Multilocus sequence typing and antigen gene sequencing in the investigation of a meningococcal disease outbreak

Pyrosequencing: sequence typing at the speed of light

Nucleotide sequencing is an established method for gaining information relating to partial gene, whole gene, or whole genome sequence. Here we describe some of the background leading to the advent of modern nucleotide sequencing and how it has led to the development of Pyrosequencing, a relatively new method for real-time nucleotide sequencing. In particular, we describe how this method can be used for typing bacterial pathogens.

Genetic diversity and carriage dynamics of Neisseria lactamica in infants

Neisseria lactamica, a harmless human commensal found predominantly in the upper respiratory tracts of infants, is closely related to Neisseria meningitidis, a pathogen of global significance. Colonization with N. lactamica may be responsible for the increase in immunity to meningococcal disease that occurs during childhood, when rates of meningococcal carriage are low. This observation has led to the suggestion that N. lactamica whole cells or components are potential constituents of novel meningococcal vaccines. However, the dynamics of carriage and population diversity of N. lactamica in children are poorly understood, presenting difficulties for the choice of representative isolates for use in vaccine development. This problem was addressed by the multilocus sequence typing of N. lactamica isolates from two longitudinal studies of bacterial carriage in infants. The studies comprised 100 and 216 subjects, with N. lactamica carriage monitored from age 4 weeks until age 96 weeks and from age 2 weeks until age 24 weeks, respectively. The maximum observed carriage rate was 44% at 56 weeks of age, with isolates obtained on multiple visits for the majority (54 of 75, 72%) of carriers. The N. lactamica isolates were genetically diverse, with 69 distinct genotypes recovered from the 75 infants. Carriage was generally long-lived, with an average rate of loss of under 1% per week during the 28 weeks following acquisition. Only 11 of the 75 infants carried more than one genotypically unique isolate during the course of the study. Some participants shared identical isolates with siblings, but none shared identical isolates with their parents. These findings have implications for the design of vaccines based on this organism.

Multilocus sequence typing reveals thatBacillus cereusstrains isolated from clinical infections have distinct phylogenetic origins

Eight strains of Bacillus cereus isolated from bacteremia and soft tissue infections were assigned to seven sequence types (STs) by multilocus sequence typing (MLST). Two strains from different locations had identical STs. The concatenated sequences of the seven STs were aligned with 65 concatenated sequences from reference STs and a neighbor-joining tree was constructed. Two strains were distantly related to all reference STs. Three strains were recovered in a clade that included Bacillus anthracis, B. cereus and rare Bacillus thuringiensis strains while the other three strains were assigned to two STs that were more closely affiliated to most of the B. thuringiensis STs. We conclude that invasive B. cereus strains do not form a single clone or clonal complex of highly virulent strains.

Distribution of serogroups and genotypes among disease-associated and carried isolates of Neisseria meningitidis from the Czech Republic, Greece, and Norway

The distribution of serogroups and multilocus sequence types (STs) in collections of disease-associated and carried meningococci from the period 1991 to 2000 in three European countries (the Czech Republic, Greece, and Norway) was investigated. A total of 314 patient isolates and 353 isolates from asymptomatic carriers were characterized. The frequency distributions of serogroups and clone complexes differed among countries and between disease and carrier isolate collections. Highly significant differentiation was seen at each housekeeping locus. A marked positive association of serogroup C with disease was evidenced. The ST-11 complex was strongly positively associated with disease; associations for other clone complexes were weaker. The genetic diversity of the clone complexes differed. A single ST dominated the ST-11 clone complex, while the ST-41/44 complex exhibited greater levels of diversity. These data robustly demonstrated differences in the distribution of meningococcal genotypes in disease and carrier isolates and among countries. Further, they indicated that differences in genotype diversity and pathogenicity exist between meningococcal clone complexes.

