Clonal diversity in Haemophilus pleuropneumoniae.

Genetic diversity among 135 isolates of nine serotypes of Haemophilus pleuropneumoniae recovered from pigs with pleuropneumonia or other invasive diseases in 14 countries was estimated by multilocus enzyme electrophoresis, which detects allelic variation in structural genes. Thirty-two multilocus genotypes (electrophoretic types [ETs]) were distinguished on the basis of allele profiles at 15 enzyme loci, and 36 distinctive combinations of ET and serotype were identified. The recovery of isolates with identical properties in widely separated geographic regions and over a 20-year period indicated that the population structure of H. pleuropneumoniae is clonal. Isolates of the same ET generally shared the same serotype and electrophoretic pattern of the outer membrane proteins, but some ETs were represented by isolates of several different serotypes, outer membrane protein patterns, or both. On average, the genetic diversity among ETs of the same serotype was 56% of the total genetic diversity in the species. Isolates of serotype 1, which are unusually pathogenic, belong to a distinctive group of clones that are closely related to clones marked by serotype 9.