Prevalence and genetic structures of Streptococcus pneumoniae serotype 6D, South Korea

Pneumococcal meningitis in Greece: A retrospective serotype surveillance study in the post-PCV13 era (2010-2020)

BACKGROUND

As Greece is a country which has introduced the 13-valent pneumococcal conjugate vaccine (PCV13) both in the infant and in the adult immunization programs, the aim of the study was to investigate age-specific and serotype-specific trends of pneumococcal meningitis over an 11-year period (2010-2020).

MATERIALS AND METHODS

Data are reported from pneumococcal meningitis cases [notified to the National Public Health Organization (NPHO)], with clinical samples and bacterial isolates sent for pneumococcal identification and serotyping at the National Meningitis Reference Laboratory (NMRL). Pneumococcal identification was performed directly on clinical samples or bacterial isolates by multiplex PCR (mPCR) assay, while serotyping was carried out by application of the Capsular Sequence Typing (CST) method with the combination of single tube PCR assays.

RESULTS

A total of 427 pneumococcal meningitis cases were notified to the NPHO between 2010 and 2020. Among those, 405 (94.8%) were microbiologically confirmed, while samples from 273 patients were sent to the NMRL for identification and/or further typing. The annual notification rate peaked at 0.47/100,000 in 2016 and since then has been decreasing. The incidence was highest in infants and in older adults. Pneumococcal serotypes were identified in 260/273 (95.2%) cases, where clinical samples were sent to the NMRL. The most prevalent serotypes (≥5%) were 3, 19A, 23B, 15B/C, 11A/D, 23A, 22F. During the study period there has been a decrease of PCV13 serotypes combined with an increase of non-PCV13 serotypes (p = 0.0045).

CONCLUSIONS

This is the first study to report serotypes for pneumococcal meningitis across all ages in the post-PCV13 era in Greece. There is a need to enhance surveillance, by close monitoring of the emerging serotypes and the impact of vaccination programs. Higher-valency PCVs may help to improve the coverage of pneumococcal disease.

Trend change of nasopharyngeal colonization with Streptococcus pneumoniae and non-typeable Haemophilus influenzae in children attending daycare centres: nationwide population-based study, South Korea 2014 and 2019

BACKGROUND

Nasopharyngeal (NP) colonization with Streptococcus pneumoniae and non-typeable Haemophilus influenzae (NTHi) is common in children, and may evolve as the source of invasive infections. In Korea, the pneumococcal conjugate vaccines (PCVs) were introduced >10 years ago, enabling the authors to study the effect of the vaccine in preventing carriage.

METHODS

NP swabs were taken and a household survey was conducted at daycare centres located in different regions of Korea in 2014 and 2019. Pneumococcal serotypes were identified using the Quellung method and sequencing. NTHi were identified based on pilA and bexA genes.

RESULTS

In total, 1460 NP swabs were obtained with pneumococcal carriage rates of 36.4-42.1% and NTHi carriage rates of 36.5-26.7%. Among children carrying pneumococci, a significant increase was seen in serotype 23A between 2014 and 2019 (from 12.6% to 22.0%; P=0.005). Children who had received PCV were at lower risk of vaccine-type carriage (2.9% vs 0.8%; P=0.005).

CONCLUSIONS

Between 2014 and 2019, the proportion of children carrying serotype 23A increased significantly, while the carriage rate of NTHi decreased. Continuous surveillance is needed to assess the long-term effects of the PCVs on carriage dynamics of pneumococcus and NTHi.

Changes in the Serotype Distribution among Antibiotic Resistant Carriage Streptococcus pneumoniae Isolates in Children after the Introduction of the Extended-Valency Pneumococcal Conjugate Vaccine