The influence of mutation, recombination, population history, and selection on patterns of genetic diversity in Neisseria meningitidis

Patterns of genetic diversity within populations of human pathogens, shaped by the ecology of host-microbe interactions, contain important information about the epidemiological history of infectious disease. Exploiting this information, however, requires a systematic approach that distinguishes the genetic signal generated by epidemiological processes from the effects of other forces, such as recombination, mutation, and population history. Here, a variety of quantitative techniques were employed to investigate multilocus sequence information from isolate collections of Neisseria meningitidis, a major cause of meningitis and septicemia world wide. This allowed quantitative evaluation of alternative explanations for the observed population structure. A coalescent-based approach was employed to estimate the rate of mutation, the rate of recombination, and the size distribution of recombination fragments from samples from disease-associated and carried meningococci obtained in the Czech Republic in 1993 and a global collection of disease-associated isolates collected globally from 1937 to 1996. The parameter estimates were used to reject a model in which genetic structure arose by chance in small populations, and analysis of molecular variation showed that geographically restricted gene flow was unlikely to be the cause of the genetic structure. The genetic differentiation between disease and carriage isolate collections indicated that, whereas certain genotypes were overrepresented among the disease-isolate collections (the "hyperinvasive" lineages), disease-associated and carried meningococci exhibited remarkably little differentiation at the level of individual nucleotide polymorphisms. In combination, these results indicated the repeated action of natural selection on meningococcal populations, possibly arising from the coevolutionary dynamic of host-pathogen interactions.

Multilocus sequence typing--what is resolved?

Nucleotide sequence-based methods for bacterial typing (multilocus sequence typing; MLST) allow rapid and global comparisons between results from different laboratories. Combining this advantage with the reduced cost of high throughput sequencing, increasing automation and the amenability of sequence data for evolutionary analysis, it seems inevitable that sequence-based typing will eventually predominate over gel-based methods such as pulsed-field gel electrophoresis (PFGE) for most bacterial species. The increasing availability of multiple genome sequences for single pathogenic species, and the recent development of many new MLST schemes, means that a re-examination of the utility of multilocus sequencing, and in particular the choice of gene loci, is now appropriate.

Identification of Neisseria meningitidis sequence type 66 in Poland

Investigation of two cases of invasive meningococcal disease within a single family revealed the presence of isolates of Neisseria meningitidis phenotype C:2b:P1.2,P1.5 belonging to sequence type (ST) 66. The ST66 clone is a single-locus variant of the widely distributed ST8 complex, which has been observed previously in Spain, Belgium, Australia and New Zealand. This hypervariable meningococcal lineage has been responsible for local epidemics worldwide. This is the first report of ST66 meningococcal isolates of this phenotype from Poland.

Dam inactivation in Neisseria meningitidis: prevalence among diverse hyperinvasive lineages

BACKGROUND

DNA adenine methyltransferase (Dam) activity is absent in many, but not all, disease isolates of Neisseria meningitidis, as a consequence of the insertion of a restriction endonuclease-encoding gene, the 'dam replacing gene' (drg) at the dam locus. Here, we report the results of a survey to assess the prevalence of drg in a globally representative panel of disease-associated meningococci.

RESULTS

Of the known meningococcal hyper-invasive lineages investigated, drg was absent in all representatives of the ST-8 and ST-11 clonal complexes tested, but uniformly present in the representatives of the other hyper-invasive lineages present in the isolate collection (the ST-1, ST-4, ST-5, ST-32 and ST-41/44 clonal complexes). The patterns of sequence diversity observed in drg were consistent with acquisition of this gene from a source organism with a different G+C content, at some time prior to the emergence of present-day meningococcal clonal complexes, followed by spread through the meningococcal population by horizontal genetic exchange. During this spread a number of alleles have arisen by mutation and intragenic recombination.

CONCLUSION

These findings are consistent with the idea that possession of the drg gene may contribute to the divergence observed among meningococcal clonal complexes, but does not have a direct mechanistic involvement in virulence.

