Listeria monocytogenes on blood agar.
Cultivation 48 hours, aerobic atmosphere, 37°C.

Colonies on blood agar are whitish gray, convex and can be confused with streptococci, enterococci or diphtheroids (particularly after 24 hours of cultivation when beta-hemolysis is quite weak). It often is overgrown on nonselective media by both Gram-positive and Gram-negative species.