This study investigated the serotype distribution and antimicrobial resistance of 3,820 nasopharyngeal Streptococcus pneumoniae isolates from infants and children who presented with respiratory symptoms at Seoul National University Children's Hospital from July 2010 to June 2015 after the introduction of the extended-valency pneumococcal conjugate vaccines (PCVs). Serotypes and antimicrobial susceptibility were determined using the Quellung reaction and E-test, respectively. S. pneumoniae was isolated from 397 (10.4%) specimens. The most common serotypes were 19A (14.0%), 23A (12.8%), 15B/C (10.7%), 11A (10.1%), 6C (7.8%), and 6A (6.3%) among the typeable pneumococci (n = 335). The PCV serotype proportions significantly decreased (59.1% in 2010/11 to 17.0% in 2014/15, P < 0.001), whereas the non-PCV serotype proportions significantly increased (40.9% in 2010/11 to 83.0% in 2014/15, P < 0.001). The non-susceptibility rates for penicillin (oral), penicillin (parenteral, non-meningitis), cefotaxime, and erythromycin were 97.8%, 22.8%, 27.7%, and 95.5%, respectively. The proportions of PCV serotypes responsible for non-susceptibility to penicillin (parenteral, non-meningitis) and multidrug resistance significantly decreased (80.8% to 21.1%, P < 0.001 and 64.3% to 12.3%, P < 0.001, respectively), whereas the non-PCV serotype proportions significantly increased (19.2% to 78.9%, P < 0.001 and 35.7% to 87.7%, P < 0.001, respectively). Serotypes 23A and 15B/C demonstrated significant proportional increase among the antibiotics resistant strains. Thus, the PCV serotype proportions decreased and the non-PCV serotype proportions increased among nasopharyngeal carriage pneumococci after the introduction of extended-valency PCVs in Korea. Antimicrobial non-susceptibility rates for penicillin and erythromycin remain high despite the decrease in the proportion of PCV serotypes responsible for antimicrobial resistance over time.

Distribution of capsular types and drug resistance patterns of invasive pediatric Streptococcus pneumoniae isolates in Teheran, Iran

OBJECTIVES

To explore the serotype distribution and drug resistance patterns of invasive pneumococcal isolates from children under 5 years of age.

METHODS

During a 32-month period, 585 clinical samples (including blood, cerebrospinal fluid (CSF), and synovial fluid) from children suspected of having meningitis, sepsis, pneumonia, or septic arthritis were analyzed using the BACTEC culture system. Positive cultures were examined using biochemical tests and lytA amplification for the identification of pneumococcal strains. The confirmed pneumococcal isolates were examined to determine capsular types using a modified sequential multiplex PCR and susceptibility to antimicrobial agents.

RESULTS

Fifty-three pneumococcal isolates were detected in the 585 clinical samples: 21 (39.6%) blood samples and 32 (60.4%) CSF samples. The most frequent serotype was 23F (24.5%), followed by serotypes 19F (18.9%), 19A (7.5%), and 9V (7.5%). Twenty-one percent of pneumococcal isolates were penicillin-non-susceptible and serotype 19A was significantly associated with resistance to penicillin.

CONCLUSIONS

This study indicated that the 13-valent pneumococcal conjugate vaccine (PCV13) could cover the majority of the invasive pneumococcal isolates. Drug-resistant and multidrug-resistant Streptococcus pneumoniae strains are circulating in Iran. Therefore, public immunization of infants using PCV13 is recommended to reduce the incidence of pneumococcal disease and pneumococcal-resistant strains in Teheran.

Early Changes in the Serotype Distribution of Invasive Pneumococcal Isolates from Children after the Introduction of Extended-valent Pneumococcal Conjugate Vaccines in Korea, 2011-2013

This study was performed to measure early changes in the serotype distribution of pneumococci isolated from children with invasive disease during the 3-year period following the introduction of 10- and 13-valent pneumococcal conjugate vaccines (PCVs) in Korea. From January 2011 to December 2013 at 25 hospitals located throughout Korea, pneumococci were isolated among children who had invasive pneumococcal disease (IPD). Serotypes were determined using the Quellung reaction, and the change in serotype distribution was analyzed. Seventy-five cases of IPD were included. Eighty percent of patients were aged 3-59 months, and 32% had a comorbidity that increased the risk of pneumococcal infection. The most common serotypes were 19A (32.0%), 10A (8.0%), and 15C (6.7%). The PCV7 serotypes (4, 6B, 9V, 14, 18C, 19F, 23F, and 6A) accounted for 14.7% of the total isolates and the PCV13 minus PCV7 types (1, 3, 5, 7F and 19A) accounted for 32.0% of the total isolates. Serotype 19A was the only serotype in the PCV13 minus PCV7 group. The proportion of serotype 19A showed decreasing tendency from 37.5% in 2011 to 22.2% in 2013 (P = 0.309), while the proportion of non-PCV13 types showed increasing tendency from 45.8% in 2011 to 72.2% in 2013 (P = 0.108). Shortly after the introduction of extended-valent PCVs in Korea, serotype 19A continued to be the most common serotype causing IPD in children. Subsequently, the proportion of 19A decreased, and non-vaccine serotypes emerged as an important cause of IPD. The impact of extended-valent vaccines must be continuously monitored.