Sequence typing reveals extensive strain diversity of the Lyme borreliosis agents Borrelia burgdorferi in North America and Borrelia afzelii in Europe

The genetic polymorphism of Borrelia burgdorferi and Borrelia afzelii, two species that cause Lyme borreliosis, was estimated by sequence typing of four loci: the rrs-rrlA intergenic spacer (IGS) and the outer-membrane-protein gene p66 on the chromosome, and the outer-membrane-protein genes ospA and ospC on plasmids. The major sources of DNA for PCR amplification and sequencing were samples of the B. burgdorferi tick vector Ixodes scapularis, collected at a field site in an endemic region of the north-eastern United States, and the B. afzelii vector Ixodes ricinus, collected at a similar site in southern Sweden. The sequences were compared with those of reference strains and skin biopsy isolates, as well as database sequences. For B. burgdorferi, 10-13 alleles for each of the 4 loci, and a total of 9 distinct clonal lineages with linkage of all 4 loci, were found. For B. afzelii, 2 loci, ospC and IGS, were examined, and 11 IGS genotypes, 12 ospC alleles, and a total of 9 linkage groups were identified. The genetic variants of B. burgdorferi and B. afzelii among samples from the field sites accounted for the greater part of the genetic diversity previously reported from larger areas of the north-eastern United States and central and northern Europe. Although ospC alleles of both species had higher nucleotide diversity than other loci, the ospC locus showed evidence of intragenic recombination and was unsuitable for phylogenetic inference. In contrast, there was no detectable recombination at the IGS locus of B. burgdorferi. Moreover, beyond the signature nucleotides that specified 10 IGS genotypes, there were additional nucleotide polymorphisms that defined a total of 24 subtypes. Maximum-likelihood and parsimony cladograms of B. burgdorferi aligned IGS sequences revealed the subtype sequences to be terminal branches of clades, and the existence of at least three monophyletic lineages within B. burgdorferi. It is concluded that B. burgdorferi and B. afzelii have greater genetic diversity than had previously been estimated, and that the IGS locus alone is sufficient for strain typing and phylogenetic studies.

Utility of multilocus sequence typing as an epidemiological tool for investigation of outbreaks of gastroenteritis caused by Campylobacter jejuni

Multilocus sequence typing (MLST) has been proven useful for the study of the global population structure of Campylobacter jejuni; however, its usefulness for the investigation of outbreaks of disease caused by C. jejuni has not been proven. In this study, MLST plus sequencing of the flaA short variable region (SVR) were applied to 47 isolates from 12 outbreaks of C. jejuni infection whose relatedness has been determined previously, and the results were compared to those of serotyping and pulsed-field gel electrophoresis (PFGE). Isolates implicated in an outbreak were indistinguishable by all four subtyping methods, with sporadic isolates being distinguished from outbreak isolates. Two sporadic isolates from one outbreak were resistant to SmaI digestion and therefore nontypeable by PFGE but were differentiated from the outbreak strain by the other methods. PFGE and flaA SVR typing were the most discriminatory methods, with discriminatory indices (DI) of 0.930 and 0.923, respectively. However, an epidemic strain from one outbreak was distinguished from the other outbreak isolates by flaA SVR typing; its flaA allele was different at five nucleotides, suggesting that this change was possibly mediated by recombination. MLST was less discriminatory than PFGE and flaA SVR typing (DI = 0.859), and many of the epidemic strains possessed common sequence types (STs) including ST-8, -21, -22, and -42. However, further discrimination within STs was achieved by flaA SVR typing or PFGE. The results from this study demonstrate that a combined approach of MLST plus flaA SVR typing provides a level of discrimination equivalent to PFGE for outbreak investigations.

Identification of multidrug-resistant Streptococcus pneumoniae strains isolated in Poland by multilocus sequence typing

Multilocus sequence typing (MLST) of 35 isolates of multidrug-resistant Streptococcus pneumoniae recovered in Poland during 1995-1996 distinguished 10 different sequence types (ST). The majority of the isolates were assigned to two Polish clones of serotypes 6B and 23F, although the international clones, Spain23F-1 and Spain9V-3, were also identified. Similar results were obtained using pulsed-field gel electrophoresis (PFGE), providing a direct comparison of these two typing methods.

