Alimentary Bacteriology Practice Test

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.