Clonal population structure of encapsulated Haemophilus influenzae.

Chromosomal genotypes of 2,209 isolates of the six polysaccharide capsule types of Haemophilus influenzae recovered from human hosts worldwide were characterized by an analysis of electrophoretically demonstrable allelic profiles at 17 metabolic enzyme loci. For 222 representative isolates, restriction fragment length polymorphism patterns produced by digestion of cap region DNA were also determined. With few exceptions, isolates belonging to individual phylogenetic lines or groups of allied lineages identified by multilocus enzyme electrophoresis had characteristic cap region restriction fragment length polymorphism patterns and characteristic combinations of cap region patterns and outer membrane protein types. The occurrence of strong associations of characters and the recovery of isolates with identical genetic properties in widely separated geographic regions and over a 40-year period indicated that the population structure of encapsulated H. influenzae is clonal. Recombination of chromosomal genes, including those mediating capsule synthesis, apparently is not a major factor in the short-term evolution of these pathogenic organisms and, therefore, may be of minor clinical significance.