Clonal complexes of Campylobacter jejuni identified by multilocus sequence typing correlate with strain associations identified by multilocus enzyme electrophoresis

Multilocus sequence typing (MLST) and pulsed-field gel electrophoresis (PFGE) with SmaI were used to subtype 55 isolates of Campylobacter jejuni from a diverse range of human and animal sources previously characterized by multilocus enzyme electrophoresis (MEE). MEE and MLST targeted 11 and 7 loci, respectively, and all loci were unique to each method. MEE, MLST, and PFGE identified 40, 37, and 48 discrete subtypes, respectively, with many of the subtypes occurring only once within the data set. Simpson's indices of diversity were calculated to be 0.979, 0.966, and 0.994 for MEE, MLST, and PFGE, respectively, demonstrating that MEE and MLST had similar discriminatory powers but that PFGE was more discriminatory. Allele diversity was higher in the MLST loci; individual single-locus diversities for the 11 MEE loci and the 7 MLST loci were 0.491 and 0.854, respectively. The clonal complexes recognized by MLST correlated with the strain associations previously recognized by MEE and contained some isolates indistinguishable by PFGE. Many clusters contained isolates from diverse geographical regions and from both humans and animals. These results demonstrate the usefulness of MLST for investigation of the global epidemiology of this important pathogen and illustrate its potential to identify indistinguishable strains or clones in geographically distinct regions.

Long-term predominance of a rare meningococcal phenotype in a small geographical area

Reported here is a predominant clone of Neisseria meningitidis B:14:P1.13 that persisted over a 6-year period in the northernmost province of Italy, where it was responsible for a higher incidence of meningococcal disease compared to the rest of the country. Genetic relatedness of isolates was confirmed by multilocus sequence typing, pulsed-field gel electrophoresis and PorA variable region typing. All strains examined belonged to the ST44 complex/lineage III. Risk factors for infection were evaluated through a case-control study conducted with 21 cases and 63 age- and sex-matched controls. Risk factors for infection in the seven patients younger than 13 years were (i) residence in a rural area, (ii) exposure to passive smoke, and (iii) living in a home with rooms rented to tourists (all odds ratios infinite). The only risk factor for the older patients was previous influenza-like illness (odds ratio, 41.9; 95% confidence interval, 1.6-1068.9). Guidelines for the early treatment of patients and public information campaigns were successfully implemented to reduce disease transmission and the case fatality rate in the region.

Nucleotide sequence-based typing of meningococci directly from clinical samples

The unpredictable characteristics of meningococcal disease (MD) make outbreaks complicated to monitor and consequently lead to high levels of public anxiety. Traditional molecular techniques have been utilized in order to understand better the epidemiology of MD, but some have disadvantages such as being highly specialized and labour-intensive, with low reproducibility. Some of these problems have been overcome by using multilocus sequence typing (MLST). This technique exploits the unambiguous nature and electronic portability of nucleotide sequencing data for the characterization of micro-organisms. The need for enhanced surveillance of MD after the introduction of serogroup C conjugate vaccines means that it is important to gain typing information from the infecting organism in the absence of a culture isolate. Here, the application of MLST for the laboratory confirmation and characterization of Neisseria meningitidis directly from clinical samples is described. This involved using a newly designed set of primers that were complementary to nucleotide sequences external to the existing MLST primers already in use for culture-based MLST of meningococci. This combination has produced a highly sensitive procedure to allow the efficient genotypic characterization of meningococci directly from clinical samples.

Molecular typing for fungi--a critical review of the possibilities and limitations of currently and future methods

Invasive fungal infections represent an increasing problem in patients with inherited and acquired immunodeficiencies. Molecular biotyping techniques, such as DNA fingerprinting, are useful tools to increase our knowledge of the pathogenic organisms that cause them, and thus to improve their treatment and develop prevention strategies. In the present review, we evaluate and discuss the possibilities and limitations of the methods currently used for biotyping strains of fungal species. These include techniques based on restriction fragment length polymorphism (RFLP) with or without hybridization to probes (Southern), PCR-based techniques, electrophoretic karyotyping (EK), and multilocus enzyme electrophoresis (MLEE). Additionally, we discuss newer techniques that are being developed for the fingerprinting of fungal strains. Among them, we review conformation-based polymorphism scanning methods, such as single-strand conformation polymorphism analysis (SSCP) and heteroduplex mobility assays, sequencing strategies such as multilocus sequence typing (MLST) and DNA microarrays.

