Clonal spread of serogroup A meningococci: a paradigm for the analysis of microevolution in bacteria

Recombination in Bacterial Genomes: Evolutionary Trends

Bacterial organisms have undergone homologous recombination (HR) and horizontal gene transfer (HGT) multiple times during their history. These processes could increase fitness to new environments, cause specialization, the emergence of new species, and changes in virulence. Therefore, comprehensive knowledge of the impact and intensity of genetic exchanges and the location of recombination hotspots on the genome is necessary for understanding the dynamics of adaptation to various conditions. To this end, we aimed to characterize the functional impact and genomic context of computationally detected recombination events by analyzing genomic studies of any bacterial species, for which events have been detected in the last 30 years. Genomic loci where the transfer of DNA was detected pertained to mobile genetic elements (MGEs) housing genes that code for proteins engaged in distinct cellular processes, such as secretion systems, toxins, infection effectors, biosynthesis enzymes, etc. We found that all inferences fall into three main lifestyle categories, namely, ecological diversification, pathogenesis, and symbiosis. The latter primarily exhibits ancestral events, thus, possibly indicating that adaptation appears to be governed by similar recombination-dependent mechanisms.

Meningococcal disease and control in China: Findings and updates from the Global Meningococcal Initiative (GMI)

The Global Meningococcal Initiative (GMI) is a global expert group, including scientists, clinicians and public health officials from a wide range of specialities. The goal of the GMI is to prevent meningococcal disease worldwide through education, research, and co-operation. The Chinese GMI roundtable meeting was held in June 2017. The GMI met with local experts to gain insight into the meningococcal disease burden in China and current prevention and vaccination strategies in place. China experienced five epidemics of serogroup A meningococcal disease (MenA) between 1938 and 1977, with peak incidence of 403/100,000 recorded in 1967. MenA incidence rates have significantly declined following the universal introduction of the MenA polysaccharide vaccine in China in the 1980s. Further, surveillance data indicates changing meningococcal epidemiology in China with the emergence of new clones of serogroup B from serogroup C clonal complex (cc) 4821 due to capsular switching, and the international spread of serogroup W cc11. The importance of carriage and herd protection for controlling meningococcal disease was highlighted with the view to introduce conjugate vaccines and serogroup B vaccines into the national immunization schedule. Improved disease surveillance and standardized laboratory techniques across and within provinces will ensure optimal epidemiological monitoring.

Emergence of a new epidemic Neisseria meningitidis serogroup A Clone in the African meningitis belt: high-resolution picture of genomic changes that mediate immune evasion

In the African “meningitis belt,” outbreaks of meningococcal meningitis occur in cycles, representing a model for the role of host-pathogen interactions in epidemic processes. The periodicity of the epidemics is not well understood, nor is it currently possible to predict them. In our longitudinal colonization and disease surveys, we have observed waves of clonal replacement with the same serogroup, suggesting that immunity to noncapsular antigens plays a significant role in natural herd immunity. Here, through comparative genomic analysis of 100 meningococcal isolates, we provide a high-resolution view of the evolutionary changes that occurred during clonal replacement of a hypervirulent meningococcal clone (ST-7) by a descendant clone (ST-2859). We show that the majority of genetic changes are due to homologous recombination of laterally acquired DNA, with more than 20% of these events involving acquisition of DNA from other species. Signals of adaptation to evade herd immunity were indicated by genomic hot spots of recombination. Most striking is the high frequency of changes involving the pgl locus, which determines the glycosylation patterns of major protein antigens. High-frequency changes were also observed for genes involved in the regulation of pilus expression and the synthesis of Maf3 adhesins, highlighting the importance of these surface features in host-pathogen interaction and immune evasion.

While established meningococcal capsule polysaccharide vaccines are protective through the induction of anticapsular antibodies, findings of our longitudinal studies in the African meningitis belt have indicated that immunity to noncapsular antigens plays a significant role in natural herd immunity. Our results show that meningococci evade herd immunity through the rapid homologous replacement of just a few key genomic loci that affect noncapsular cell surface components. Identification of recombination hot spots thus represents an eminent approach to gain insight into targets of protective natural immune responses. Moreover, our results highlight the role of the dynamics of the protein glycosylation repertoire in immune evasion by Neisseria meningitidis. These results have major implications for the design of next-generation protein-based subunit vaccines.

