Which organism produces pale pink colonies on culture media and is associated with disease in livestock?

• A. Arcanobacterium hemolyticum
• B. Actinomyces naeslundii
• C. Gardnerella vaginalis
• D. Rhodococcus equi

Answer: (D)

Pale pink to salmon-colored colonies on routine culture media point to Rhodococcus equi, a gram-positive bacterium that pigment-producing colonies. This organism is a well-known pathogen of livestock, especially young animals like foals, where it causes pneumonia and septicemia; it can also affect pigs and cattle. The distinctive pink pigment comes from carotenoids it produces, helping differentiate it from many other gram-positive rods that form white or gray colonies. In a veterinary context, seeing pink pigmented colonies from clinical or environmental samples, particularly from sick young livestock, strongly suggests Rhodococcus equi. The other organisms listed do not characteristically produce pink pigmented colonies and are associated with different hosts or sites of infection, so they don’t fit the combination of pigment and livestock disease as well.