High-throughput epidemiologic typing in clinical microbiology

Mapping, and ultimately preventing, the dissemination of infectious agents is an important topic in public health. Newly developed molecular-microbiological methods have contributed significantly to recent advances in the efficient tracking of the nosocomial and environmental spread of microbial pathogens. Not only has the application of novel technologies led to improved understanding of microbial epidemiology, but the concepts of population structure and dynamics of many of the medically significant microorganisms have advanced significantly also. Currently, genetic identification of microbes is also within the reach of clinical microbiology laboratory professionals including those without specialized technology research interests. This review summarizes the possibilities for high-throughput molecular-microbiological typing in adequately equipped medical microbiology laboratories from both clinical and fundamental research perspectives. First, the development and application of methods for large-scale comparative typing of serially isolated microbial strains are discussed. The outcome of studies employing these methods allows for long-term epidemiologic surveillance of infectious diseases. Second, recent methods enable an almost nucleotide-by-nucleotide genetic comparison of smaller numbers of strains, thereby facilitating the identification of the genetic basis of, for instance, medically relevant microbiological traits. Whereas the first approach provides insights into the dynamic spread of infectious agents, the second provides insights into intragenomic dynamics and genetic functionality. The current state of technology is summarized, and future perspectives are sketched.

Independent subsets of amplified fragments from the genome of Neisseria meningitidis identify the same invasive clones of ET37 and ET5

To determine whether fluorescent amplified-fragment length polymorphism (FAFLP) analysis is an unbiased genome sampling technique, data were analysed from three different primer combinations, amplifying three independent fragment subsets from 123 isolates of Neisseria meningitidis. Using these data, dendrograms were generated with near-identical topologies that identified the same invasive clones of ET37 and ET5 and also identified the same outbreak clusters.

Detection and genotyping of meningococci using a nested PCR approach

An effective vaccine against Neisseria meningitidis serogroup B is required. Outer-membrane protein vaccines have been developed, which may provide protection against common circulating serotypes and serosubtypes in some countries. However, limited genosubtyping data are available because most laboratories use mAbs directed against a limited number of specific serotypes and serosubtypes and laboratories do not genosubtype directly from body fluids due to the lack of a sensitive PCR method. A nested PCR was therefore developed that enables the amplification of the porA gene directly from clinical samples and has the required sensitivity for nucleotide sequencing of the three main variable regions, VR1, VR2 and VR3. Data were compared with those from culture-based nucleotide sequencing, and the use of this method increased the availability of genosubtype information by 45 %, thereby indicating the impact that this methodology has on the data provided and the implications for vaccine design.

Phylogenetic Evidence for Frequent Positive Selection and Recombination in the Meningococcal Surface Antigen PorB

Previous estimates of rates of synonymous (d(S)) and nonsynonymous (d(N)) substitution among Neisseria meningitidis gene sequences suggested that the surface loops of the variable outer membrane protein PorB were under only weak selection pressure from the host immune response. These findings were consistent with studies indicating that PorB variants were not always protective in immunological and microbiological assays and questioned the suitability of this protein as a vaccine component. PorB, which is expressed at high levels on the surface of the meningococcus, has been implicated in mechanisms of pathogenesis and has also been used as a typing target in epidemiological investigations. In this work, using more precise estimates of selection pressures and recombination rates, we have shown that some residues in the surface loops of PorB are under very strong positive selection, as great as that observed in human immunodeficiency virus-1 surface glycoproteins, whereas amino acids within the loops and the membrane-spanning regions of the protein are under purifying selection, presumably because of structural constraints. Congruence tests showed that recombination occurred at a rate that was not sufficient to erase all phylogenetic similarity and did not greatly bias selection analysis. Homology models of PorB structure indicated that many strongly selected sites encoded residues that were predicted to be exposed to host immune responses, implying that this protein is under strong immune selection and requires further examination as a potential vaccine candidate. These data show that phylogenetic inference can be used to complement immunological and biochemical data in the choice of vaccine candidates.

