Recombinational exchanges at the capsular polysaccharide biosynthetic locus lead to frequent serotype changes among natural isolates of Streptococcus pneumoniae

Multilocus sequence typing

Multilocus sequence typing (MLST) provides a new approach to molecular epidemiology that can identify and track the global spread of virulent or antibiotic-resistant isolates of bacterial pathogens using the Internet. MLST databases, together with interrogation software, are available for Neisseria meningitidis and Streptococcus pneumoniae and databases for Streptococcus pyogenes and Staphylococcus aureus will be released shortly.

Genotypic survey of recent beta-lactam-resistant pneumococcal nasopharyngeal isolates from asymptomatic children in Chile

To assess pneumococcal strain variability among young asymptomatic carriers in Chile, we used serotyping, antibiotic susceptibility testing, and genotyping to analyze 68 multidrug-resistant pneumococcal isolates recovered from 54 asymptomatic children 6 to 48 months of age. The isolates represented capsular serotypes 19F (43 isolates), 14 (14 isolates), 23F (7 isolates), 6B (3 isolates), and 6A (1 isolate). Genotypic analysis, which included pulsed-field gel electrophoresis (PFGE) of chromosomal digests, penicillin binding protein (PBP) gene fingerprinting, and dhf gene fingerprinting, revealed that the isolates represented six different genetic lineages. Clear circumstantial evidence of capsular switching was seen within each of four of the genetically related sets. The majority of the isolates, consisting of the 43 19F isolates and 2 type 6B isolates, appeared to represent a genetically highly related set distinct from previously characterized pneumococcal strains. Each of three other genetically defined lineages was closely related to one of the previously characterized clones Spain(6B)-2, France(9V)-3, or Spain(23F)-1. A fifth lineage was comprised of four type 23F isolates that, by the techniques used for this study, were genetically indistinguishable from three recent type 19F sterile-site isolates from the United States. Finally, a sixth lineage was represented by a single type 23F isolate which had a unique PFGE type and unique PBP and dhf gene fingerprints.

Population biology of Streptococcus pneumoniae isolated from oropharyngeal carriage and invasive disease

The population structure of Streptococcus pneumoniae in a sample of 134 carried antibiotic-susceptible isolates, and 53 resistant and susceptible invasive isolates, was examined using a DNA-based version of multilocus enzyme electrophoresis: multilocus restriction typing (MLRT). This involved RFLP analysis of PCR products generated from nine loci of housekeeping genes located around the pneumococcal chromosome. The combination of alleles at each of the nine loci gave an allelic profile or restriction type (RT). All carried (throat or nasopharyngeal) isolates from children or adults in Oxford and Manchester, UK, and from an HIV-seropositive cohort in Nairobi, Kenya, showed an epidemic population structure. Twelve carried clonal groups, each with different serotypes, were identified at both locations within the UK. Almost all of the carried clones examined (16/17) were found to possess identical RTs or sequence types (STs) to invasive isolates, indicating that frequently carried clones are also associated with cases of invasive disease. As expected from previous studies, the population of 53 invasive, mainly penicillin-resistant, isolates was also found to be at linkage equilibrium. Serotype switching was identified among 14% of RTs that possessed two or more members, or 5.7% of individual isolates within these RTs. In support of a population structure in which there is frequent recombination, there is also clear evidence that the trpA/B locus within pneumococci has evolved by horizontal gene transfer. A non-serotypable isolate from an HIV-seropositive patient in Kenya was clearly genetically distinct from other strains studied, with unique alleles at eight out of nine loci examined. However, it was initially identified as a pneumococcus by a 16S RNA gene probe (Gen-Probe), optochin susceptibility and the presence of pneumolysin and autolysin.

Functional analysis of glycosyltransferase genes from Lactococcus lactis and other gram-positive cocci: complementation, expression, and diversity

Sixteen exopolysaccharide (EPS)-producing Lactococcus lactis strains were analyzed for the chemical compositions of their EPSs and the locations, sequences, and organization of the eps genes involved in EPS biosynthesis. This allowed the grouping of these strains into three major groups, representatives of which were studied in detail. Previously, we have characterized the eps gene cluster of strain NIZO B40 (group I) and determined the function of three of its glycosyltransferase (GTF) genes. Fragments of the eps gene clusters of strains NIZO B35 (group II) and NIZO B891 (group III) were cloned, and these encoded the NIZO B35 priming galactosyltransferase, the NIZO B891 priming glucosyltransferase, and the NIZO B891 galactosyltransferase involved in the second step of repeating-unit synthesis. The NIZO B40 priming glucosyltransferase gene epsD was replaced with an erythromycin resistance gene, and this resulted in loss of EPS production. This epsD deletion was complemented with priming GTF genes from gram-positive organisms with known function and substrate specificity. Although no EPS production was found with priming galactosyltransferase genes from L. lactis or Streptococcus thermophilus, complementation with priming glucosyltransferase genes involved in L. lactis EPS and Streptococcus pneumoniae capsule biosynthesis could completely restore or even increase EPS production in L. lactis.

