Salmonella typhimurium and Salmonella enteritidis strains

Salmonella is a facultative intracellular pathogen that causes a variety of infectious diseases. The most common of such diseases is gastroenteritis, with bacterial multiplication in intestinal submucosae and diarrhea, caused by the inflammatory response and perhaps also by toxins. In specific hosts, adapted salmonellae produce systemic diseases such as typhoid and paratyphoid fevers in humans. If host defenses are impaired, as in elderly or AIDS patients, Salmonella can enter the bloodstream and cause septicemia, which is often fatal [47]. A recent proposal based on molecular relationships classifies Salmonella into two species, Salmonella enterica and Salmonella bongori [40]. In turn, S. enterica is divided into seven phylogenetic groups, subspecies I, II, IIIa, IIIb, IV, VI and VII. Subspecies I includes 1,367 serovars, some of which are commonly isolated from infected birds and mammals, including humans. The other subspecies mainly colonize coldblooded vertebrates. Following common practice, we will name the serovars as if they were species, i.e. Salmonella typhi rather than S. enterica ser. typhi. Virulence factors responsible for pathogenicity in enteric bacteria are often encoded by plasmids, as in Escherichia coli, Yersinia spp. and Shigella spp. In Salmonella, the existence of plasmid-borne virulence genes was first suggested in 1982, but current evidence suggests that the contribution of virulence plasmids to pathogenesis in Salmonella is less important than in the aforementioned bacteria. Virulence plasmids have been found in only a few serovars of Salmonella belonging to subspecies I, particularly those showing host adaptation (Table 1). These plasmids are 50 to 90 kb in size, and have been called “serovar-specific plasmids” (see refs. [7, 20, 21] for reviews). Not every isolate of a plasmid-bearing serovar carries the virulence plasmid [11]. The virulence plasmid of Salmonella is important for bacterial multiplication in the reticulo-endothelial system of warm-blooded vertebrates. Only a 7.8 kb region, spv (Salmonella plasmid virulence), is necessary to confer the virulent phenotype. Other loci of the plasmid, involved in the biosynthesis of fimbriae and in serum resistance, may play a role in other stages of the infection process [20, 21]. Salmonella typhi, which causes typhoid fever in humans, and other serovars less often involved in similar human diseases (paratyphoid fever), such as Salmonella paratyphi A, Salmonella paratyphi B, and Salmonella sendai, lack virulence plasmids (and the spv region). Salmonella paratyphi C is an exception, but the contribution of its virulence plasmid to systemic diseases is unknown [7, 11]. The importance of the virulence plasmid in septicemic diseases caused by non-typhoid serovars can only be inferred from indirect evidence: plasmids are more frequently found in Salmonella typhimurium and Salmonella enteritidis strains Rafael Rotger Josep Casadesús