Bacteriology
Changes of serotype and genotype in Streptococcus pneumoniae isolates from a Korean hospital in 2007

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Abstract

We investigated the change in clones and serotypes of Streptococcus pneumoniae isolates in a Korean tertiary-care hospital. Serotypes of S. pneumoniae isolates were determined by the capsular quellung method, and in vitro susceptibility testing was performed by broth microdilution method. Multilocus sequence typing was performed to determine the genotypes of the S. pneumoniae isolates. The erm(B) and mef(A) genes in erythromycin-resistant isolates were also detected using the duplex polymerase chain reaction method. During the 2 periods assayed (1998–2000 and 2007), 7-valent pneumococcal conjugate vaccine (PCV7) serotypes decreased significantly from 58.3% to 30.9% (P = 0.001). Especially, serotypes 19F and 23F decreased significantly from 31.7% to 8.5% (P < 0.0001) and from 20.0% to 7.4% (P = 0.021), respectively. In contrast to the other PCV7 serotypes, serotype 14 coupled with CC554 emerged in 2007, which may indicate no effect of PCV7 against serotype 14 isolates from Korea and the possibility of a different subtype. Of the non- PCV7 serotypes, serotype 19A increased from 8.3% to 14.9% (P = 0.227) and serotype 15 increased from 0% to 8.5% (P = 0.023). The increase of serotype 19A was due to the expansion of a preexisting clone with serotype 19A, ST320. However, S. pneumoniae isolates of serotype 15 showed diverse STs. Our data may provide helpful information in local vaccine serotype expansion or replacement in Korea.

Introduction

Streptococcus pneumoniae continues to be one of the most important bacterial pathogens, causing invasive infections worldwide especially in infants and children. High antimicrobial resistance rates in S. pneumoniae isolates, which may be due to the selection pressure by considerable use of antibiotics and the clonal dissemination of multidrug-resistant strains, are of great concern (Adam, 2002). As a preventive strategy with a significant public health impact, 7-valent pneumococcal conjugate vaccine (PCV7) against serotypes 4, 6B, 9V, 14, 18C, 19F, and 23F has recently been introduced in the United States and several other countries.

Since the introduction of PCV7 in the United States in 2000, a significant decrease in invasive pneumococcal disease (IPD) among children has been reported (Centers for Disease Control and Prevention, 2005, Whitney et al., 2003). Instead, the incidence of IPD by nonvaccine serotypes increased in the United States and other countries since the introduction of PCV7 (Hicks et al., 2007, Muñoz-Almagro et al., 2008). Especially, the increase of serotype 19A is the main cause of this phenomenon (Hicks et al., 2009; Pai et al., 2005). Although serotype 19A belongs to the same serogroup with serotype 19F, a member of PCV7, PCV7 is not effective against IPD by serotype 19A (Whitney et al., 2006). Several hypotheses have been suggested for the increase in serotype 19A since the introduction of PCV7. First, a preexisting clone of serotype 19A might have expanded (Muñoz-Almagro et al., 2008). Second, capsular switching by recombination of serotype 19A might have occurred in successful clones associated with PCV7 serotypes (Brueggemann et al., 2007, Moore et al., 2008). Third, new clone(s) of serotype 19A may have been introduced into the population (Klugman, 2002).

Korea is one of the hot spots with regard to antimicrobial resistance of S. pneumoniae (Song et al., 2004). It has been reported that penicillin nonsusceptibility was more than 60% (in former criteria, that is, MIC ≥0.12 mg/L) and erythromycin resistance was more than 80% in Korea (Song et al., 2004). In Korea, CC81 (Spain23F-1) and CC271 (Taiwan19F-14) were known to be major multidrug-resistant pneumococcal clonal complexes (Ko and Song, 2004, Song et al., 2004).

PCV7 was introduced in November 2003 in Korea, although PCV7 was not introduced as a routine vaccination program. Although informally, some reports noted that 78% of newly born children in Korea have been vaccinated through the private market (http://www.equitybulls.com/admin/news2006/news_det.asp?id=22998) in 2006. To our knowledge, only limited reports have compared the serotype distribution change since the introduction of PCV7 in Korea (Choi et al., 2008). In that study, serotype 19A also increased in a Korean tertiary-care hospital even before the use of PCV7, unlike in the United States. They demonstrated that multidrug-resistant ST320 isolates were responsible for the expansion of serotype 19A (Choi et al., 2008). However, more investigations are required to evaluate the effect of pneumococcal vaccination and to draw an effective vaccination strategy in Korea.

In this article, we report the changes of pneumococcal serotype, clone distribution, and antimicrobial resistance between 1998 to 2000 and 2007 in Korea.

Section snippets

Pneumococcal isolates

A total of 154 S. pneumoniae isolates from a tertiary-care hospital in Korea (Samsung Medical Center, Seoul, South Korea) were investigated. A total of 60 and 94 nonduplicate isolates were collected in 1998 to 2000 and 2007, respectively. More than half of the isolates were from sputum (83 isolates), followed by blood (23 isolates), tracheal aspirate (16 isolates), nasal swab (11 isolates), and pus (4 isolates). The other isolates were from broncho alveolar lavage (BAL) fluid, bile, bronchial

Serotypes

Among the 60 S. pneumoniae isolates obtained in 1998 to 2000, 58.3% showed PCV7 serotypes. These serotypes decreased to 30.9% in 2007 (P = 0.001) (Table 1). Serotype 19F decreased from 31.7% to 8.5%, and serotype 23F decreased significantly from 20.0% to 7.4% (P < 0.0001 and P = 0.021, respectively). In contrast, serotype 14 was found in 6 isolates in 2007, whereas no isolates with serotype 14 were identified in 1998 to 2000 (P = 0.082). Accompanying the decrease in pneumococcal isolates with

Discussion

Most reports from the United States and other countries have reported a decrease of IPD due to S. pneumoniae but a relative increase of non-PCV7 serotypes, especially 19A, after the introduction of PCV7 (Hanage et al., 2007, Hicks et al., 2007, Muñoz-Almagro et al., 2008, Pai et al., 2005, Pichichero and Casey, 2007). In Korea, a recent study reported an increase of serotype 19A, even before the introduction of PCV7 (Choi et al., 2008). In this study, we found an increase of serotype 19A in

Acknowledgments

This study was supported in part by the Samsung Biomedical Research Institute (CA-72172 and BA-71022), Seoul, South Korea, and by the Asian-Pacific Research Foundation for Infectious Diseases, Seoul, South Korea.

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    J.H. Song and J.Y. Baek contributed equally to this work as the 1st authors.

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