Repeated extragenic sequences in prokaryotic genomes: a proposal for the origin and dynamics of the RUP element in Streptococcus pneumoniae

A survey of all Streptococcus pneumoniae GenBank/EMBL DNA sequence entries and of the public domain sequence (representing more than 90% of the genome) of an S. pneumoniae type 4 strain allowed identification of 108 copies of a 107-bp-long highly repeated intergenic element called RUP (for repeat unit of pneumococcus). Several features of the element, revealed in this study, led to the proposal that RUP is an insertion sequence (IS)-derivative that could still be mobile. Among these features are: (1) a highly significant homology between the terminal inverted repeats (IRs) of RUPs and of IS630-Spn1, a new putative IS of S. pneumoniae; and (2) insertion at a TA dinucleotide, a characteristic target of several members of the IS630 family. Trans-mobilization of RUP is therefore proposed to be mediated by the transposase of IS630-Spn1. To account for the observation that RUPs are distributed among four subtypes which exhibit different degrees of sequence homogeneity, a scenario is invoked based on successive stages of RUP mobility and non-mobility, depending on whether an active transposase is present or absent. In the latter situation, an active transposase could be reintroduced into the species through natural transformation. Examination of sequences flanking RUP revealed a preferential association with ISs. It also provided evidence that RUPs promote sequence rearrangements, thereby contributing to genome flexibility. The possibility that RUP preferentially targets transforming DNA of foreign origin and subsequently favours disruption/rearrangement of exogenous sequences is discussed.

The three major Spanish clones of penicillin-resistant Streptococcus pneumoniae are the most common clones recovered in recent cases of meningitis in Spain

One hundred six isolates of Streptococcus pneumoniae recovered in Spain from patients with meningitis in 1997 and 1998 were characterized by multilocus sequence typing. A heterogeneous collection of genotypes was associated with meningitis in Spain: 65 different sequence types were resolved and, even at a genetic distance of 0.43, there were 37 distinct lineages. Thirty-eight percent of the isolates, including all isolates of serotypes 6B, 9V, 14, and 23F, were resistant to penicillin, and 24% of the isolates were members of the three major Spanish penicillin-resistant or multidrug-resistant clones of serotypes 6B, 9V, and 23F or serotype variants of these clones. These three clones (MICs, 1 to 2 microg of penicillin/ml) were the most common clones associated with pneumococcal meningitis in Spain during 1997 and 1998. Only two of the other clones associated with meningitis were penicillin resistant (MICs, 0.12 to 0.5 microg/ml). One of the two most prevalent penicillin-susceptible clones causing meningitis (serotype 3) has not been detected outside of Spain, whereas the other (serotype 18C) has been recovered from patients with meningitis in the United Kingdom, The Netherlands, and Denmark. The prevalence of meningitis caused by isolates of the three major Spanish penicillin-resistant or multiply antibiotic-resistant clones, which are now globally distributed, is disturbing and clearly establishes their ability to cause life-threatening disease.

Characterization of the type 8 capsular gene cluster of Streptococcus pneumoniae

The complete nucleotide sequence of the capsular gene cluster (cap8) responsible for the biosynthesis of the capsular polysaccharide of Streptococcus pneumoniae type 8 has been determined. The cap8 gene cluster, located between the genes dexB and aliA, is composed of 12 open reading frames. A 14.7-kb DNA fragment embracing the cap8 genes was sufficient to transform an unencapsulated type 3 S. pneumoniae strain to a strain with the type 8 capsule. A possible scenario for the evolution of pneumococcal types 2 and 8 is outlined.

