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Chapter 10 : Salmonella Species
Category: Applied and Industrial Microbiology; Food Microbiology
Salmonella spp. are facultatively anaerobic gram-negative rod-shaped bacteria belonging to the family Enterobacteriaceae. The biochemical identification of foodborne and clinical Salmonella isolates is generally coupled to serological confirmation, a complex and labor-intensive technique involving the agglutination of bacterial surface antigens with Salmonella-specific antibodies. National epidemiologic registries continue to underscore the importance of Salmonella spp. as a leading cause of foodborne bacterial illnesses in humans, where reported incidents of foodborne salmonellosis tend to dwarf those associated with other foodborne bacterial pathogens. The antibiotic resistance genes currently present in bacterial pathogens may originate from antibiotic-producing bacteria present in soil. The major global impact of typhoid and paratyphoid salmonellae on human health led to the early development of parenteral vaccines consisting of heat-, alcohol-, or acetone-killed cells. Salmonella invasion genes are organized into contiguous and functionally related loci, Salmonella pathogenicity islands (SPI1), located within a 40 to 50-kb segment of chromosomal DNA (i.e., pathogenicity island) that encodes determinant factors for the facilitated entry of salmonellae into host cells. The presence of virulence plasmids within the Salmonella genus is limited and has been confirmed for serovars Typhimurium, Dublin, Gallinarum-Pullorum, Enteritidis, Choleraesuis, and Abortusovis. This chapter provides a brief overview of the operating characteristics and sensitivity of polymerase chain reaction (PCR) technology, which is rapidly gaining application in food microbiology.
Genetic map of Salmonella enterica serovar Typhimurium DT104 multidrug resistance locus, SGI1. Regions of SGI1 with deletions, insertions, or an alternate gene cassette(s) within the class 1 integron are indicated within the figure ( 65 , 132 – 134 ).
Genetic map of S. enterica virulence loci ( 65a , 315 , 316 , 408a ). Genomic islands are depicted on the chromosome relative to the positions of housekeeping genes or specific tRNAs. All 10 Salmonella pathogenicity islands, including SPI6 to SPI10, which are only present in Salmonella serovars associated with enteric fever, are illustrated as circles, with serovar Typhi genomic islands attached to the map via arrows with dotted lines. Prophage genomes are also included in the map and are designated by the “lollipop” symbol shown. Squares denote adhesins/fimbrial operons. More detailed genetic organization of SPI1-5 is given in Fig. 10.3 .
Genetic organization of virulence genes present in SPI1-5 and virulence plasmid. The genes and their organization into operons are shown, with arrows demarcating operons and their directions of transcription. The known functions of genes are depicted in this illustration, with shading or pattern designating the functions as described in the key at the bottom of the figure. Those genes with no known function are shown as white boxes. Type III effectors are bacterial proteins injected into host cell cytoplasm via the type III secretion system.
Genetic maps of serovar Typhimurium GIFSY prophages. The detailed genetic maps presented in this figure highlight putative virulence genes within the genomes of two lambda-like prophages present in the S. enterica serovar Typhimurium LT2 genome ( 69 ). One of these genes, sseI, is injected into the host cytoplasm via the type III secretion system present in SPI1.
Mechanism for Salmonella invasion of host epithelial cells.