Clinical and microbiological characterization of serotype 6D pneumococcal infections in South Korea

BACKGROUND

The prevalence of Serotype 6D Streptococcus pneumoniae was reported relatively high in South Korea. Since the introduction of 7-valent pneumococcal conjugate vaccine (PCV7), serotype replacement was observed. This study was designed to better clarify genetic diversity of pneumococcal serotype 6D and its clinical characteristics after introduction of PCV7 in 2000.

METHODS

We performed serotyping analysis with 1298 pneumococcal isolates from clinical specimens in South Korea from 2004 to 2011. Multilocus sequence typing was performed, and minimal inhibitory concentration was determined for the available serotype 6D and nontypeable (NT) pneumococcal isolates during the 2006-2007 period.

RESULTS

The proportion of serotype 6D pneumococci increased from 0.8% (2004-2007) to 2.9% (2008-2011) of all clinical pneumococcal isolates, accounting for 14.9% of serogroup 6 pneumococci in South Korea. NT pneumococci markedly increased to 13.3% during 2006-2007 in advance of the increase in serotype 6D. Among the 26 available serotype 6D pneumococcal isolates, ST282 was predominant (23 isolates, 88.5%). The STs of NT pneumococci (26 isolates) were diverse, but clonal complex 271 was the dominant clone. The oral penicillin non-susceptibility rate was 92.3% (24 among 26 isolates) for both serotype 6D and NT pneumococci. The ceftriaxone non-susceptibility rates of serotype 6D and NT pneumococci were 7.7% and 3.8%, respectively.

CONCLUSION

ST228(6D) strain expanded, particularly among old adults with comorbidities in South Korea. Both antibiotic and PCV7 pressure might have contributed to the selective increase of NT and serotype 6D pneumococci.

Diversity of Pneumolysin and Pneumococcal Histidine Triad Protein D of Streptococcus pneumoniae Isolated from Invasive Diseases in Korean Children

Pneumolysin (Ply) and pneumococcal histidine triad protein D (PhtD) are candidate proteins for a next-generation pneumococcal vaccine. We aimed to analyze the genetic diversity and antigenic heterogeneity of Ply and PhtD for 173 pneumococci isolated from invasive diseases in Korean children. Allele was designated based on the variation of amino acid sequence. Antigenicity was predicted by the amino acid hydrophobicity of the region. There were seven and 39 allele types for the ply and phtD genes, respectively. The nucleotide sequence identity was 97.2%-99.9% for ply and 91.4%-98.0% for phtD gene. Only minor variations in hydrophobicity were noted among the antigenicity plots of Ply and PhtD. Overall, the allele types of the ply and phtD genes were remarkably homogeneous, and the antigenic diversity of the corresponding proteins was very limited. The Ply and PhtD could be useful antigens for universal pneumococcal vaccines.

Genomics Reveals the Worldwide Distribution of Multidrug-Resistant Serotype 6E Pneumococci

The pneumococcus is a leading pathogen infecting children and adults. Safe, effective vaccines exist, and they work by inducing antibodies to the polysaccharide capsule (unique for each serotype) that surrounds the cell; however, current vaccines are limited by the fact that only a few of the nearly 100 antigenically distinct serotypes are included in the formulations. Within the serotypes, serogroup 6 pneumococci are a frequent cause of serious disease and common colonizers of the nasopharynx in children. Serotype 6E was first reported in 2004 but was thought to be rare; however, we and others have detected serotype 6E among recent pneumococcal collections. Therefore, we analyzed a diverse data set of ∼1,000 serogroup 6 genomes, assessed the prevalence and distribution of serotype 6E, analyzed the genetic diversity among serogroup 6 pneumococci, and investigated whether pneumococcal conjugate vaccine-induced serotype 6A and 6B antibodies mediate the killing of serotype 6E pneumococci. We found that 43% of all genomes were of serotype 6E, and they were recovered worldwide from healthy children and patients of all ages with pneumococcal disease. Four genetic lineages, three of which were multidrug resistant, described ∼90% of the serotype 6E pneumococci. Serological assays demonstrated that vaccine-induced serotype 6B antibodies were able to elicit killing of serotype 6E pneumococci. We also revealed three major genetic clusters of serotype 6A capsular sequences, discovered a new hybrid 6C/6E serotype, and identified 44 examples of serotype switching. Therefore, while vaccines appear to offer protection against serotype 6E, genetic variants may reduce vaccine efficacy in the longer term because of the emergence of serotypes that can evade vaccine-induced immunity.

