Comparison of Capsular Genes of Streptococcus pneumoniae Serotype 6A, 6B, 6C, and 6D Isolates

Abstract

Recently, Streptococcus pneumoniae serotypes 6C and 6D have been identified. It is thought that they emerged by the replacement of wciN ␤ in the capsular loci of serotypes 6A and 6B, respectively. However, their evolution has not been unveiled yet. To investigate the evolution of four serotypes of S. pneumoniae serogroup 6, four genes of the capsular polysaccharide synthesis (cps) locus, wchA, wciN, wciO, and wciP, of isolates of S. pneumoniae serotypes 6A, 6B, 6C, and 6D were sequenced. Multilocus sequence typing (MLST) was performed to investigate their genetic backgrounds. The wchA gene of serotype 6C and 6D isolates was distinct from that of serotype 6A and 6B isolates, which may suggest cotransfer of wchA with wciN ␤ . Otherwise, serotypes 6C and 6D displayed different genetic backgrounds from serotypes 6A and 6B, which was suggested by MLST analysis. In addition, serotype 6C isolates showed distinct wciP polymorphisms from other serotypes, which also indicated that serotype 6C had not recently originated from serotype 6A. Although serotype 6D shared the same amino acid polymorphisms of wciO with serotype 6B, wciP of serotype 6D differed from that of serotype 6B. The data indicate the implausibility of the scenario of a recent emergence of the cps locus of serotype 6D by genetic recombination between serotypes 6B and 6C. In addition, five serotype 6A and 6B isolates (6X group) displayed cps loci distinct from those of other isolates. The cps locus homogeneity and similar sequence types in MLST analysis suggest that most of the 6X group of isolates originated from the same ancestor and that the entire cps locus might have recently been transferred from an unknown origin. Serotype 6B isolates showed two or more cps locus subtypes, indicating a recombination-mediated mosaic structure of the cps locus of serotype 6B. The collective data favor the emergence of cps loci of serotypes 6A, 6B, 6C, and 6D by complicated recombination.Streptococcus pneumoniae is an important human pathogen responsible for several invasive and noninvasive bacterial diseases. According to the World Health Organization (WHO), S. pneumoniae is responsible for approximately 1.6 million deaths each year, with nearly 1 million involving children Ͻ5 years of age living in developing countries (16). So far, more than 90 different capsular serotypes, including newly identified serotypes 6C, 6D, and 11E, have been recognized (5, 9, 13).S. pneumoniae serogroup 6 isolates are commonly found in invasive diseases. Both serotypes 6A and 6B have a capsule composed of oligosaccharide subunits of rhamnose-ribitolgalactose-glucose (1, 10). Genetic studies have revealed the near identity of the capsular loci of serotypes 6A and 6B, with only a single nucleotide difference in the wciP gene (G instead of A at residue 584), resulting in an amino acid substitution (Ser195Asn) in the encoding sequence and different rhamnose-ribitol linkages (1 3 3 in 6A and 1 3 4 in 6B) (10). In addition, there are two other polymorphisms between the two serotypes: Ala192 and ...

Multidrug-Resistant Streptococcus pneumoniae Serotype 6D Clones in South Korea

“…Bratcher et al (5) proposed that serotype 6C has been produced by introduction of a DNA fragment spanning the capsular polysaccharide synthesis (cps) locus irrespective of serotypes 6A and serotype 6D has resulted from a recombination between the cps loci of serotypes 6B and 6C. In addition, we also showed the emergence of cps loci of serotypes 6C and 6D by complicated recombination (23).…”

“…pneumoniae serotype 6D isolates have been found in several countries and are relatively abundant in South Korea (8,23). In our study, two main clones (ST282 and ST3171) have been identified among S. pneumoniae serotype 6D isolates from Korea.…”

“…The structure of the 11 genes around cps locus evidenced the close relatedness between ST282 6A and ST81 6A . The complete sequence identity of four genes of the cps locus (wchA, wciN, wciO, and wciP) between ST282 6D and ST3171 6D isolates was described in our previous study (23). ST282…”

“…It has been postulated that serotypes 6C and 6D emerged by the replacement of wciN ␤ in the capsular loci of serotypes 6A and 6B, respectively (3,22). However, two recent papers reported that the serotypes 6C and 6D emerged just by the replacement of the wciN ␤ and wciP mutation (5,23). Bratcher et al (5) proposed that serotype 6C has been produced by introduction of a DNA fragment spanning the capsular polysaccharide synthesis (cps) locus irrespective of serotypes 6A and serotype 6D has resulted from a recombination between the cps loci of serotypes 6B and 6C.…”

Capsular Gene Sequences and Genotypes of “Serotype 6E” Streptococcus pneumoniae Isolates

“…n a previous study, we identified a distinct group, group 6X, among serogroup 6 Streptococcus pneumoniae isolates (1). These group 6X strains were hypothesized to be similar to class 2 isolates in previous reports (2,3) and capsular subtype 6B-III in a study by Elberse et al (4).…”