Functional genomics studies of the human pathogen Neisseria meningitidis

Neisseria meningitidis is a strictly human pathogen and is one of the major causes of septicemia and meningitis worldwide. Functional genomics approaches have been extensively applied to study how N. meningitidis adapts to grow and survive in different human niches encountered during the infection. DNA microarrays performed in in vitro and ex vivo conditions have revealed changes in the transcriptome profiles of N. meningitidis upon interaction with human cells and after incubation in human serum and blood. Mutagenesis studies allowed detecting mutants in genes crucial for N. meningitidis colonization and systemic infection. The analysis of N. meningitidis genomes has been also successful in the identification of vaccine candidates used to develop an effective protein-based vaccine. The application of all these approaches revealed the potential to identify new virulence factors and vaccine candidates and to assign functions to previously uncharacterized genes providing new insights in the biology and pathogenesis of N. meningitidis.

Genealogical typing of Neisseria meningitidis

Despite the increasing popularity of multilocus sequence typing (MLST), the most appropriate method for characterizing bacterial variation and facilitating epidemiological investigations remains a matter of debate. Here, we propose that different typing schemes should be compared on the basis of their power to infer clonal relationships and investigate the utility of sequence data for genealogical reconstruction by exploiting new statistical tools and data from 20 housekeeping loci for 93 isolates of the bacterial pathogen Neisseria meningitidis. Our analysis demonstrated that all but one of the hyperinvasive isolates established by multilocus enzyme electrophoresis and MLST were grouped into one of six genealogical lineages, each of which contained substantial variation. Due to the confounding effect of recombination, evolutionary relationships among these lineages remained unclear, even using 20 loci. Analyses of the seven loci in the standard MLST scheme using the same methods reproduced this classification, but were unable to support finer inferences concerning the relationships between the members within each complex.

Meningococcal vaccine in travelers

PURPOSE OF REVIEW

New vaccines to prevent meningococcal disease have been licensed in recent years. It is therefore timely to discuss current vaccine strategies pertinent to international travelers in relation to the changing epidemiology.

RECENT FINDINGS

Serogroup W135 achieved epidemic status in Africa in 2002, and then largely disappeared over a short time period. The year 2006 saw a marked epidemic rise in meningitis attack rates across the meningitis belt in Africa. This rise was mainly due to a new serogroup A strain, indicating that a new meningitis epidemic wave is beginning in Africa. Epidemics are also spreading south of the meningitis belt, including the Greater Lakes Area (Burundi, Rwanda, Republic of Tanzania). The new quadrivalent conjugate meningococcal vaccine is now licensed in North America but not elsewhere. In most other industrialized countries, the serogroup C conjugate vaccine is licensed. Plain polysaccharide quadrivalent vaccines are available almost worldwide.

SUMMARY

Quadrivalent meningococcal vaccination is a visa requirement for Hajj and Umrah pilgrims to Saudi Arabia. Travelers to the meningitis belt during the dry season should be advised to receive meningococcal vaccine that covers all four serogroups. This recommendation should be extended to the Greater Lake Area, because of recent epidemics. Vaccine choices depend on availability.

The analysis of Neisseria meningitidis proteomes: Reference maps and their applications

Neisseria meningitidis is an encapsulated Gram-negative bacterium responsible for significant morbidity and mortality worldwide. The availability of meningococcal genome sequences in combination with the rapid growth of proteomic techniques and other high-throughput methods, provided new approaches to the analysis of bacterial system biology. This review considers the meningococcal reference maps so far published as a starting point aimed to elucidate bacterial physiology and pathogenicity, paying particular attention to proteins with potential vaccine and diagnostic applications.

Australia's century of meningococcal disease: development and the changing ecology of an accidental pathogen

Trends in meningococcal disease (MD) over the 20th century in Australia, as in other industrialised countries, have been characterised by epidemics during the two World Wars, a transient rise in incidence in the 1950s followed by endemic disease, and in the 1980s the emergence of a sustained hypersporadic phase. Epidemics occur at times of social upheaval and among marginalised populations, and resolve when living conditions improve. Periodic serogroup A epidemics have been replaced since the 1950s by endemic disease caused mainly by serogroups B and C meningococci. The current hypersporadic plateau in Australia, as in other industrialised countries, is associated with the intercontinental spread of hypervirulent clones of meningococci. The conjugate serogroup C vaccine has reduced the incidence of MD and carriage rates of serogroup C meningococci. However, the vaccine is expensive and its long-term impact on the emergence of non-vaccine strains and on nasopharyngeal microecology is unknown. A rising incidence of MD should not be viewed as the action of a virulent microbe exploiting a vulnerable population, but as the emergence of an "accidental pathogen" from an evolving host-microbial ecology. While it is essential to monitor the impact of vaccines on this ecology, we must find ways that can optimise our coexistence with microbes.