Capsular polysaccharide gene diversity of pneumococcal serotypes 6A, 6B, 6C, and 6D

This study was performed to better understand the genetic diversity and evolutionary relatedness of pneumococcal serotypes 6A, 6B, 6C, and 6D. Multi-locus sequence typing (MLST) was performed for 160 serogroup 6 isolates from clinical specimens collected from children between 1991 and 2010. We identified 38 sequence types (STs) comprising five clonal complexes with 12 singletons. Although most STs were confined to a single serotype, some STs were shared by two serotypes, and one ST was shared by three serotypes. Many STs of serotype 6A showed genetic relatedness with those of serotype 6C or 6D in eBURST analysis. Five capsular polysaccharide (cps) genes - wchA, wciO, wciP, wzy, and wzx - were analysed in 74 isolates from our clinical samples and in 36 isolates from GenBank. There were several profiles and clades in each serotype on the analysis of the concatenated sequences of the five cps genes. Small genetic distances between serotypes 6A and 6B and between serotypes 6C and 6D were observed while serotype 6B with an indel sequence formed a distinct clade. When comparing the individual cps genes between the serotypes, there was also a high level of similarity in the wchA and wciO gene sequences between serotype 6C and serotype 6D. On the other hand, serotypes 6A and 6D had the most highly similar wzy and wzx gene sequences. The wzy sequences of serotype 6C were nearly identical (99.6%) to those of serotype 6A clade II strains. In conclusion, we revealed the diversity of the genetic background and cps sequences in each pneumococcal serotype of serogroup 6. Pneumococcal serotype diversity might be attributable to complex serial mutation and recombination events.

Serotype distribution and susceptibility to penicillin and erythromycin among noninvasive or colonization isolates of Streptococcus pneumoniae in northern Japan: a cross-sectional study in the pre-PCV7 routine immunization period

Distribution of serotypes, prevalence of resistance to penicillin and/or erythromycin (EM), and its genetic traits were analyzed for a total of 1,061 noninvasive or colonization isolates of Streptococcus pneumoniae (998 and 61 isolates from children and adults, respectively) in Hokkaido, northern main island of Japan, in the year 2011, the pre-PCV7 routine immunization period. Serotype deduction was performed by sequential multiplex polymerase chain reaction (PCR), employing mutagenic PCR-restriction fragment length polymorphism for discrimination of 6A/C and 6B/D. Unaltered three PBP genes and macrolide resistance genes erm(B) and mef(A/E) were detected by multiplex PCR. Among isolates from children, 25 serotypes, including the prevalent types 6B (17.5%), 19F (15.6%), 23F (12.2%), and 6C (11.6%), were identified, revealing the PCV7 and PCV13 coverage rates as 48.2% and 60.3%, respectively, while serotype 3 was the most frequent (19.0%) among isolates from adults. Most of the pediatric isolates (96.8%) exhibited resistance to EM (minimum inhibitory concentration [MIC], ≥1 μg/ml), with a higher prevalence of erm(B) (67.2%) than mef(A/E) (39.7%). erm(B) was associated with high-level EM resistance (MIC, ≥128 μg/ml) and distributed at high detection rates to major serotypes 23F (85.2%) and 6B (85.1%), as well as minor serotypes 3, 10A, 14, 15B, 15C, 19A, and 23A (>90%). While penicillin-resistant S. pneumoniae (PRSP) (penicillin G-MIC, 2-3 μg/ml) was detected in 7.8% of isolates from children, the most common PBP gene genotype was gPRSP (three altered genes pbp1a, 2x, and 2b; 38.3%), which was detected at higher rates (>60%) in the dominant serotypes 23F, 6B, and 19F, and minor serotypes 6D and 15A. Dominant serotypes in the S. pneumoniae isolates were generally similar to those reported for invasive strains, despite lower coverage rates by PCV7/13. The importance of further surveillance on incidence and drug resistance in the post-PCV7 period was suggested for non-PCV7/13 serotypes 6C, 6D, 10A, 15A, 15B, 15C, 23A, and 35B.

