Analysis of Moraxella catarrhalis by DNA typing: Evidence for a distinct subpopulation associated with virulence traits

Two DNA typing methods, probe-generated restriction fragment length polymor phism analysis and single-adapter amplified fragment length polymorphism an alysis, were used to study the genetic relationships among 90 Moraxella cat arrhalis strains. Both methods were found to be highly concordant, generati ng a dendrogram with 2 main branches. The division of the M. catarrhalis po pulation into 2 subspecies was supported by analysis of the 16S rRNA sequen ces. Both beta-lactamase-positive and beta-lactamase-negative strains were found in all main branches, suggesting horizontal transfer of the beta-lact amase gene. In contrast, 2 virulence traits, complement resistance and adhe rence to epithelial cells, were strongly associated with 1 of the 2 subspec ies. The branch depth suggested that complement-resistant adherent strains diverged from a common ancestor more recently than did complement-sensitive nonadherent strains. These findings suggest the existence of subpopulation s of M. catarrhalis that differ in virulence, and they may have implication s for vaccine development.