CAMLPR Microbiology Practice Test

This pattern is characteristic of Listeria monocytogenes. It is a small Gram-positive, non-spore-forming bacillus that is catalase positive, which helps distinguish it from many Gram-positive rods that are catalase negative. At room temperature, Listeria is motile and exhibits a distinctive tumbling motion in a wet mount; on semi-solid agar it spreads with an umbrella-like pattern due to radial motility. It is also beta-hemolytic on blood agar, producing a clear zone around colonies. The combination of Gram-positive staining, catalase positivity, tumbling motility in liquid, umbrella motility on agar, and beta-hemolysis is the classic profile for this organism and helps separate it from other similar-looking bacteria, such as Nocardia (which is filamentous and weakly acid-fast), Corynebacterium/Propionibacterium (which are often non-motile or differ in hemolysis and appearance), and Haemophilus influenzae (which is Gram-negative). This motility behavior and hemolysis pattern reflect its unique biology and pathogenic potential, including intracellular lifestyle and foodborne transmission.