Serotype distribution and antibiotic resistance of Streptococcus pneumoniae isolated from invasive infections after optional use of the 7-valent conjugate vaccine in Korea, 2006-2010

This study examined the serotype distribution and antimicrobial resistance of pneumococcal isolates from invasive infections in children between 2006 and 2010, when the 7-valent pneumococcal conjugate vaccine (PCV7) was offered as an optional vaccine in Korea. Among 140 isolates collected from 8 centers, the common serotypes were 19A (22.9%), 19 F (12.1%), and 6B (8.6%). Between 2006 and 2010, PCV7 serotypes decreased from 62.5% to 21.4% (P = 0.002), whereas three 13-valent pneumococcal conjugate vaccine (PCV13)-specific serotypes (3, 6A, and 19A) increased from 18.8% to 42.9% (P = 0.016). Among 102 multidrug-resistant isolates, the proportion of PCV7 serotypes decreased from 65.2% to 21.7% (P = 0.001), and 3 PCV13-specific serotypes increased from 17.4% to 47.8% (P = 0.008). Optional PCV7 vaccination has influenced the proportion of PCV7 serotypes in Korea, resulting in a decrease, whereas the proportions of 3 PCV13-specific serotypes, particularly 19A, have increased.

[Drastic changes in serotypes of carried pneumococci due to an increased vaccination rate in Hungary]

Introduction of the conjugate pneumococcal vaccine into the voluntary childhood vaccine program in Hungary in April 2009 resulted in a sharp increase of the vaccination rate. However, changes in serotypes as a consequence of vaccination should be considered.

AIMS

The aim of the authors was to compare pneumococci isolated from children with high-level and low-level vaccination rates.

METHODS

Nasal specimens from 854 children attending 20 nurseries at various locations in Hungary have been collected since 2009. The serotypes, antibiotic susceptibility and genetic relatedness of the isolated pneumococci were determined.

RESULTS

324 strains were isolated, and the carriage rate was 37.94%. The strains were sensitive to most antibiotics, except for macrolides. A definite suppression of vaccine types was detected during these 3 years, from the initial 78.85% to 35.30%.

CONCLUSIONS

The authors conclude that the results reflect the efficacy of the vaccine, which underlines the need for the inclusion of pneumococcal vaccine into the list of obligatory vaccines.

The 7-valent pneumococcal conjugate vaccine elicits cross-functional opsonophagocytic killing responses to Streptococcus pneumoniae serotype 6D in children

Background

We investigated the immune response to serogroup 6 with the opsonophagocytic killing assay (OPKA) in children aged 12–23 months of age after immunization with the 7-valent pneumococcal conjugate vaccine (PCV7) containing serotype 6B.

Methods

Blood samples were obtained from 59 children who had blood sampling for medical examination. Immunization status against PCV7 was confirmed by immunization records and samples were categorized according to immunization status into a booster, primary, or control group. The OPKA was performed for serotypes 6A, 6B, 6C, and 6D.

Results

Subjects with no previous PCV7 immunization history showed opsonic activity for serogroup 6 in 5-30% (according to serotype). In subjects vaccinated with a 3-dose primary series, 81% showed opsonic activity for serotypes 6B and 6D, and 29% showed opsonic activity for serotypes 6A and 6C. Among subjects vaccinated with a booster dose, all subjects had opsonic activity against serotype 6B. Subjects in the booster group with opsonic activity against serotypes 6A, 6C, and 6D were 100%, 78%, and 89%, respectively.

Conclusions

In subjects aged 12–23 months, an immune response is elicited after a primary series of immunizations with PCV7 for serotypes 6B and 6D, and a booster dose enhances a cross reactive immune response against serotypes 6A, 6C and 6D.