The establishment of Neisseria meningitidis serogroup W135 of the clonal complex ET-37/ST-11 as an epidemic clone and the persistence of serogroup A isolates in Burkina Faso

We analyzed 48 invasive isolates of Neisseria meningitidis that were isolated from meningitis cases in Burkina Faso (April 2002 to April 2003). Thirty-nine of these isolates had the phenotype (serogroup:serotype:serosubtype) W135:2a:P1.5,2, eight isolates were A:4:P1.9 and one isolate was nongroupable:nonserotypable:nonserosubtypable. Genotyping of meningococcal isolates showed that W135 isolates belonged to the sequence type (ST)-11. The nongroupable isolate was of genogroup W135 and belonged to ST-192. Isolates of serogroup A belonged to ST-2859 (a member of the subgroup III/ST-5 clonal complex). W135 (ST-11) isolates involved in meningitis outbreaks in Burkina Faso differed from those involved in the Hajj-2000 associated outbreak by their pulsed-field gel electrophoresis profile. These data confirm the changing epidemiology of meningococcal infection in Burkina Faso with the establishment and expansion of serogroup W135 N. meningitidis strains of the ET-37/ST-11 clonal complex, as well as the emergence of a new clone within the subgroup III/ST-5 clonal complex.

Meningococcal disease in international travel: vaccine strategies

International travel and migration facilitate the rapid intercontinental spread of meningococcal disease. Serogroup A, and to a lesser extent serogroup C, have been responsible for pandemics in the past (mainly in Africa), but in recent years there was an international outbreak due to W135 related to the Hajj pilgrimage. The high carriage rates, persistence and transmissibility, in combination with the high case fatality rate of the Hajj-associated W135 outbreak clone, certainly raise considerable concern about the public health consequences of widespread dissemination of this organism and the potential for future epidemics. Indeed, the now evolving W135 epidemic in Africa mandates that the bivalent meningococcal vaccine should be replaced by the tetravalent meningococcal vaccine, covering A, C, Y and W135 serogroups. The currently available polysaccharide tetravalent meningococcal vaccine, albeit associated with high seroconversion and efficacy rates, has several shortcomings: it is not immunogenic in young children, duration of protective immunity is short, and it has minimal or no effect on nasopharyngeal carriage and therefore transmission of the organism. Immunogenicity of polysaccharide vaccines can be improved by chemical conjugation to a protein carrier, thereby eliciting a T-cell-dependent antibody response. In contrast to polysaccharide vaccines, conjugate vaccines are immunogenic in young infants, induce long-term protection, and reduce nasopharyngeal carriage. The tetravalent conjugate vaccine will be a leap forward in the control of meningococcal epidemics in affected countries. It will also boost the uptake of meningococcal vaccines in travelers, because the duration of protection is longer and it eliminates the problem of immune hyporesponsiveness of serogroup C with repeated dosing. The small risk of travel-associated disease for the general traveler and the unpredictable nature of epidemics make it difficult to provide evidence-based vaccine recommendations. The current recommendation is to vaccinate all Hajj pilgrims, travelers to areas with current outbreaks, travelers to the sub-Saharan meningitis belt, and high-risk individuals (i.e., those with immunodeficiencies).

Continuing diversification of Neisseria meningitidis W135 as a primary cause of meningococcal disease after emergence of the serogroup in 2000

The occurrence of a clonal outbreak of serogroup W135 (of the electrophoretic type 37 [ET-37] clonal complex) meningococcal disease among Hajj pilgrims in 2000 has led to enhanced surveillance of the evolution of this particular serogroup, formerly considered rare, in invasive infections. Since the first case of meningococcal disease due to a serogroup W135 strain was detected in France in 1994, all isolates were characterized phenotypically. We further used phenotypic and genotypic approaches to type the 101 serogroup W135 strains isolated from patients with invasive meningococcal diseases in France in 2001 and 2002. Overall, 55% of these isolates had Hajj strain-related phenotypes (60 and 52% in 2001 and 2002, respectively), although only 45% belonged to the ET-37 clonal complex. Moreover, pulsed-field gel electrophoresis of the ET-37 clonal complex isolates showed that only 32% of the serogroup W135 isolates were indistinguishable from the 2000 Hajj-related strain. Our results suggest the continuous emergence of new genetic lineages of serogroup W135 independently of the 2000 global outbreak.

