What is the appearance of colonies on Bismuth-sulfite agar?

The appearance of colonies on Bismuth-sulfite agar is characterized by black colonies with a green sheen. This agar is specifically formulated to isolate and differentiate Salmonella typhi and Salmonella paratyphi from other enteric bacteria. The black coloration arises from the reduction of bismuth sulfite, which occurs when these bacteria metabolize sulfate, leading to the precipitation of black bismuth sulfide. The green sheen that may be observed is a result of iridescence due to the specific conditions of the medium and the growth of the Salmonella species, which further aids in distinguishing them from other non-target organisms that might appear in a mixed culture. The distinctive black color and sheen serve as important visual cues for microbiologists when identifying these organisms in a laboratory setting.