Haemophilus that meet the species requirements for H. influenzae may or may not have acapsule. Those that do are divided into six serotypes, designated a to f, based on the cap-sular polysaccharide antigen. The type b capsule is made up of a polymer of ribose, ribitol, and phosphate, called polyribitol phosphate (PRP). These surface polysaccha-rides are strongly associated with virulence, particularly H. influenzae type b (Hib). The nonencapsulated, and thus nontypable, H. influenzae can be classified by a number of typ-ing schemes based on outer membrane proteins and other factors. These protein systems can also be applied to capsulated H. influenzae but have no particular association with virulence.
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