Genetic lineages and their traits in Neisseria meningitidis

Neisseria meningitidis is a model organism for the study of bacterial population biology, for genome sequencing and pathogenicity research. In the recent years, our group has identified a variety of markers for hypervirulent lineages of meningococci, which in part could be validated for typing purposes. Furthermore, carrier strain collections of meningococci and N. lactamica were studied by multilocus sequence typing, and elucidated the impressive genetic variability of those species. Characterisation of meningococcal carrier strains allowed to define the capsule null locus (cnl) of meningococci, which frequently occurs among carrier isolates and renders strains constitutively unencapsulated. This finding poses the question about the yet unclear role of the meningococcal polysaccharide capsule in transmission and carriage. O-acetylation of the meningococcal polysaccharides is another variably expressed trait in meningococci. We identified the genes responsible for O-acetylation of the serogroup C, W-135 and Y capsules, and provided the genetic basis for understanding the variability of O-acetylation patterns in meningococci. The oatC and oatWY genes proved to be the first genes identified to be responsible for O-acetylation of polysialic acid.

Meningococcal disease and travel

Meningococcal disease continues to be a worldwide problem. This review examines the impact meningococcal disease has on international travel and vice versa the impact international travel has on the intercontinental spread of meningococci. The risk of meningococcal disease to the endemic population differs from that of travellers. The best documented risk of meningococcal disease among travellers has been in Hajj pilgrims for Mecca and Madina in Saudi Arabia. In response to the recent Hajj associated outbreak of W135 meningococcal disease, quadrivalent meningococcal vaccine (against serogroups A/C/Y/W135) became a visa requirement. In view of increasing worldwide reports of Y and W135 meningococcal disease, there should be a switch in recommendation from the bivalent (against A& C) to the quadrivalent vaccine for all travellers.

Transcriptome analysis of Neisseria meningitidis during infection

Neisseria meningitidis is the cause of septicemia and meningococcal meningitis. During the course of infection, N. meningitidis encounters multiple environments within its host, which makes rapid adaptation to environmental changes a crucial factor for neisserial pathogenicity. Employing oligonucleotide-based DNA microarrays, we analyzed the transcriptome of N. meningitidis during two key steps of meningococcal infection, i.e., the interaction with epithelial cells (HeLa cells) and endothelial cells (human brain microvascular endothelial cells). Seventy-two genes were differentially regulated after contact with epithelial cells, and 48 genes were differentially regulated after contact with endothelial cells, including a considerable proportion of well-known virulence genes. While a considerable number of genes were in concordance between bacteria adherent to both cell types, we identified several open reading frames that were differentially regulated in only one system. The data obtained with this novel approach may provide insight into the pathogenicity mechanisms of N. meningitidis and could demonstrate the importance of gene regulation on the transcriptional level during different stages of meningococcal infection.

Pathogenesis, Therapy, and Prevention of Meningococcal Sepsis

Neisseria meningitidis (meningococcus), an exclusive pathogen of humans, is the cause of sepsis (meningococcemia) and meningitis, often in otherwise healthy individuals. Several hundred thousand cases of meningococcal disease occur worldwide each year, a number that is frequently accentuated by epidemic outbreaks. In recent years, significant advances, fueled by new molecular approaches and genome sequencing projects, have improved our understanding of the pathogenesis of meningococcal disease and have led to progress in the development of the next generation of meningococcal vaccines. However, the mortality of meningococcal disease remains 10% to 15% for all cases, and is up to 40% in patients with severe sepsis. This review summarizes current knowledge of the pathogenesis, therapy, and prevention of meningococcal disease with emphasis on meningococcal sepsis.

The duality of virulence and transmissibility in Neisseria meningitidis

Neisseria meningitidis is a commensal bacterium of the human nasopharynx that occasionally provokes invasive disease. Carriage strains of N. meningitidis are heterogeneous, more frequent in nature and are transmitted among carriers. Disease is not a part of this transmission cycle and is caused by virulent strains. N. meningitidis is highly variable and variants that are modified in their virulence and/or transmissibility are continually generated. These events probably occur frequently, thus explaining not only the heterogeneous nature of meningococcal populations in carriers but probably also the evolutionary success of this human-restricted bacterium.

Evidence of recombination in Porphyromonas gingivalis and random distribution of putative virulence markers

The association of Porphyromonas gingivalis to periodontal disease is not clearly understood. Similar proportions of P. gingivalis may be cultivated from both inactive and actively degrading periodontal pockets. Differences in virulence among strains of P. gingivalis exist, but the molecular reason for this remains unknown. We examined the population structure of P. gingivalis to obtain a framework in which to study pathogenicity in relation to evolution. Phylogenetic trees derived from the sequencing of fragments of four housekeeping genes, ahp, thy, rmlB, and infB, in 57 strains were completely different with no correlation between clustering of strains in the four dendrograms. Combining the various alleles of the four gene fragments sequenced resulted in 41 different sequence types. The index of association, I(A), based on a single representative of each sequence type was 0.143 +/- 0.202, indicating a population at linkage equilibrium. Inclusion of all isolates for the calculation of I(A) resulted in a value of 0.206 +/- 0.171. This suggests an epidemic population structure supported by the finding of genetically identical strains in different parts of the world. We observed a random distribution of two virulence-associated mobile genetic elements, the ragB locus and the insertion sequence IS1598, among 132 strains tested. In conclusion, P. gingivalis has a nonclonal population structure characterized by frequent recombination. Our study suggests that particular genotypes, possibly with increased pathogenic potential, may spread successfully in the human population.