Epidemiology of Streptococcus pneumoniae serogroup 6 isolates from IPD in children and adults in Germany

This study presents serogroup 6 isolates from invasive pneumococcal disease (IPD) before and after the recommendation for childhood pneumococcal conjugate vaccination in Germany (July 2006). A total of 19,299 (children: 3508, adults: 15,791) isolates were serotyped. Serogroup 6 isolates accounted for 9.5% (children) and 6.7% (adults), respectively. 548 isolates had serotype 6A, 558 had serotype 6B, 285 had serotype 6C, and 4 had serotype 6D. Among children, serotype 6B was most prevalent (7.5% of isolates) before vaccination, followed by 6A and 6C. After the 7-valent pneumococcal conjugate vaccine (PCV7), the prevalence of serotype 6B significantly decreased (p = 0.040), a pattern which continued in the higher-valent PCV period (PCV10, PCV13). Serotype 6A prevalence showed a slight increase directly after the start of PCV7 vaccination, followed by a decrease which continued throughout the PCV10/13 period. Serotype 6C prevalence remained low. Serotype 6D was not found among IPD isolates from children. Among adults, prevalence of both 6A and 6B decreased, with 6B reaching statistical significance (p = 0.045) and 6A showing a small increase in 2011–2012. Serotype 6C prevalence was 1.5% or lower before vaccination, but increased post-vaccination to 3.6% in 2011/12 (p = 0.031). Four serotype 6D isolates were found post-PCV7 childhood vaccination, and two post-PCV10/13. Antibiotic resistance was found mainly in serotype 6B; serotype 6A showed lower resistance rates. Serotype 6C isolates only showed resistance among adults; serotype 6D isolates showed no resistance. Multilocus sequence typing showed that sequence type (ST) 1692 was the most prevalent serotype 6C clone. Thirty-two other STs were found among serotype 6C isolates, of which 12 have not been previously reported. The four serotype 6D isolates had ST 948, ST 2185 and two new STs: 8422 and 8442. Two serogroup 6 isolates could not be assigned to a serotype, but had STs common to serogroup 6.

Baseline epidemiology and genetic structure of Streptococcus pneumoniae serotype 6D in southern Israel prior to introduction of pneumococcal conjugate vaccines

We characterized Streptococcus pneumoniae serotype 6D from among previously identified S. pneumoniae serotype 6B strains from Jewish and Bedouin children in southern Israel during a decade before vaccination. S. pneumoniae serotype 6D isolates constituted 6.7% of the presumed S. pneumoniae serotype 6B isolates. S. pneumoniae serotype 6D strains belonged to 20 sequence types that were differentially distributed between the two ethnic groups.

Clinical implications of pneumococcal serotypes: invasive disease potential, clinical presentations, and antibiotic resistance

Streptococcus pneumoniae can asymptomatically colonize the nasopharynx and cause a diverse range of illnesses. This clinical spectrum from colonization to invasive pneumococcal disease (IPD) appears to depend on the pneumococcal capsular serotype rather than the genetic background. According to a literature review, serotypes 1, 4, 5, 7F, 8, 12F, 14, 18C, and 19A are more likely to cause IPD. Although serotypes 1 and 19A are the predominant causes of invasive pneumococcal pneumonia, serotype 14 remains one of the most common etiologic agents of non-bacteremic pneumonia in adults, even after 7-valent pneumococcal conjugate vaccine (PCV7) introduction. Serotypes 1, 3, and 19A pneumococci are likely to cause empyema and hemolytic uremic syndrome. Serotype 1 pneumococcal meningitis is prevalent in the African meningitis belt, with a high fatality rate. In contrast to the capsule type, genotype is more closely associated with antibiotic resistance. CC320/271 strains expressing serotype 19A are multidrug-resistant (MDR) and prevalent worldwide in the era of PCV7. Several clones of MDR serotype 6C pneumococci emerged, and a MDR 6D clone (ST282) has been identified in Korea. Since the pneumococcal epidemiology of capsule types varies geographically and temporally, a nationwide serosurveillance system is vital to establishing appropriate vaccination strategies for each country.

