This is a differential medium. It tests for organisms’ abilities to ferment glucose and lactose to acid and acid plus gas end products. It also allows for identification of sulfur reducers. This media is commonly used to separate lactose fermenting members of the family Enterobacteriaceae (e.g. Escherichia coli) from members that do not ferment lactose, like Shigella dysenteriae. These lactose nonfermenting enterics generally tend to be the more serious pathogens of the the gastrointestinal tract.
The first differential ingredient, glucose, is in very short supply. Organisms capable of fermenting this sugar will use it up within the first few hours of incubation. Glucose fermentation will create acidic byproducts that will turn the phenol red indicator in the media yelllow. Thus, after the first few hours of incubation, the tube will be entirely yellow. At this point, when the glucose has been all used up, the organism must choose another food source. If the organism can ferment lactose, this is the sugar it will choose. Lactose fermentation will continue to produce acidic byproducts and the media will remain yellow (picture on the far left below). If gas is produced as a result of glucose or lactose fermentation, then fissures will appear in the agar or the agar will be lifted off the bottom of the tube.
If an organism cannot use lactose as a food source it will be forced to use the amino acids / proteins in the media. The deamination of the amino acids creates NH3, a weak base, which causes the medium to become alkaline. The alkaline pH causes the phenol red indicator to begin to turn red. Since the incubation time is short (18-24 h), only the slant has a chance to turn red and not the entire tube. Thus an organism that can ferment glucose but not lactose, will produce a red slant and a yellow butt in a KIA tube (second from the left below). These organisms are the more serious pathogens of the GIT such as Shigella dysenteriae.
If an organism is capable of using neither glucose nor lactose, the organism will use solely amino acids / proteins. The slant of the tube will be red and the color of the butt will remain unchanged (picture on the far right below). Pseudomonas aeruginosa is an example of a nonfermenter.
KIA tubes are also capable of detecting the production of H2S. It is seen as a black precipitate (second picture from the right). Sometimes the black precipitate obscures the butt of the tube. In such cases, the organisms should be considered positive for glucose fermentation (yellow butt). Proteus mirabilis (pictured here, second from right) is a glucose positive, lactose negative, sulfur reducing enteric.
