Capsulation in distantly related strains of Haemophilus influenzae type b: genetic drift and gene transfer at the capsulation locus

Abstract

Among natural populations of capsulate Haemophilus influenzae, clones of strains with type b capsular polysaccharide are found in each of two widely separated phylogenetic divisions. The chromosomal capsulation locus found in strains from either division has a three-segment organization, with serotype-specific DNA nested between elements common to all serotypes, but pairwise comparison of the segments between the divisions suggests that they have distinct phylogenetic histories. Genes clustered in one of the non-serotype-specific segments appear to have diverged from an ancestral element, reflected in 12% nucleotide sequence divergence in one homologous pair. In contrast, genes conferring the capacity to produce type-specific polysaccharide exhibit no such divergence, and we speculate that these have been subject more recently to horizontal transfer within the bacterial population. Clinically important capsulate gram-negative bacteria share a common organization of their capsulation loci, arguing convergence on a successful arrangement of genes. In H. influenzae this appears to have allowed the occasional exchange of serotype-specific capsulation genes between strains, a event of potential clinical importance in this major bacterial pathogen.Haemophilus influenzae is the commonest bacterial cause of meningitis and other life-threatening infections in young children in the United Kingdom and the United States (23), with a lifetime risk of invasive infection of about 1 in 400. More than 95% of these infections are caused by serotype b strains (25), and the type-specific capsular polysaccharide is a major determinant of virulence (18).A recent study of the population diversity of capsulate H. influenzae has shown that there are highly genetically divergent strains sharing serotype b (20, 21). Based on the electrophoretic mobilities of 17 metabolic enzymes, strains have been ascribed a multilocus electrotype, and cluster analysis of these electrotypes has revealed two primary phylogenetic divisions, I and II, separated by a genetic distance of 0.66, representing differences between electrotypes, on the average, at 10 of the 17 loci assayed (Fig. 1). Although more than 98% of type b strains have electrotypes in division I, there is a small cluster, recovered over many years from widely separated geographical sources, with electrotypes in division II. Notwithstanding the substantial genetic distance separating these strains from the majority of type b isolates, chemical and spectroscopic analyses of the respective capsular polysaccharides have shown them to be identical in composition and linkages (3, 4; W. Egan, personal communication). Here we report the characterization of the chromosomal capsulation locus (cap) in a type b strain from phylogenetic division II. Our results indicate that cap loci in type b strains from the two divisions are homologous but that different segments of cap have different phylogenetic histories. The results also suggest that the horizontal transfer of type-specific capsulation genes has ...

Pharyngeal Colonization Dynamics of Haemophilus influenzae and Haemophilus haemolyticus in Healthy Adult Carriers

“…influenzae is a naturally competent organism and possesses approximately 1,400 copies (54) of the 9-bp uptake signal sequence that allows genetic exchange by transformation and homologous recombination. Intraspecies (28,29) and interspecies (30) gene transfer has been demonstrated in H. influenzae. High rates of recombination have also been demonstrated in otitis media isolates of NTHI (5), and hypermutability mutants have been associated with high genetic variability among H. influenzae isolates from the airways of patients with cystic fibrosis (49).…”

Genetic variation of dTDP-l-rhamnose pathway genes in Salmonella enterica The GenBank accession numbers for the sequences reported in this paper are AF279615–AF279625 for the rml gene sets and AF279626–AF279648 for the rmlB gene fragments.

“…This pattern has been observed in the gene clusters of several structurally related O antigens in S. enterica (Brown et al, 1992 ;Jiang et al, 1991 ;Liu et al, 1991 ;Wang et al, 1992) and CPS (Coffey et al, 1998 ;Frosch et al, 1989 ;Kroll & Moxon, 1990 ;Kroll et al, 1989) as well as E. coli group II K antigens (Roberts, 1996). It has been hypothesized, based on these studies, that the outer conserved genes play a role in mediating the exchange of central serogroup-specific genes.…”

Characterization of Encapsulated and Noncapsulated Haemophilus influenzae and Determination of Phylogenetic Relationships by Multilocus Sequence Typing

“…The strong association between capsular serotype and genotype observed in studies using both MLST and MLEE also indicates that capsular genes have very rarely become established in new lineages as a result of horizontal transfer and recombination at the capsular biosynthetic locus (24,(26)(27)(28). The presence of the type a and b capsular genes in the distantly related lineages of group I and group II indicates that these genes were distributed horizontally at some time in the past (21,28), but this process appears to be rarer than in S. pneumoniae and N. meningitidis, where isolates with very different genotypes frequently have the same capsular serotype (serogroup), and variation in capsular serotype (serogroup) is found even within isolates of the same clone (4,6).…”