A purple slant and blackened butt in lysine iron agar (LIA) indicate which bacterium?

In lysine iron agar (LIA), a purple slant and blackened butt indicate specific metabolic activities of the bacteria being tested. The purple slant results from the alkaline reaction due to the decarboxylation of lysine, which is a characteristic of certain enteric bacteria, including some Salmonella species. The blackened butt occurs due to the production of hydrogen sulfide (H2S), which reacts with iron salts in the medium to form ferrous sulfide, thus causing the black coloration.

Salmonella spp. are known for their ability to produce H2S and can decarboxylate lysine, leading to the observed reactions in LIA. Therefore, the combination of a purple slant and a black butt on LIA is indicative of Salmonella spp. This contrast can help distinguish them from other bacteria that might exhibit different reactions in this medium.