODc and 2ODc indicates weak biofilm production; A value between 2ODc and 4ODc indicates moderate production; while an OD greater than 4ODc indicates strong biofilm production. Results. Salmonella spp. was detected in 17/103 samples (16.5%). The identified serotypes were S. Derby (10/17; 58.8%), S. Typhimurium (1/17; 5.9%), S. Typhimurium 4,[5],12:i: - (3/17; 17.6%), S. Enteritidis (1/17; 5.9%), and S. Bredeney (2/17; 11.8%). All isolates were found to be sensitive to the action of CTX and MEM. 47% (8/17) of the Salmonella spp. isolates were weak biofilm producers, while the remaining isolates showed no biofilm production. Most of the biofilm-producing strains belonged to serotype S. Derby (7/8) and the single strain of S. Enteritidis. Conclusions. The detection of Salmonella spp. in pig mesenteric lymph nodes highlights the potential risk of contamination in the pork supply chain. Although all isolates were found to be sensitive to the antimicrobials considered, the presence of biofilm-producing strains, albeit weak, suggests an increased ability to persist in the processing environment. The results obtained highlight the importance of monitoring Salmonella spp. in the primary production phase and their characteristics, in order to improve control and risk reduction strategies throughout the entire production chain.