The K antigen of the Enterobacteriaceae is which of the following?

• A. The somatic antigen
• B. Located on the flagellum
• C. Heat labile
• D. The antigen used to group Shigella

Answer: (C)

The K antigen is the capsule antigen of Enterobacteriaceae—the capsule is a layer of polysaccharide outside the cell wall that helps bacteria resist phagocytosis. In serologic testing, capsule antigens are described as heat-labile, meaning heating can disrupt or destroy the capsule and its antigenic properties. This distinguishes K antigens from the somatic O antigen (located in the cell wall lipopolysaccharide, which is heat-stable) and from the H antigen (the flagellar protein, which is also heat-labile but corresponds to the flagellum rather than the capsule). Shigella grouping relies mainly on O antigens, not K. So the K antigen being heat-labile correctly identifies it as the capsule antigen that loses its reactivity when heated.