Analysis of the prevaccine population of noncapsulate Haemophilus influenzae and identification of a putative epidemic clone

For six months prior to the introduction of Haemophilus influenzae serotype b vaccines, all noncapsulate Haemophilus influenzae received by our laboratory were characterised by biotyping, antibiogram, outer-membrane protein profiling, and ribotyping, Simpson's index of diversity (SID) showed the population was heterogeneous with multiple clones, The study identified a clone within noncapsulate Haemophilus influenzae biotype II that caused more disease than other strains. This clone was shown to have previously caused two outbreaks of respiratory disease and to possess a small extrachromosomal plasmid encoding ampicillin resistance. The study shows that describing the diversity within a bacterial population with SID may negate the need for retrospective subtyping comparisons.