Growth in a 48-hour semisolid agar stab culture at room temperature reveals lateral filamentous growth away from the stab near the top of the medium. This observation is most characteristic of which organism?

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

Answer: (D)

Motility patterns in semisolid agar at room temperature reveal how some bacteria actively move using their flagella. Listeria monocytogenes is motile under these conditions and shows an umbrella-like, lateral filamentous growth that spreads away from the inoculation line toward the surface of the medium. This surface-directed spreading is a classic signature of Listeria when observed in a 0.4%–0.5% agar stab incubated at room temperature. The other organisms listed either grow nonmotile under these conditions or do not produce this kind of surface spreading, so they don’t match the observed pattern.