To cross the epithelial barrier, Salmonella can also use dendritic cells, the cells that are able to move between adjacent intestinal epithelial cells without disrupting the tight junctions.

Once intracellular, Salmonella remains trapped in a vacuolar compartment and, unlike Shigella, does not escape into the cytoplasm. There, flagellin induces production of proinflammatory cytokines such as IL-8. Salmonella also produces flagellin, the monomeric subunit of the bacterial flagellae, and is able to translocate it into host cell cytosol using the SPI-1 TTSS. These events alter actin dynamics at the point of bacterial entry and subsequent bacterial uptake. The TTSS pore complex, SipB/SipD, and the SPI-1 effector proteins (SipA, SopB, SopD, and SopE/E2) then stimulate a host signaling cascade and cytoskeletal changes involving activation of host proteins such as Cdc42 and Rac. Like Shigella (see preceding), Salmonella enters host cells by a ‘triggering mechanism’ that involves cytoskeletal alterations resulting in the formation of actin-rich membrane ruffles that engulf the pathogen.

Invasion of epithelial cells is mediated by TTSS and the secreted effector proteins encoded by the 40 kb Salmonella Pathogenicity Island 1 (SPI-1) located on the Salmonella chromosome. Subsequently, the bacteria either invade the intestinal epithelial cells or get internalized by the M cells of the Peyer’s patches. Invasion is initiated by attachment of the pathogen to the intestinal epithelial cells by different types of fimbriae including type 1 fimbriae, long polar fimbriae (Lpf), and the thin aggregative fimbriae (curli). Salmonella infections develop in both the small and large intestines. The disease is self-limiting and usually lasts no longer than 3–7 days, but can lead to serious complications in immunocompromised hosts such as HIV patients. Disease symptoms usually appear within 6–48 h of ingestion of the bacteria and include nausea, vomiting, abdominal cramps, and diarrhea. The infectious dose in adult volunteers has been shown to be 10 6. The infection usually occurs after ingestion of contaminated water or foods of animal origin such as meat, poultry, eggs, and dairy products. The nontyphoidal S almonella is the most common bacterial pathogen associated with food-borne outbreaks. Nontyphoidal Salmonella infections occur at a high rate in both developed and developing countries. enterica serovars typhimurium, enteritidis, newport and heidelberg, referred to as nontyphoidal Salmonella, cause gastroenteritis (salmonellosis), whereas the serovars typhi, paratyphi, and sendai cause enteric fever (see the section following on S. enterica causes a variety of infections in diverse animal hosts including humans, domestic animals, and birds, with infections in humans being caused mostly by serovars in subspecies I. enterica, is further subdivided into six subspecies with approximately 2500 serovars (serotypes). are Gram-negative facultatively anaerobic rods. Sansonetti, in Encyclopedia of Microbiology (Third Edition), 2009 Salmonella ( nontyphoidal serovars)

Gram negative rods