A test tube containing motility test medium that was needle inoculated with the motile bacterium Proteus mirabilis, then incubated at 35°C for 26 hours. This medium is semisolid, allowing bacteria to move in it. The addition of a tetrazolium salt allows tracking of bacterial movement through the medium, as bacterial metabolism reduces the colorless soluble tetrazolium salt to its insoluble, dark red form. The fuzzy, indistinct red line of bacterial growth shows that the motile P. mirabilis has moved from its initial inoculation along a single stab line outward in all directions. Motility testing is very useful in the differentiation and identification of bacteria such as the enterics and gram-positive rods. (Labeled view) (Janelle Hare, Morehead State University, Morehead, KY)