Molecular serotyping of rough Salmonella strains isolated from Danish pork production

Salmonella is an important zoonotic pathogen and meat, including pork, is one of the main sources of salmonellosis. Surveillance of Salmonella in the meat production chain is therefore essential to increase food safety. To enable e.g. source attribution and to reveal the source of outbreaks methods for subtyping is necessary and serotyping is one of the most commonly used approaches for Salmonella. Traditionally, serotyping is done by slide agglutination using antisera aiming at two cell surface antigens: somatic (O) lipopolysaccharides (LPS) and flagellar (H). Some Salmonella strains have incomplete LPS structures, referred to as rough strains, or do not express H antigens, therefore serotyping by agglutination cannot be performed on these isolates. This results in data gaps when subtyping data is needed. To overcome this obstacle, serotypes can be determined on DNA level, i.e. by determining the presence of genes encoding the O and H antigens using molecular serotyping. The aim of this study was to use molecular serotyping on rough Salmonella strains isolated from pork production in Denmark and to assess if the serotypes obtained differed from serotypes found for strains isolated during the same time period from similar sources. A number of rough Salmonella strains (n = 211) isolated from pig carcasses during 2005-2012 were analyzed with molecular serotyping employing Luminex technology. Results show that molecular serotyping enabled serovar identification in 168 of the strains (80%). The most common serotypes were Typhimurium (n=92; 44% of the 211 strains), Derby (n=40; 19%), 4,(5),12:i:- (n=22; 10%), Infantis (n=8; 4%) and others (n=6; 3%). Results for strains (n = 1233) isolated during the same years from similar sources where typing using traditional serotyping was possible (smooth strains) showed a similar pattern in serotypes found, where Typhimurium (n=547; 44%), Derby (n=399; 32%) and 4,(5),12:i:- (n = 72; 6%) also were the most common serotypes. Studies are in progress to serotype the remaining 43 rough strains using other DNA based techniques to further assess the difference in results between molecular and traditional serotyping. In conclusion, our preliminary data suggests that molecular serotyping of rough Salmonella strains mirrored the serotypes obtained using traditional serotyping for smooth strains.