MIcrobiology Why would we be most likely to see black precip

MIcrobiology: Why would we be most likely to see black precipitate at the bottom of the KIA tube? In other words, what about that environment increases the need for hydrogen sulfide reduction, versus the top of the slant?

Solution

Kligler\'s Iron agar is the differential medium used to distinguish between lactose and nonlactose fermenters among Enterobacteriaceae members. Phenol red indicator is used to differentiate these organisms. The lactose fermenters produce lacti acid and therefore, the slant turns red to yellow. Presence of non-lactose fermenters does not change the color of the slant. The H₂S producing organisms produce black color throughout the butt of the slant or form a ring at the top of the butt. The H₂S production at the bottom of the tube indicate the organisms are anaerobes or microaerophiles that live in absence or very low oxygen concentrations. Mainly anaerobic organisms use H₂S as source of energy production during respiration. The organisms that form a black ring at top of butt are microaerophilic or facultative aerobes.