Comparative genetics of capsular polysaccharide biosynthesis in Streptococcus pneumoniae types belonging to serogroup 19

The genetic basis for the structural diversity of capsule polysaccharide (CPS) in Streptococcus pneumoniae serogroup 19 (consisting of types 19F, 19A, 19B, and 19C) has been determined for the first time. In this study, the genetic basis for the 19A and 19C serotypes is described, and the structures of all four serogroup 19 cps loci and their flanking sequences are compared. Transformation studies show that the structural difference between the 19A and 19F CPSs is likely to be a consequence of differences between their respective polysaccharide polymerase genes (cps19aI and cps19fI). The CPS of type 19C differs from that of type 19B by the addition of glucose. We have identified a single gene difference between the two cps loci (cps19cS), which is likely to encode a glucosyl transferase. The arrangement of the genes within the cps19 loci is highly conserved, with 13 genes (cps19A to -H and cps19K to -O) common to all four serogroup 19 members. These cps genes encode functions required for the synthesis of the shared trisaccharide component of the group 19 CPS repeat unit structures. Furthermore, the genetic differences between the group 19 cps loci identified are consistent with the CPS structures of the individual serotypes. Functions have been assigned to nearly all of the cps19 gene products, based on either gene complementation or similarity to other proteins with known functions, and putative biosynthetic pathways for production of all four group 19 CPSs have been proposed.

Evidence of clonal dissemination of multidrug-resistant Streptococcus pneumoniae in Hong Kong

The relationship between the phenotypic and genotypic characteristics of 105 penicillin-intermediate or -resistant Streptococcus pneumoniae isolates saved during 1994 to 1997 at the Prince of Wales Hospital and Pamela Youde Nethersole Eastern Hospital, Hong Kong, was studied. The pbp genes for penicillin-binding proteins 1a, 2b, and 2x for each isolate were amplified by PCR, and the products were digested with restriction enzymes HinfI and AluI. A combination of the pulsed-field gel electrophoresis (PFGE) profiles, pbp fingerprints, and phenotypic characteristics of capsular types and antibiograms enabled these isolates to be divided into four major groups. Seventy-four percent (78 of 105) of the strains, belonging to serotypes 23F, 19F, and 14, showed indistinguishable pbp fingerprint patterns (group A1, 1-1-1, 1-1-1), with PFGE patterns belonging to group A and its subtypes, suggesting that these strains were closely related. Eighty-three percent (65 of 78) of these isolates were also resistant to tetracycline, erythromycin, chloramphenicol, and trimethoprim. The type 23F isolates were indistinguishable from representative strains of the Spanish 23F clone by these molecular methods, indicating that these strains may be variants of the Spanish 23F clone. Serotype 6B accounted for 19% (20 of 105) of the isolates with reduced penicillin susceptibility and was made up of variants belonging to four different pbp fingerprint groups with the PFGE pattern group B, the predominant group being indistinguishable from that of the Spanish 6B clone. Other PFGE and fingerprint groups were mainly obtained from penicillin-susceptible strains of various serotypes. The results suggest that the rapid emergence of drug-resistant S. pneumoniae in Hong Kong has been due to the rapid dissemination of several successful clones.

A single gene (tts) located outside the cap locus directs the formation of Streptococcus pneumoniae type 37 capsular polysaccharide. Type 37 pneumococci are natural, genetically binary strains

The molecular aspects of the type 37 pneumococcal capsular biosynthesis, a homopolysaccharide composed of sophorosyl units (beta-d-Glc-(1-->2)-beta-d-Glc) linked by beta-1,3 bonds, have been studied. Remarkably, the biosynthesis of the type 37 capsule is driven by a single gene (tts) located far apart from the cap locus responsible for capsular formation in all of the types characterized to date in Streptococcus pneumoniae. However, a cap37 locus virtually identical to the cap33f cluster has been found in type 37 strains, although some of its genes are inactivated by mutations. The tts gene has been sequenced and its transcription start point determined. Tts shows sequence motifs characteristic of cellulose synthases and other beta-glycosyltransferases. Insertion of the tts gene into the pneumococcal DNA causes a noticeable genome reorganization in such a way that genes normally separated by more than 350 kb in the chromosome are located together in clinical isolates of type 37. Encapsulated pneumococcal strains belonging to 10 different serotypes (or serogroups) transformed with tts synthesized type 37 polysaccharide, leading to the formation of strains that display the binary type of capsule. Type 37 pneumococcus constitutes the first case of a natural, genetically binary strain and represents a novel alternative to the mechanisms of intertype transformation.

Are horizontal transfers involved in the evolution of the Streptococcus thermophilus exopolysaccharide synthesis loci?

