In a 48-hour semisolid agar stab culture incubated at room temperature, lateral filamentous growth away from the stab near the top is most characteristic of which organism?

• A. Rhodococcus sp.
• B. Corynebacterium urealyticum
• C. Enterococcus faecalis
• D. Listeria monocytogenes

Answer: (D)

Room-temperature motility in a semisolid agar stab is being assessed. Listeria monocytogenes is motile at room temperature because its flagella are expressed under these conditions, causing the bacteria to swim away from the inoculation line. This yields a distinctive lateral filamentous spread radiating away from the stab and expanding toward the surface, where oxygen and nutrients are readily available. The growth pattern seen here reflects active swimming motility near the agar surface, producing filaments that extend outward rather than staying confined to the stab line. The other organisms listed are not motile under these conditions, so they would not produce this radial, surface-oriented spreading pattern.