Nucleotide sequence-based typing of bacteria and the impact of automation

DNA-based typing methods are increasingly important for the characterisation of bacteria. They are used to monitor the epidemiology of pathogens with public health significance and also to help understand the evolution and population biology of bacteria. However, these methods require accuracy and reproducibility and are often of a high-throughput nature. Laboratory automation is therefore the key to the successful implementation of such methods. This review describes the impact of automation on DNA-based typing methods, particularly multi-locus sequence typing (MLST), and the method components that can be automated.

Multilocus sequence typing of Bordetella pertussis based on surface protein genes

Despite more than 50 years of vaccination, Bordetella pertussis has remained endemic in The Netherlands, causing epidemic outbreaks every 3 to 5 years. Strain variation may play a role in the persistence of B. pertussis and was studied by sequencing 15 genes coding for surface proteins, including genes for all five components of acellular pertussis vaccines: pertussis toxin (Ptx), pertactin (Prn), filamentous hemagglutinin, and fimbriae (Fim2 and Fim3). A low level of allelic variation was observed, confirming a recent evolutionary origin of B. pertussis. In modern isolates, polymorphism was observed only in prn, ptxS1, ptxS3, and tcfA. Polymorphism in ptxS1, ptxS3, and tcfA was used to categorize isolates in multilocus sequence types (MLSTs). Analysis of Dutch isolates from 1949 to 1999 revealed five MLSTs, which showed a highly dynamic temporal behavior. We observed significant changes in the MLSTs after the introduction of pertussis vaccination in The Netherlands. Epidemic years were found to be associated with the expansion of MLST-4 or MLST-5. MLST-5 showed a remarkable expansion from 10% in 1997 to 80% in 1999. The MLST analysis was extended to a number of widely separated geographic regions: Finland, Italy, Japan, and the United States. MLST-4 and MLST-5 were found to dominate in Italy and the United States. In Finland and Japan, MLST-3 and MLST-2, respectively, were predominant. Thus, although each region showed distinctive MLST frequencies, in three of the five regions MLST-4 and MLST-5 were predominant. These types may represent newly emerged, successful clones. The identification of highly successful clones may shed light on the question of how B. pertussis is able to maintain itself in vaccinated populations.

Many carried meningococci lack the genes required for capsule synthesis and transport

Of 830 Neisseria meningitidis isolates obtained from healthy carriers in Bavaria, Germany, 136 (16.4%) lacked the operons necessary for the synthesis, lipid modification, and transport of capsular polysaccharide. These operons were replaced by a non-coding intergenic region either 113 or 114 bp in length, termed here the capsule null locus (cnl). Comparisons of the nucleotide sequence of this region in the meningococcus and its acapsulate relatives, Neisseria gonorrhoeae and Neisseria lactamica, revealed six distinct sequence variants (cnl-1 to cnl-6), with a total of 10 nucleotide substitutions and three indels. With the exception of one 4 bp insertion, which was unique to a gonococcal isolate, all of the individual sequence changes were present in the N. lactamica isolates examined. The meningococcal isolates with a cnl belonged to one of four otherwise genetically diverse genetic groupings: the ST-53 and ST-1117 complexes (75 isolates); the ST-845 complex (12 isolates); the ST-198 and 1136 complexes (46 isolates), and the ST-44 complex (one isolate). These data demonstrated that a substantial proportion of carried meningococci were incapable of capsule production, that the cnl circulated within Neisseria populations by horizontal genetic exchange, and that the expression of a polysaccharide capsule was not a requirement for person-to-person transmission of certain meningococcal lineages.

