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Traditional routine diagnostic microbiology on Salmonella consists generally of culture-based detection and identification of the microorganism on species level, and differentiation into certain phenotypes by serological methods.
Further discrimination is achieved by antimicrobial resistance as well as phage typing, e.g. in the case of surveillance and epidemiology studies. In addition, within the past two decades microbial genotyping has tremendously improved our knowledge of how Salmonella can transmit from animals through the food chain to humans. This thesis contributes to facilitating the detection of Salmonella and to gaining a better knowledge of the spread of Salmonella from animals to humans in three respects: firstly, a rapid and sensitive detection method for Salmonella from food samples has been developed based on real-time PCR and extensively validated. Secondly, a new approach for low-number enumeration of Salmonella was developed as a proof-of-principle and thirdly, the transmission pathways of epidemiologically important Salmonella enterica serovars from livestock through food to humans were investigated by phenotypic and genotypic methods. By virulence and antimicrobial resistance typing in combination with epidemiological data the potential hazard for humans of certain genotypes has been estimated.
The validated real-time PCR detection method for Salmonella, which has been officially approved in Germany, is as accurate as traditional reference ISO 6579:2002 and places in the hands of food diagnostic laboratories a rapid, robust and cost-effective screening tool. Primers and TaqMan probe target specifically sequences within the ttr locus of Salmonella. The analytical assay was the basis for the quantification of salmonellae in low numbers in cork borer samples from pigs in combination with an eight-hour enrichment step where most bacterial growth is in the log phase. The proof-of-principle can be easily applied to other food samples by adaptation of the enrichment time.
The characterization of the pathogenicity gene repertoire of two frequently isolated S. enterica serovars, 4,12:d:- and multidrug-resistant Paratyphi B d-tartrate + (O:5 antigen negative), from poultry estimated the potential hazard for humans as rather low compared to that of other broad-host-range S. enterica serovars (e.g. Enteritidis). In contrast, S. enterica serovars 4,,12:i:- and Derby frequently found in pigs and transmitted by the one vehicle pork could be identified to be a major infection source for humans.
The prevalence of S. enterica serovar 4,,12:i:- has continuously increased especially in pigs and humans since the beginning of the 2000s. Two different subtypes of the monophasic serovar have been identified as prevalent in Germany. The genetic background of the subtypes and serovar is highly similar to that of S. enterica serovar Typhimurium but lacking functional genes in the genome encoding the phase two flagellum antigen H2:1,2.
Overall certain subtypes or clonal lineages of the S. enterica serovars investigated, some of them highly multidrug-resistant, represent a risk for human health through transmission by food. To prevent these serovars from entering the food chain and from the potential dissemination of antimicrobial resistance determinants to related microorganisms, both the farm and food production levels should be subject to rigorous Salmonella control measures.