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.