Prevalence and Genetic Structures of Streptococcus pneumoniae Serotype 6D, South Korea

DHHS/CDC Emerging Infectious Diseases
Volume 16, Number 11–November 2010

Eun Hwa Choi, Hoan Jong Lee, Comments to Author Eun Young Cho, Chi Eun Oh, Byung Wook Eun, Jina Lee, and Min Ja Kim
Author affiliations: Seoul National University Children's Hospital, Seoul, South Korea (E.H. Choi, H.J. Lee, E.Y. Cho, C.E. Oh, B.W. Eun); Seoul National University College of Medicine, Seoul (E.H. Choi, H.J. Lee); Kosin University College of Medicine, Busan, South Korea (C.E. Oh); Gachon University Gil Hospital, Incheon, South Korea (B.W. Eun); Seoul National University Bundang Hospital, Seongnam, South Korea (J. Lee); and Korea University College of Medicine, Seoul (M.J. Kim)

Abstract
To determine prevalence and genetic structures of new serotype 6D strains of pneumococci, we examined isolates from diverse clinical specimens in South Korea during 1991–2008. Fourteen serotype 6D strains accounted for 10.4% of serogroup 6 pneumococci from blood, sputum, nasopharynx, and throat samples. Serotype 6D strains consisted of 3 sequence types.
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After the discovery and characterization of 6C through genetic and biochemical studies, a new experimental serotype, 6X1 (later named 6D), was created by mutating the critical nucleotide in the wciP gene of the 6C capsule gene locus or by inserting the wciN_β gene into the 6B capsule gene locus (3). However, this putative serotype, 6D was thought to not exist in nature until recently, when 2 studies found 6D strains in nasopharyngeal aspirates from children in Fiji during 2004–2007 (5) and in 2 nasopharyngeal aspirates from children in South Korea in 2008 (6). Although serotype 6C has only recently been described, several studies indicate that serotype 6C pneumococci have been circulating in many countries, including the United States, the Netherlands, Australia, Israel, and South Africa (7–10). However, reports of naturally occurring serotype 6D pneumococci are limited.

We investigated the prevalence of serotypes 6C and 6D in 2 collections of pneumococci isolated from clinical specimens in South Korea. We compared the genetic diversity and antimicrobial drug susceptibility patterns of the 4 serotypes, 6A, 6B, 6C, and 6D.

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Conclusions

We identified 14 naturally occurring serotype 6D strains among 134 serogroup 6 pneumococci collected from diverse clinical specimens in South Korea during 1991–2008. The prevalence rate of serotype 6D among serogroup 6 isolates was 10.4%, slightly higher than that of serotype 6C (4.5%). Although serotype 6D was only recently discovered, we demonstrated that serotype 6D strains have been circulating since at least 1996. Serotype 6D was identified from various clinical sources, including blood, sputum, throat swab, and nasopharynx specimens, contrasting with findings of 2 previous studies (5,6).

The genetic structures of serotype 6D pneumococci in the MLST database (www.mlst.net) were single isolates of ST4241 (Australia); ST982, ST4190, ST5085, and ST5086 (China); and 2 isolates of ST282 (South Korea). Of those, 3 strains from China (ST982, ST5085, and ST5086) were closely related to the ST3171 strain from South Korea. This cluster of serotype 6D strains was associated with serotype 6A and 6B isolates from 3 countries in Asia. A single isolate of ST4241 was related to STs associated mostly with serotype 6B, but the ST4170 strain did not seem to be linked to other STs. This study demonstrated that 7 serotype 6D strains of ST189 and 3 serotype 6D strains of ST282 were related to clonal complex 81, which had previously been associated with only serotype 6A isolated from South Korea. However, this clonal complex also included several STs associated with many other global serotypes, such as 23F, 19F, and 19A. Although the mechanism is not completely clear, available data indicate that capsular switching from serotypes 6A, 23F, 19F, or 19A to serotype 6D is possible; this switching could occur in addition to replacement of the wciN_β gene into the 6B capsule gene locus. A previous study indicated capsular switching as the possible event for formation of serotype 6C isolates (14).

In a recent study, factor 6d antiserum was validated for accurate serotyping of 6C (10) and is now commercially available, but antiserum for detection of 6D has not yet been developed. Further studies will be required to investigate the prevalence and genetic relatedness of serotype 6D pneumococci in different countries and to evaluate the effect of pneumococcal conjugate vaccine on serotype distribution.