Prevalence of Salmonella enterica in Italian salami and evaluation of isolates survival in a human macrophage-like (U937) cell line

Salmonella is the second pathogen reported in the European Union (EU) as causative agents of human foodborne diseases (EFSA and ECDC, 2015). In 2014, S. Enteritidis and S. Typhimurium were the two more frequently isolated serovariants in confirmed human cases. The main source of contamination is food of animal origin, especially pig meat, which is largerly consumed in EU (Devine, 2003); in Italy, in particular, the consumption of a wide range of salami produced with pork is very common. Pigs are important reservoir of infection for humans as they are asymptomatic carriers of broad host-range serovars of Salmonella (Fedorka-Cray et al., 2000). From April to December 2015, 155 ground pork and fat mixture samples (corresponding to 155 different batches) used for salami production were tested for Salmonella.All samples were provided by four production plants located in Emilia-Romagna region, northern Italy. Salami batches were tested only in case their ground raw mixture was positive for Salmonella. The detection of Salmonella was performed using a molecular method based on the Real-Time polymerase chain reaction (PCR) and positive samples were confirmed by the ISO 6579:2002 bacteriological standard method . The enumeration of Salmonella in the Real-Time PCR positive samples was performed following the miniaturized Most Probable Number (MPN) technique according to ISO 6579-2:2012. Since the ability of Salmonella to cause systemic disease has been assessed in mice according to their survival and growth in murine cell lines, , we evaluated the capacity of some Salmonella isolatesto survive and replicate within a human macrophages-like cell line (U937). This study was performed to investigate the potential invasiveness for humans of Salmonella strains found in ready-to-eat pork products. Salmonella was isolated from 24/46 (52.2%) of raw mixture samples and from 16/59 (27.1%) of cured salami; eight Salmonella serovars were identified, with S. Derby (12/24; 50.0%) as the most common. The MPN enumeration of Salmonella in raw mixture ranged from 160 MPN/g to < 1.3 MPN/g, while in cured salami it ranged from 8.7 MPN/g to 1.3 MPN/g in seven samples (43.7%), and was < 1.3 MPN/g in the remaining nine (56.3%). Three of the four tested strains of S. Typhimurium 4,[5],12:i:- were able to survive and growth in the human macrophage-like (U937) cell line. On the contrary, one isolate of S. Typhimurium 4,[5],12:i:- and one strain of S. Infantis were not able to to survive.