Novel pneumococcal serotypes 6C and 6D: anomaly or harbinger

Clinical use of the 7-valent pneumococcal protein conjugate (PCV7) vaccine, which includes serotypes 4, 6B, 9V, 14, 18C, 19F, and 23F, dramatically reduced invasive pneumococcal disease (IPD); however, the effectiveness was diminished due to serotype shift. Although shift due to known serotypes was anticipated, shift by misidentified serotypes was unexpected. We describe the experience with newly recognized serotypes 6C and 6D, which were mistyped as serotypes 6A and 6B, respectively. Although serotype 6D caused only occasional infections, IPD due to serotype 6C disease expanded in the PCV7 era. Subsequent studies showed that PCV7 provided cross-protection against serotype 6A but not serotype 6C. The 13-valent pneumococcal protein conjugate (PCV13) vaccine, which includes PCV7 serotypes plus serotypes 1, 3, 5, 6A, 7F, 19A, may provide protection against IPD due to serotypes 6C and 6D. Regardless, this narrative illustrates the potential impact of unrecognized serotypes on the efficacy of a serotype-specific vaccine.

Changes in serotype distribution and antibiotic resistance of nasopharyngeal isolates of Streptococcus pneumoniae from children in Korea, after optional use of the 7-valent conjugate vaccine

We investigated serotype distribution and antimicrobial resistance of pneumococcal carriage isolates from children after optional immunization with the 7-valent pneumococcal conjugate vaccine (PCV7) in Korea. From June 2009 to June 2010, 205 (16.5%) pneumococcal isolates were obtained from 1,243 nasopharyngeal aspirates of infants and children at Seoul National University Children's Hospital, Korea. Serotype was determined by Quellung reaction and antibiotic susceptibility was tested by E-test. The results were compared to previous studies done in the pre-PCV7 period. In this study, the most common serotypes were 6A (15.3%), 19A (14.7%), 19F (10.2%), 35B (7.3%), and 6D (5.6%). The proportion of PCV7 serotypes decreased from 61.9% to 23.8% (P < 0.001). The overall penicillin nonsusceptibility rate increased from 83.5% to 95.4% (P = 0.001). This study demonstrates the impact of optional PCV7 vaccination in Korea; the proportion of all PCV7 serotypes except 19F decreased while antimicrobial resistant serotypes 6A and 19A further increased.

Multidrug-resistant Streptococcus pneumoniae serotype 6D clones in South Korea

To investigate the characteristics of main Streptococcus pneumoniae clones of serotype 6D (ST282 and ST3171) in South Korea, antimicrobial susceptibility testing was performed, and 11 genes around the cps locus were sequenced on ST282(6D), ST3171(6D), and ST81(6A) isolates. The antimicrobial susceptibility patterns were very similar between clones belonging to the same clonal complex, ST81(6A) and ST282(6D); nonsusceptibilities to penicillin and cefuroxime, high MICs of ceftriaxone, and high resistance rates to trimethoprim-sulfamethoxazole. However, ST3171(6D) isolates showed resistance to only macrolides and clindamycin. The sequences of 11 genes around the cps locus indicated the same genetic backgrounds between the ST81(6A) and ST282(6D) isolates. On the other hand, ST3171(6D) isolates showed nucleotide and amino acid differences from ST81(6A) and ST282(6D) isolates in most genes, indicating a different genetic background. The mosaic structure of dexB gene in ST282(6D) isolates indicated that recombination might occur in the dexB gene. Our results suggest that the multidrug-resistant ST282(6D) pneumococcal clone has emerged by serial genetic recombination, including capsular switch.

Streptococcus pneumoniae type determination by multiplex polymerase chain reaction

The purpose of this study was to develop pneumococcal typing by multiplex PCR and compare it with conventional serotyping by quellung reaction. Pneumococcal strains used in this study included 77 isolates from clinical specimens collected from children at Seoul National University Children's Hospital from 2006 to 2010. These strains were selected as they represented 26 different serotypes previously determined by quellung reaction. Molecular type was determined by 8 sequential multiplex PCR assays. Bacterial DNA extracted from cultured colonies was used as a template for PCR, and primers used in this study were based on cps operon sequences. Types 6A, 6B, 6C, and 6D were assigned based on the presence of wciN(β) and/or wciP genes in 2 simplex PCRs and sequencing. All 77 isolates were successfully typed by multiplex PCR assays. Determined types were as follows: 1, 3, 4, 5, 6A, 6B, 6C, 6D, 7C, 7F, 9V, 10A, 11A, 12F, 13, 14, 15A, 15B/15C, 19A, 19F, 20, 22F, 23A, 23F, 34, 35B, and 37. The results according to the PCR assays were in complete concordance with those determined by conventional quellung reaction. The multiplex PCR assay is highly reliable and potentially reduces reliance upon conventional serotyping.