The panmictic nature ofNeisseria meningitidisserogroup B during a period of endemic disease in Canada

Three hundred and one (301) strains of Neisseria meningitidis serogroup B, isolated from patients with meningococcal disease during the years 1994–1996, were subjected to multilocus enzyme electrophoresis, serotyping, and serosubtyping. Based on the analyses of 14 enzyme loci, 177 electrophoretic types (ETs) were identified. Of these, 136 were represented by single isolates and 41 were represented by multiple isolates (range 2–31). The mean genetic diversity for isolates was 0.444 and for ETs was 0.440. The index of association (IA) between loci was 0.530 ± 0.08 for isolates and 0.256 ± 0.10 for ETs. Cluster analysis revealed the presence of 39 lineages each represented by a single ET or clusters of ETs. The most common serotypes were 4, 15, and 14 and accounted for 84 (28.0%), 53 (17.6%), and 32 (10.6%) of the isolates, respectively, and were dispersed amongst 46 ETs (1–122), 35 ETs (3–165), and 26 ETs (18–76), respectively. The 109 (36.6%) nontypable (NT) isolates were amongst 74 ETs (6–177). The mean genetic diversity for serotypes 4, 15, and 14 and NT isolates was 0.368, 0.371, 0.343, and 0.442, respectively, and for ETs was 0.363, 0.354, 0.397, and 0.440, respectively. Combinations of serotypes and serosubtypes (number of isolates) that occurred most frequently were 4:P1.14 (17), 14:P1.16 (16), NT:P1.16 (16), 15:P1.16 (13), and NT:P1.13 (13). The majority of group B disease in Canada during 1994–1996 was caused by meningococci of considerable genetic diversity, and reflects a situation of endemic disease. However, the results also indicate that organisms belonging to the ET-5 complex, which has been responsible for outbreaks of group B disease globally for several decades, have been introduced into the country.Key words: meningococcal, genotypes, serotypes, serosubtypes, Neisseria meningitidis.

Interactions of pathogenic neisseriae with epithelial cell membranes

The closely related bacterial pathogens Neisseria gonorrhoeae (gonococci, GC) and N. meningitidis (meningococci, MC) initiate infection at human mucosal epithelia. Colonization begins at apical epithelial surfaces with a multistep adhesion cascade, followed by invasion of the host cell, intracellular persistence, transcytosis, and exit. These activities are modulated by the interaction of a panoply of virulence factors with their cognate host cell receptors, and signals are sent from pathogen to host and host to pathogen at multiple stages of the adhesion cascade. Recent advances place us on the verge of understanding the colonization process at a molecular level of detail. In this review we describe the Neisseria virulence factors in the context of epithelial cell biology, placing special emphasis on the signaling functions of type IV pili, pilus-based twitching motility, and the Opa and Opc outermembrane adhesin/invasin proteins. We also summarize what is known about bacterial intracellular trafficking and growth. With the accelerated integration of tools from cell biology, biochemistry, biophysics, and genomics, experimentation in the next few years should bring unprecedented insights into the interactions of Neisseriae with their host.

Epidemiology and pathogenesis of Neisseria meningitidis

Neisseria meningitidis, an exclusive pathogen of humans, remains the leading worldwide cause of meningitis and fatal sepsis, usually in otherwise healthy individuals. In recent years, significant advances have improved our understanding of the epidemiology and genetic basis of meningococcal disease and led to progress in the development of the next generation of meningococcal vaccines. This review summarizes current knowledge of the human susceptibility to and the epidemiology and molecular pathogenesis of meningococcal disease.

Epidemiology of bacterial meningitis

Bacterial meningitis caused by Haemophilus influenzae type B (Hib) has almost disappeared from the United States, Canada, and other countries that have implemented routine vaccination with Hib conjugate vaccines. The overall incidence of meningitis in these countries has declined by more than 50%, and the age distribution of susceptibility has shifted, so that the disease is now more common in adults than in children. Another new feature of the epidemiology of bacterial meningitis has been the occurrence of clusters of meningococcal disease. Such clusters have been school related, mainly in adolescents, and most clusters have been associated with a clone of group C, serotype 2a. The role of cigarette smoking as a risk factor for bacterial meningitis has been confirmed and adds urgency to the efforts to control smoking in adolescents and young adults.

