Colonies that resemble droplets of mercury are characteristic of which organism?

• A. Bordetella pertussis
• B. Burkholderia cepacia
• C. Campylobacter jejuni
• D. Yersinia pestis

Answer: (A)

Colony appearance on specialized media is a key clue in identifying Bordetella pertussis. On Bordet-Gengou or Regan-Lowe media, this organism forms very small, smooth, shiny colonies that resemble droplets of mercury—a description that practitioners memorize because it stands out from other bacteria. The mercury-drop look comes from the way B. pertussis grows slowly and produces a highly glossy, pearl-like colony, which helps distinguish it from other respiratory pathogens cultured on the same plates.

Other organisms listed do not produce this mercury-like appearance. Burkholderia cepacia tends to form moist colonies that may appear metallic or wrinkled on selective media; Campylobacter jejuni forms tiny, translucent, often curved colonies that reflect its microaerophilic, fastidious nature; Yersinia pestis can produce gray-white, convex colonies and has other distinctive growth patterns on specific media. So the mercury-drop colony description is most characteristic of Bordetella pertussis.