Check for motility by observing migration of the inoculum from the stab line through the semisolid medium. The liberation of hydrogen sulfide is indicated by a blackening of the medium. Indole production is determined after the addition of indole reagent. The medium is used primarily for differentiation of gram negative enteric bacteria. The media are poured as slants and are inoculated with a stab to the butt followed by a streak of the slant surface.
Phenol red is present as an indicator.
Do not tighten the cap on the tube. If the bacteria are nonfermenters , such as Pseudomonas, they can grow on the slant by the aerobic degradation of protein components in the medium to alkaline products. The slant and the butt will remain red. If the bacteria can ferment dextrose , but not sucrose or lactose, acid is produced in the slant and the butt and the medium turns yellow. The dextrose is used up, however, within 12 hours.