Microevolution through DNA exchange among strains of Neisseria meningitidis isolated during an outbreak in the Czech Republic

Neisseria meningitidis is a highly variable bacterium. Indeed, N. meningitidis is naturally competent for transformation, and horizontal DNA exchange between strains may lead to mosaic genetic loci in N. meningitidis. We studied such an exchange in nature during an epidemic provoked by N. meningitidis. This epidemic started in the Czech Republic in 1993 and the original epidemic clone was shown to have the antigenic formula (serogroup:serotype:serosubtype) C:2a:P1.2,5. We analysed 145 meningococcal strains isolated in the Czech Republic between 1993 and 1997 using serological and genetic typing methods (multilocus enzyme electrophoresis and polymorphism of pilA and pilD genes). This analysis showed that genetic exchange between epidemic and endemic strains had occurred. Exchanges involved mostly surface-exposed structures such as the capsule, giving rise to new meningococcal variants. The expansion of these variants should be kept under close surveillance.

Molecular epidemiology of recent belgian isolates of Neisseria meningitidis serogroup B

In Belgium an increase in the incidence of meningococcal disease has been noted since the early 1990s. Four hundred twenty clinical strains isolated during the period from 1990 to 1995, along with a set of 30 European reference strains, and 20 Dutch isolates were examined by random-primer and repetitive-motif-based PCR. A subset was investigated by multilocus enzyme electrophoresis and pulsed-field gel electrophoresis. The data were compared with results obtained by serotyping (M. Van Looveren, F. Carion, P. Vandamme, and H. Goossens, Clin. Microbiol. Infect. 4:224-228, 1998). Both phenotypic and molecular epidemiological data suggest that the lineage III of Neisseria meningitidis, first encountered in The Netherlands in about 1980, has been introduced in Belgium. The epidemic clone, as defined by oligonucleotide D8635-primed PCR, encompasses mainly phenotypes B:4:P1.4 and B:nontypeable:P1.4, but strains with several other phenotypes were also encountered. Therefore, serotyping alone would underestimate the prevalence of the epidemic clone.

Microevolution during epidemic spread of Neisseria meningitidis

Similar to many other naturally transformable bacteria, Neisseria meningitidis has yielded many examples where horizontal genetic exchange has resulted in genetic variation of individual strains. Epidemic strains are purified of genetic variants due to bottlenecks during the spread from country to country, resulting in clonal descent. Occasionally, clonal replacement also occurs during epidemic spread. These processes occur rapidly in serogroup A meningococci; such that after their descent from a common ancestor, clonally related bacteria have diversified at numerous loci within the last decades.

Microevolution and epidemic spread of serogroup A Neisseria meningitidis--a review

An extensive and representative strain collection of serogroup A Neisseria meningitidis was established. These bacteria were obtained from different endemic and epidemic/pandemic sources and include strains from diseased patients and healthy carriers. The genetic relationships of the bacteria were defined by multi-locus enzyme electrophoresis and sequence polymorphisms of genetically variable antigens have been analyzed in closely-related groupings. The results are interpreted as reflecting a balance of recombination events, which disrupt clonal relationships, and sequential bottlenecks, which purify the bacterial population of genetic variants during epidemic spread.

Transformation competence and type-4 pilus biogenesis in Neisseria gonorrhoeae--a review

In Neisseria gonorrhoea (Ngo), the processes of type-4 pilus biogenesis and DNA transformation are functionally linked and play a pivotal role in the life style of this strictly human pathogen. The assembly of pili from its main subunit pilin (PilE) is a prerequisite for gonococcal infection since it allows the first contact to epithelial cells in conjunction with the pilus tip-associated PilC protein. While the components of the pilus and its assembly machinery are either directly or indirectly involved in the transport of DNA across the outer membrane, other factors unrelated to pilus biogenesis appear to facilitate further DNA transfer across the murein layer (ComL, Tpc) and the inner membrane (ComA) before the transforming DNA is rescued in the recipient bacterial chromosome in a RecA-dependent manner. Interestingly, PilE is essential for the first step of transformation, i.e., DNA uptake, and is itself also subject to transformation-mediated phase and antigenic variation. This short-term adaptive mechanism allows Ngo to cope with changing micro-environments in the host as well as to escape the immune response during the course of infection. Given the fact that Ngo has no ecological niche other than man, horizontal genetic exchange is essential for a successful co-evolution with the host. Horizontal exchange gives rise to heterogeneous populations harboring clones which better withstand selective forces within the host. Such extended horizontal exchange is reflected by a high genome plasticity, the existence of mosaic genes and a low linkage disequilibrium of genetic loci within the neisserial population. This led to the concept that rather than regarding individual Neisseria species as independent traits, they comprise a collective of species interconnected via horizontal exchange and relying on a common gene pool.

