Salmonella Overview



Salmonella is a genus of bacteria that includes S. typhi and S. enteritidis. These bacteria are gram negative rods, which do not ferment lactose, and oxidase negative. However, these bacteria DO produce hydrogen sulfide, which produces a black precipitate on iron agar. Salmonella is a facultative intracellular bacteria that is motile under the microscope. Since the bacteria is acid labile, it is easily destroyed by stomach acid, and requires a large inoculum or infectious dose in order to cause disease.  Finally, the common pathogenesis for a Salmonella infection is that the bacteria invades the GI tract via the M cells of Peyer's patches in the intestines.

Key Points

  • Salmonella spp. (FA 2019 p144)
    • Divided into typhoidal (S. typhi) and non-typhoidal (S. enteritidis) types
    • Common Characteristics
      • Salmonella and Shigella spp. are all gram-negative rods, non-lactose fermenters, oxidase negative, and invade the GI tract via M cells of Peyer patches
      • Gram-negative rod
        • First test in algorithm, narrows scope of bacteria
      • Does not ferment lactose
        • Second test in algorithm
      • Oxidase negative
        • Third test in algorithm
      • Produces H2S
        • Fourth test in algorithm, narrows to Salmonella spp.
        • Contrast vs. Shigella, which does not produce H2S
      • Has flagella (motile)
        • All Salmonella have flagella and swim (“salmon swim”)
        • Contrast w Shigella - nonmotile
      • Intracellular
        • Replication only occurs inside infected cells
      • Acid Labile (destroyed by stomach acid)
        • Infectious dose (ID50) is high
          • Requires a large inoculum (dose)
    • Common Pathogenesis
      • Penetrates the intestinal mucosa, and travels to mesenteric lymph nodes, where it multiplies and is phagocytosed by macrophages within which it can survive.
      • Invade GI tract via M cells of Peyer patches
        • both species of Salmonella and Shigella invade the GI tract through this mechanism