Multilocus sequence typing for characterization of clinical and environmental salmonella strains

Multilocus sequence typing (MLST) based on the 16S RNA, pduF, glnA, and manB genes was developed for Salmonella, and its discriminatory ability was compared to those of pulsed-field gel electrophoresis (PFGE) and serotyping. PFGE differentiated several strains undifferentiable by serotyping, and 78 distinct PFGE types were identified among 231 Salmonella isolates grouped into 22 serotypes and 12 strains of undetermined serotype. The strains of several PFGE types were further differentiated by MLST, which suggests that the discriminatory ability of MLST for the typing of Salmonella is better than that of serotyping and/or PFGE typing. manB-based sequence typing identified two distinct genetic clusters containing 32 of 54 (59%) clinical isolates whose manB gene sequences were analyzed. The G+C contents and Splitstree analysis of the manB, glnA, and pduF genes of Salmonella indicated that the genes differ in their evolutionary origins and that recombination played a significant role in their evolution.

Molecular typing of selected Enterococcus faecalis isolates: pilot study using multilocus sequence typing and pulsed-field gel electrophoresis

The present study compared the recently developed multilocus sequence typing (MLST) approach with a well-established molecular typing technique, pulsed-field gel electrophoresis (PFGE), for subspecies differentiation of Enterococcus faecalis isolates. We sequenced intragenic regions of three E. faecalis antigen-encoding genes (ace, encoding a collagen and laminin adhesin; efaA, encoding an endocarditis antigen; and salA, encoding a cell wall associated antigen) and one housekeeping gene (pyrC) of 22 E. faecalis isolates chosen largely for their temporal and geographical diversity, but also including some outbreak isolates. MLST analysis of polymorphic regions of these four genes identified 13 distinct sequence types (STs) with different allelic profiles; the composite sequences generated from the four sequenced gene fragments of individual isolates showed 98.3 to 100% identity among the 22 isolates. We also found that the allelic profiles from two sequences, ace and salA, were sufficient to distinguish all 13 STs of this study. The 13 STs corresponded to 12 different PFGE types, with one previously designated PFGE clone (a widespread U.S. clone of beta-lactamase-producing isolates) being classified into two highly related STs which differed at 2 of 2,894 bases, both in the same allele. MLST also confirmed the clonal relationships among the isolates of two other PFGE clonal groups, including vancomycin resistant isolates. Thus, this pilot study with representative E. faecalis isolates suggests that, similar to PFGE, the sequence-based typing method may be useful for differentiating isolates of E. faecalis to the subspecies level in addition to identifying outbreak isolates.

Semiautomation of multilocus sequence typing for the characterization of clinical isolates of Neisseria meningitidis

The Scottish Meningococcus and Pneumococcus Reference Laboratory (SMPRL) provides a national service for the laboratory confirmation of meningococcal and pneumococcal disease in Scotland. Part of this service includes the serogrouping of meningococcal isolates followed by typing and subtyping. The procedures for this are labor-intensive but important for the identification of linked cases and the surveillance of disease so that effective public health measures can be taken. However, different strains of meningococci, such as those within the electrophoretic type 37 complex, occurring during case clusters of disease are now indistinguishable by current methods. The SMPRL has started using multilocus sequence typing (MLST) as a routine method for the characterization of isolates of Neisseria meningitidis. MLST produces nucleotide sequence data of seven housekeeping genes providing results that are useful for public health management. However, the method is laborious and time-consuming and therefore lends itself towards automation. The SMPRL therefore developed a semiautomated method for MLST using a 96-well format liquid handler and an automated DNA sequencer. Semiautomated MLST is now provided as a reference service for Scotland. This work describes the methodology required for the characterization of N. meningitidis and highlights its usefulness for public health intervention.