Genetic analysis of a meningococcal population based on polymorphism of the pilA-pilB locus: a molecular approach for meningococcal epidemiology

The genetic relationships between 88 meningococcal strains were analyzed by using the polymorphism of the pilA gene and the multilocus enzyme electrophoresis. While a good agreement was observed, correlation with antigenic formula (serogroup, serotype, and serosubtype) was incomplete. The inadequacy of serological classification alone in outbreak surveillance may be overcome by DNA-based approaches.

Arginine-, hypoxanthine-, uracil-requiring isolates of Neisseria gonorrhoeae are a clonal lineage with a non-clonal population

Multilocus enzyme electrophoresis has shown that a collection of 101 arginine-, hypoxanthine-, uracil-requiring (AHU-) isolates of Neisseria gonorrhoeae, recovered over a 39 year period from the UK and Denmark, were of a single electrophoretic type (91% of strains), or differed from the predominant electrophoretic type at only a single locus. The striking uniformity of the AHU-isolates, and the correlation between auxotype, serovar and overall genetic background, contrasts with previous studies of gonococcal populations (that included very few AHU-strains), and a small sample of non-AHU-isolates studied here, which demonstrated a non-clonal population structure and a lack of association between auxotype, serovar and genetic background. There was no marked difference in the ability of AHU-isolates to be transformed with their own DNA, or with DNA from gonococci of other auxotypes, and the relative genetic stability of AHU-isolates does not appear to be due to a defect in their ability to be transformed. An alternative possibility is that AHU-gonococci recombine with other lineages, but that the resulting recombinants are not maintained in the population. This would occur, for example, if AHU-gonococci competed poorly in mixed infections, within which effective recombination between lineages occurs, and are usually only transmitted from individuals who are singly infected with an AHU-strain. The association between AHU-gonococci and asymptomatic infections may lead to an increased rate of transmission of these strains which under this scenario would be needed to prevent them from being lost from the population.

A comparison of the nucleotide sequences of the adk and recA genes of pathogenic and commensal Neisseria species: evidence for extensive interspecies recombination within adk

The sequences of the adenylate kinase gene (adk) and the RecA gene (recA) were determined from the same isolates of Neisseria gonorrhoeae, N. meningitidis, N. lactamica, N. polysaccharea, N. cinerea, N. mucosa, N. pharyngis var. flava, N. flavescens, and N. animalis. The patterns of sequence divergence observed at adk and recA were very different. Dendrograms constructed from the recA data using two different algorithms were statistically robust and were congruent with each other and with the relationships between the species previously proposed using other data. In contrast, the dendrograms derived from the adk data were noncogruent with each other, and with those from the recA data, and were statistically poorly supported. These results, along with the uniform distribution of pairwise sequence divergences between the species at adk, suggest there has been a history of interspecies recombination within the adk gene of the human Neisseria species which has obscured the phylogenetic relationships between the species. This view was supported by Sawyer's runs test, and the Index of Association (IA) between codons, which provided significant evidence for interspecies recombination between the adk genes from the human Neisseria species, but no evidence of interspecies recombination between the recA sequences.

The porA alleles are identical in subgroup III serogroup A Neisseria meningitidis strains isolated in China in the 1960s and 1980s

The porA genes from serogroup A, subgroup III strains isolated in the People's Republic of China in 1966 and in 1984 and 1985 were amplified by PCR at an annealing temperature of 75 degrees C. The DNA sequences (5 strains) and the restriction patterns generated by MspI (14 strains) were identical, unlike the results reported by Peixuan et al. (Z. Peixuan, H. Xujing, and X. Li, J. Clin. Microbiol. 33:458-462, 1995). Furthermore, PCR products which were amplified at an annealing temperature of 60 degrees C, as described by Peixuan et al., were heterogeneous in our study.

Molecular population genetic analysis of emerged bacterial pathogens: selected insights

Research in bacterial population genetics has increased in the last 10 years. Population genetic theory and tools and related strategies have been used to investigate bacterial pathogens that have contributed to recent episodes of temporal variation in disease frequency and severity. A common theme demonstrated by these analyses is that distinct bacterial clones are responsible for disease outbreaks and increases in infection frequency. Many of these clones are characterized by unique combinations of virulence genes or alleles of virulence genes. Because substantial interclonal variance exists in relative virulence, molecular population genetic studies have led to the concept that the unit of bacterial pathogenicity is the clone or cell line. Continued new insights into host parasite interactions at the molecular level will be achieved by combining clonal analysis of bacterial pathogens with large-scale comparative sequencing of virulence genes.