A 32.5kb variable locus of the Streptococcus thermophilus CNRZ368 chromosome, the eps locus, contains 25 ORF and seven insertion sequences (IS). The putative products of 17 ORF are related to proteins involved in the synthesis of polysaccharides in various bacteria. The two distal regions and a small central region of the eps locus are constant and present in all or almost all of the S. thermophilus strains tested. The other regions are variable and present in only some S. thermophilus strains tested, particularly in the closely related strains CNRZ368 and A054. A 13.6kb variable region of the eps locus of S. thermophilus CNRZ368 contains two ORF that are almost identical to epsL and orfY of the eps locus of Lactococcus lactis NIZOB40 and seven IS belonging to four different families, ISS1, IS981, IS1193 and IS1194. Five of these sequences were probably acquired by horizontal transfer from L. lactis (Bourgoin, F., et al., 1996. Gene 178, 15-23). Three probes of this 13.6kb region hybridized with the DNA of several L. lactis strains tested. A specific probe for another sequence within the S. thermophilus eps locus, epsF, hybridized with the DNA of one of the L. lactis strains tested. Sequence comparisons also suggest that five ORF of the eps locus have a mosaic structure and probably result from recombinations between sequences that are 10 to 50% divergent. The chimeric structure of the eps locus suggests a very complex evolution. This evolution probably involves both the acquisition of the 13.6kb region from L. lactis by horizontal transfer and exchanges within the S. thermophilus species.

A high incidence of prophage carriage among natural isolates of Streptococcus pneumoniae

The majority (591 of 791, or 76%) of Streptococcus pneumoniae clinical isolates examined showed the presence of two or more chromosomal SmaI fragments that hybridized with the lytA-specific DNA probe. Only one of these fragments, frequently having an approximate molecular size of 90 kb, was shown to carry the genetic determinant of the pneumococcal autolysin (N-acetylmuramic acid-L-alanine amidase). Strains carrying multiple copies of lytA homologues included both antibiotic-susceptible and -resistant isolates as well as a number of different serotypes and strains recovered from geographic sites on three continents. Mitomycin C treatment of strains carrying several lytA-hybridizing fragments caused the appearance of extrachromosomal DNA hybridizing to the lytA gene, followed by lysis of the bacteria. Such lysates contained phage particles detectable by electron microscopy. The findings suggest that the lytA-hybridizing fragments in excess of the host lytA represent components of pneumococcal bacteriophages. The high proportion of clinical isolates carrying multiple copies of lytA indicates the widespread occurrence of lysogeny, which may contribute to genetic variation in natural populations of pneumococci.

Analysis of the 5' portion of the type 19A capsule locus identifies two classes of cpsC, cpsD, and cpsE genes in Streptococcus pneumoniae

Analysis of the sequence data obtained from the 5' portion of the Streptococcus pneumoniae type 19A capsular polysaccharide biosynthesis locus (cps19a) revealed that the first seven genes are homologous to the first seven genes in the type 19F (cps19f) locus. The former genes were designated cps19aA to -G and were 70 to 90% identical to their cps19f counterparts. Southern hybridization analysis of the cps loci from various S. pneumoniae serotypes with probes specific for the cps19aC, cps19aD, and cps19aE genes indicated a hybridization pattern complementary to that previously reported for cps19fC, cps19fD, and cps19fE. That is, all serotypes tested contained high-stringency homologues of either the cps19aC to -E genes or the cps19fC to -E genes, but not both. On this basis S. pneumoniae cps loci can be divided into two distinct classes. Long-range PCR was used to amplify the cps regions between cpsB and aliA from a variety of pneumococcal serotypes. Direct sequencing of the 5' end of these PCR products, and phylogenetic analysis of the sequence data, confirmed the presence of the two distinct classes of cpsC. Whereas members within one class are greater than 95% identical to each other, the DNA sequence identity between the two classes is only approximately 70%.