Multilocus sequence typing system for Campylobacter jejuni

The gram-negative bacterium Campylobacter jejuni has extensive reservoirs in livestock and the environment and is a frequent cause of gastroenteritis in humans. To date, the lack of (i) methods suitable for population genetic analysis and (ii) a universally accepted nomenclature has hindered studies of the epidemiology and population biology of this organism. Here, a multilocus sequence typing (MLST) system for this organism is described, which exploits the genetic variation present in seven housekeeping loci to determine the genetic relationships among isolates. The MLST system was established using 194 C. jejuni isolates of diverse origins, from humans, animals, and the environment. The allelic profiles, or sequence types (STs), of these isolates were deposited on the Internet (http://mlst.zoo.ox.ac.uk), forming a virtual isolate collection which could be continually expanded. These data indicated that C. jejuni is genetically diverse, with a weakly clonal population structure, and that intra- and interspecies horizontal genetic exchange was common. Of the 155 STs observed, 51 (26% of the isolate collection) were unique, with the remainder of the collection being categorized into 11 lineages or clonal complexes of related STs with between 2 and 56 members. In some cases membership in a given lineage or ST correlated with the possession of a particular Penner HS serotype. Application of this approach to further isolate collections will enable an integrated global picture of C. jejuni epidemiology to be established and will permit more detailed studies of the population genetics of this organism.

Carried meningococci in the Czech Republic: a diverse recombining population

Population and evolutionary analyses of pathogenic bacteria are frequently hindered by sampling strategies that concentrate on isolates from patients with invasive disease. This is especially so for the gram-negative diplococcus Neisseria meningitidis, a cause of septicemia and meningitis worldwide. Meningococcal isolate collections almost exclusively comprise organisms originating from patients with invasive meningococcal disease, although this bacterium is a commensal inhabitant of the human nasopharynx and very rarely causes pathological effects. In the present study, molecular biology-based techniques were used to establish the genetic relationships of 156 meningococci isolated from healthy young adults in the Czech Republic during 1993. None of the individuals sampled had known links to patients with invasive disease. Multilocus sequence typing (MLST) showed that the bacterial population was highly diverse, comprising 71 different sequence types (STs) which were assigned to 34 distinct complexes or lineages. Three previously identified hyperinvasive lineages were present: 26 isolates (17%) belonged to the ST-41 complex (lineage 3); 4 (2.6%) belonged to the ST-11 (electrophoretic type [ET-37]) complex, and 1 (0.6%) belonged to the ST-32 (ET-5) complex. The data were consistent with the view that most nucleotide sequence diversity resulted from the reassortment of alleles by horizontal genetic exchange.

Fluorescent amplified-fragment length polymorphism genotyping of Neisseria meningitidis identifies clones associated with invasive disease

Fluorescent amplified-fragment length polymorphism (FAFLP), a genotyping technique with phylogenetic significance, was applied to 123 isolates of Neisseria meningitidis. Nine of these were from an outbreak in a British university; 9 were from a recent outbreak in Pontypridd, Glamorgan; 15 were from sporadic cases of meningococcal disease; 26 were from the National Collection of Type Cultures; 58 were carrier isolates from Ironville, Derbyshire; 1 was a disease isolate from Ironville; and five were representatives of invasive clones of N. meningitidis. FAFLP analysis results were compared with previously published multilocus sequence typing (MLST) and pulsed-field gel electrophoresis (PFGE) results. FAFLP was able to identify hypervirulent, hyperendemic lineages (invasive clones) of N. meningitidis as well as did MLST. PFGE did not discriminate between two strains from the outbreak that were classified as similar but distinct by FAFLP. The results suggest that high resolution of N. meningitidis for outbreak and other epidemiological analyses is more cost efficient by FAFLP than by sequencing procedures.

High-throughput sequencing in the population analysis of bacterial pathogens of humans

High-throughput nucleotide sequence determination technologies present new opportunities for studies of bacterial pathogens by enabling the accumulation of large volumes of biodiversity information from isolate collections. Population studies, which combine these data with epidemiological, phylogenetic, and evolutionary concepts, provide insights into the behaviour of pathogens that are unavailable from other approaches as they address questions of relevance to pathogenesis from the perspective of the infectious organism rather from that of the host. Hypothesis-driven analyses applied to these data permit the determination of microbial population diversity and structure, the identification of the mechanisms of genetic change in bacterial populations, and the generation of models of pathogen evolution. The nucleotide sequence-based population studies performed to date demonstrate a spectrum of nucleotide sequence diversity, population structure, and evolutionary mechanisms among pathogenic bacteria. The rapid development of nucleotide sequence determination and analysis techniques provides the tools necessary for the prosecution of population studies on an increasing number of bacterial pathogens.