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

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)

In serotyping, the two main surface antigens used to distinguish strains are the O and H antigens. The O antigen is the somatic polysaccharide part of the lipopolysaccharide on the outer membrane of Gram-negative bacteria, and the H antigen is the flagellar protein (flagellin) that makes up the bacterial whip. By using antisera that specifically react with these antigens, laboratories identify a strain by its O–H combination, such as O157:H7. This combination acts like a unique fingerprint that differentiates strains and helps track outbreaks. Some bacteria can switch which H antigen they express through flagellar phase variation, adding another layer of specificity. The other choices mix up what each antigen represents and are not how serotyping is defined—O is not a flagellar antigen, H is not the somatic LPS, and capsular or cytoplasmic components aren’t the basis for this O–H serotyping system.

In serotyping, the two main surface antigens used to distinguish strains are the O and H antigens. The O antigen is the somatic polysaccharide part of the lipopolysaccharide on the outer membrane of Gram-negative bacteria, and the H antigen is the flagellar protein (flagellin) that makes up the bacterial whip. By using antisera that specifically react with these antigens, laboratories identify a strain by its O–H combination, such as O157:H7. This combination acts like a unique fingerprint that differentiates strains and helps track outbreaks. Some bacteria can switch which H antigen they express through flagellar phase variation, adding another layer of specificity. The other choices mix up what each antigen represents and are not how serotyping is defined—O is not a flagellar antigen, H is not the somatic LPS, and capsular or cytoplasmic components aren’t the basis for this O–H serotyping system.