The epidemiology of pneumococcal infection in children in the developing world

Pneumonia causes about three million deaths a year in young children, nearly all of which are in developing countries. Streptococcus pneumoniae (the pneumococcus) is the most important bacterial cause of pneumonia in young children and so is likely to be responsible for a high proportion of these deaths. The pneumococcus is also responsible for a substantial proportion of the 100,000-500,000 deaths that occur from meningitis in children each year. The incidence of invasive pneumococcal disease in children in the developing world is several times higher than in industrialized countries. This discrepancy may, in part, be due to socio-economic differences but genetic factors may also play a role. Children with sickle cell disease have a substantially increased risk of invasive pneumococcal infection and a search is being made for other possible genetic risk factors. Infection with human immunodeficiency virus (HIV) also predisposes to invasive pneumococcal disease and so the incidence of this disease in young children is expected to rise as increasing numbers of African and Asian children are born with a perinatally acquired HIV infection. Until recently, pneumococcal infections could be treated effectively with penicillin, a cheap and safe antibiotic. However, pneumococci that are resistant to penicillin are becoming prevalent in many countries, necessitating a change to more costly antibiotics which may be beyond the reach of the health services of poor, developing countries. The spread of antibiotic resistance has provided an added stimulus to the development of vaccines that might be able to prevent pneumococcal disease in infants. Recently developed polysaccharide-protein conjugate vaccines show promise and are now undergoing field trials. How deployment of these vaccines will influence the balance between invasive pneumococcal infections and asymptomatic nasopharyngeal carriage of pneumococci is uncertain.

A novel mechanism of phase variation of virulence in Staphylococcus epidermidis: evidence for control of the polysaccharide intercellular adhesin synthesis by alternating insertion and excision of the insertion sequence element IS256

Biofilm formation of Staphylococcus epidermidis on smooth polymer surfaces has been shown to be mediated by the ica operon. Upon activation of this operon, a polysaccharide intercellular adhesin (PIA) is synthesized that supports bacterial cell-to-cell contacts and triggers the production of thick, multilayered biofilms. Thus, the ica gene cluster represents a genetic determinant that significantly contributes to the virulence of specific Staphylococcus epidermidis strains. PIA synthesis has been reported recently to undergo a phase variation process. In this study, biofilm-forming Staphylococcus epidermidis strains and their PIA-negative phase variants were analysed genetically to investigate the molecular mechanisms of phase variation. We have characterized biofilm-negative variants by Southern hybridization with ica-specific probes, polymerase chain reaction and nucleotide sequencing. The data obtained in these analyses suggested that in approximately 30% of the variants the missing biofilm formation was due to the inactivation of either the icaA or the icaC gene by the insertion of the insertion sequence element IS256. Furthermore, it was shown that the transposition of IS256 into the ica operon is a reversible process. After repeated passages of the PIA-negative insertional mutants, the biofilm-forming phenotype could be restored. Nucleotide sequence analyses of the revertants confirmed the complete excision of IS256, including the initially duplicated 8 bp target sites. These results elucidate, for the first time, a molecular mechanism mediating phase variation in staphylcocci, and they demonstrate that a naturally occurring insertion sequence element is actively involved in the modulation of expression of a Staphylococcus virulence factor.

Identification and characterization of the cps locus of Streptococcus suis serotype 2: the capsule protects against phagocytosis and is an important virulence factor

To study the role of the capsule of Streptococcus suis serotype 2 in virulence, we generated two isogenic mutants disturbed in capsule production. For that purpose, we first cloned and characterized a major part of the capsular polysaccharide biosynthesis (cps) locus of S. suis serotype 2. Based on the established sequence, 14 open reading frames (ORFs), designated Orf2Z, Orf2Y, Orf2X, and Cps2A to Cps2K, were identified. Twelve ORFs belonged to a single transcriptional unit. The gene products of 11 of these ORFs showed similarity to proteins involved in polysaccharide biosynthesis of other gram-positive microorganisms. Nonencapsulated isogenic mutants were generated in the cps2B and cps2EF genes by insertional mutagenesis. In contrast to the wild-type S. suis serotype 2 strain, the nonencapsulated strains were highly sensitive to ingestion by porcine alveolar lung macrophages in vitro. More importantly, the nonencapsulated mutant strains were completely avirulent in young germfree pigs after intranasal inoculation. These observations indicate that the capsule of S. suis serotype 2 plays an essential role in the pathogenesis of S. suis serotype 2 infections.

The type 2 capsule locus of Streptococcus pneumoniae

The type 2 capsule locus of Streptococcus pneumoniae was characterized in Avery's strain D39, which is the parent strain of the standard transformation recipients currently used in pneumococcal research and is largely used as a virulent strain in studies on the pathogenesis of pneumococcal infections. The capsule locus was sequenced by using a 21.7-kb PCR fragment from the D39 genome as a template. Sequence data analysis showed the presence of 18 open reading frames, 17 of which have the same direction of transcription and all of which are potentially involved in capsule biosynthesis. It was also shown that R36A and R6, which are unencapsulated (rough) derivatives of D39, carry a 7,504-bp deletion involving nine capsule genes.

