In bacterial serotyping, what are the O and H antigens and how are they used to identify strains?

• A. O antigen is the somatic LPS antigen; H antigen is the flagellar antigen; combinations (e.g., O157:H7) identify specific strains.
• B. O antigen is the flagellar antigen; H antigen is the somatic LPS antigen; combinations (e.g., O157:H7) identify specific strains.
• C. O antigen and H antigen are both capsular antigens; combinations identify species.
• D. O antigen is the cytoplasmic antigen; H antigen is an outer membrane protein; combinations identify plasmids.

Answer: (A)

The main idea being tested is how serotyping uses surface antigens to identify bacterial strains. The O antigen is the somatic component, the O-polysaccharide part of the outer membrane lipopolysaccharide. The H antigen is the flagellar component, the flagellin protein that makes up the tail-like structure. By typing bacteria for both of these surface components, you get a dual signature—an O type and an H type (for example, O157:H7)—that identifies a specific strain. This combination is useful because many strains share an O or an H type on their own, but the exact pairing defines a distinct strain, aiding epidemiology and outbreak tracking. Some organisms can be non-motile and lack an H antigen, but the standard approach relies on the O type in combination with the H type when possible.