Evolution and virulence of serogroup 6 pneumococci on a global scale - PubMed (original) (raw)

Evolution and virulence of serogroup 6 pneumococci on a global scale

D Ashley Robinson et al. J Bacteriol. 2002 Nov.

Abstract

To study the evolution and virulence of pneumococcal populations, we used multilocus sequence typing to identify the major clones among 212 carriage and invasive isolates expressing capsular serogroup 6 from 39 countries. The global population consisted of 8 major complexes and 6 minor complexes of related clones and 32 clones of diverse origin. Surprisingly, serotype 6A clones evolved by mutation nearly as often as by recombination, whereas serotype 6B clones evolved almost exclusively by recombination (P = 0.0029). This is the first report of population genetic differences among serotypes of this species. The largest clonal complex was associated with invasive disease (P = 0.019) and included a common ancestor for five previously identified drug-resistant clones. The putative ancestors of the major clonal complexes were represented by a greater proportion of carriage isolates than were their descendents (P = 0.001), and the ancestors tended to be less virulent than their descendents in a mouse model of infection. These data suggested that virulent serogroup 6 clones have evolved multiple times from less-virulent ancestral clones.

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Figures

FIG. 1.

FIG. 1.

Evolutionary relationships among clones of serogroup 6. CCs are defined as groups where each member's ST is identical at five of seven loci to at least one other ST in the group. The numbers inside the circles are the designation given to each unique ST. Sizes of the circles are proportional to the number of isolates in that ST. White circles represent serotype 6A, and black circles represent serotype 6B. Underlined numbers are STs from multiple countries. Outlined in grey are STs of drug-resistant reference clones. Length of branch indicate the number of allelic differences between STs. Short branches are SLVs, and long branches are DLVs. Dotted branches indicate ambiguity of relationship between the STs. This occurs when an ST defines a secondary ancestral ST but is not a SLV of another ancestral ST. Major CCs are named for the ST of the presumed ancestral ST of that group. Minor CCs are not named since the ancestors are uncertain. Singletons differ from other STs at three or more loci. A scale is given that indicates the number of isolates in an ST by the diameter of the circle.

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