Vaccines to prevent respiratory infection: opportunities on the near and far horizon

Illnesses caused by respiratory pathogens result in great loss of life, suffering and commitment of resources for treatment. That the suffering and loss of life can be prevented through immunization has already been clearly shown with existing vaccines, such as those for Haemophilus influenzae type b, Streptococcus pneumoniae, and influenza. The emergence of drug-resistant pathogens is making reliance on therapy more expensive and perhaps less successful, accentuating the need to focus on prevention. Although several effective vaccines to prevent respiratory infections currently exist, they are underutilized globally. Improvements in immunogenicity, efficacy, and ease of administration, and lowering the costs of some of the existing vaccines would augment the potential for prevention worldwide. The greatest opportunities for the prevention of respiratory infections will rest with vaccines that will become available in the future.

Identification of three major clones of multiply antibiotic-resistant Streptococcus pneumoniae in Taiwanese hospitals by multilocus sequence typing

In this paper we demonstrate the advantages of a new molecular typing procedure, multilocus sequence typing, for the unambiguous characterization of penicillin-resistant pneumococci. The sequences of approximately 450-bp fragments of seven housekeeping genes were determined for 74 penicillin-resistant Taiwanese isolates of Streptococcus pneumoniae (MIC of penicillin > 0.5 microgram/ml). The combination of alleles at the seven loci defined an allelic profile for each strain, and a dendrogram, based on the pairwise mismatches in allelic profiles, grouped 86% of the isolates into one of three penicillin-resistant clones for which the MICs of penicillin were 1 to 2 microgram/ml. Isolates within each clone had identical alleles at all seven loci or differed at only a single locus, and the fingerprints of their pbp1A, pbp2B, and pbp2X genes were uniform. Isolates of the Taiwan-19F clone and the Taiwan-23F clone were resistant to penicillin, tetracycline, and erythromycin but were susceptible to chloramphenicol. A second serotype 23F clone and serotype 19F variants of this clone were resistant to penicillin, tetracycline, chloramphenicol, and, in some cases, erythromycin. Comparisons of the allelic profiles of the three major clones with those of reference isolates of the known penicillin-resistant clones showed that the Taiwan-19F and Taiwan-23F clones were previously undescribed, whereas the second serotype 23F clone was indistinguishable from the Spanish multidrug-resistant serotype 23F clone. Single isolates of the Spanish penicillin-resistant serotype 9V clone and the Spanish multidrug-resistant serotype 6B clone were also identified in the collection.

A multilocus sequence typing scheme for Streptococcus pneumoniae: identification of clones associated with serious invasive disease

The population biology of Streptococcus pneumoniae is poorly understood. Most of the important issues could be addressed by the molecular characterization of large, well sampled populations from carriage and from the different manifestations of pneumococcal disease. The authors have therefore developed a pneumococcal multilocus sequence typing scheme and database by sequencing approximately 450 bp fragments of seven housekeeping loci from 295 isolates. The combination of alleles at the seven loci provided an allelic profile, or sequence type (ST), and the relatedness between isolates was obtained by constructing a dendrogram from the matrix of pairwise differences between STs. The typing scheme was validated using pneumococci of known genetic relatedness and could resolve >6 billion STs. Among 274 isolates from recent cases of invasive pneumococcal disease in eight countries, 143 STs were resolved, but 12 STs contained at least five isolates (range 5-21 isolates). The repeated recovery of indistinguishable isolates from invasive disease in different countries implies that these STs define strains with an increased capacity to cause invasive disease. The relationship between STs and serotypes suggested that, in the longer term, capsular genes have been distributed horizontally within the pneumococcal population, but in the short term, expansion of clones occurs with only occasional changes of serotype. The multilocus sequence typing scheme provides a powerful new approach to the characterization of pneumococci, since it provides molecular typing data that are electronically portable between laboratories, and which can be used to probe aspects of the population and evolutionary biology of these organisms. A Web site for the molecular characterization of pneumococci by MLST is available (http ://mlst.zoo.ox.ac.uk).

Molecular characterization of the complete 23F capsular polysaccharide locus of Streptococcus pneumoniae

The complete DNA sequence of the capsular locus 23F of Streptococcus pneumoniae is presented. The 18.6-kb cps23f locus is composed of 18 open reading frames flanked at the 5' and 3' ends by the genes dexB and aliA, an arrangement similar to those of some of the other identified cps loci.