SAARS: Salmonella spp. (microbiology)

Typically non-lactose-fermenting colonies isolated from stool cultures can be distinguished as possible Salmonella or Shigella colonies according to the following phenotypic characteristics:

Salmonella spp. are enteric Gram-negative rods which are usually recovered from stool specimens, although depending on the presentation they may be found in the blood or in any other organ system after hematogenous dissemination. Salmonella produce a non-lactose-fermenting colony on MacConkey agar and are generally biochemically active, including being motile and lysine decarboxylase positive. Unlike Shigella spp., Salmonella does produce H₂S from sodium thiosulfate.

The salmonellae are grouped on the basis of heat-stable somatic O antigens and are further subdivided into serotypes based on their flagellar H antigens. Additionally blocking Vi and K antigens may be present which mask the O antigens. These blocking antigens are heat-labile, and boiling the organism will often unmask the O antigens.

E. coli and Salmonella spp. are malonate-negative; Arizona is malonate-positive and H₂S-positive.