The physical map of the chromosome of a serogroup A strain of Neisseria meningitidis shows complex rearrangements relative to the chromosomes of the two mapped strains of the closely related species N. gonorrhoeae

A physical map of the chromosome of N. meningitidis Z2491 (serogroup A, subgroup IV-1) has been constructed. Z2491 DNA was digested with NheI, SpeI, SgfI, PacI, BglII, or PmeI, resulting in a limited number of fragments that were resolved by contour-clamped homogeneous electric field (CHEF) electrophoresis. The estimated genome size for this strain was 2,226 kb. To construct the map, probes corresponding to single-copy genes or sequences were used on Southern blots of chromosomal DNA digested with the different mapping enzymes and subjected to CHEF electrophoresis. By determining which fragments from different digests hybridized to each specific probe, it was possible to walk back and forth between digests to form a circular macrorestriction map. The intervals between mapped restriction sites range from 10 to 143 kb in size. A total of 117 markers have been placed on the map; 75 represent identified genes, with the remaining markers defined by anonymous cloned fragments of neisserial DNA. Comparison of the arrangement of genetic loci in Z2491 with that in gonococcal strain FA1090, for which a physical map was previously constructed, revealed complex genomic rearrangements between the two strains. Although gene order is generally conserved over much of the chromosome, a region of approximately 500 kb shows translocation and/or inversion of multiple blocks of markers between the two strains. Even within the relatively conserved portions of the maps, several genetic markers are in different positions in Z2491 and FA1090.

Sex in Escherichia coli does not disrupt the clonal structure of the population: evidence from random amplified polymorphic DNA and restriction-fragment-length polymorphism

Analysis of the Escherichia coli population by multilocus enzyme electrophoresis (MLEE) has established its clonal organization, but there is increasing evidence that horizontal DNA transfer occurs in E. coli. We have assessed the genetic structure of the species E. coli and determined the extent to which recombination can affect the clonal structure of bacteria. A panel of 72 E. coli strains from the ECOR collection was characterized by random amplified polymorphic DNA (RAPD) and restriction-fragment-length polymorphism (RFLP) of the ribosomal RNA gene (rrn) regions. These strains have been characterized by MLEE and are assumed to reflect the range of genotypic variation in the species as a whole. Statistical analysis, including factorial analysis of correspondence (FAC) and hierarchical classifications, established that the data obtained with the three genetic markers are mutually corroborative, thus providing compelling evidence that horizontal transfer does not disrupt the clonal organization of the population. However, there is a gradient of correlation between the different classifications which ranges from the highly clonal structure of B2 group strains causing extraintestinal infections in humans to the less-stringent structure of B1 group strains that came mainly from nonprimate mammals. This group (B1) appears to be the framework from which the remaining non-A group strains have emerged. These results indicate that RAPD analysis is well suited to intraspecies characterization of E. coli. Lastly, treating the RAPD data by FAC allowed description of subgroup-specific DNA fragments which can be used, in a strategy comparable to positional cloning, to isolate virulence genes.

Molecular typing of Neisseria meningitidis serogroup A using the polymerase chain reaction and restriction endonuclease pattern analysis

A new molecular typing method for identification and characterization of Neisseria meningitidis is reported. Chromosomal DNA from 20 well-documented meningococcal strains of serogroup A originating from France, Central African Republic, Sudan and Burkina Faso were amplified using the polymerase chain reaction. Primers designed in this study were located in the pilA/pilB locus which has been shown to be conserved in the genus Neisseria. The amplified fragments were subjected to restriction endonuclease analysis using three different enzymes, and the restriction endonuclease patterns obtained were compared. Clonal isolates clustered together in distinct restriction endonuclease patterns which are described in this study and coincided with electrotypes as determined by multi-locus enzyme electrophoresis. This DNA-based typing system for meningococci may be useful for epidemiological studies.

Emergence of a new virulent clone within the electrophoretic type 5 complex of serogroup B meningococci in Norway

An increase in B:15:P1.12 meningococci among isolates from patients with Neisseria meningitidis infection in Norway in recent years led to further characterization of such strains. Between 1987 and 1992, B:15:P1.12 strains constituted 9.8% (24 strains) of B:15 isolates. The B:15:P1.12 strains belonged to the electrophoretic type 5 (ET-5) complex, but 17 (71%) strains were a new clone (ET-5c) not found elsewhere in the world. All but one strain of ET-5c were responsible for a localized outbreak of systemic meningococcal disease in western Norway. A novel monoclonal antibody (202,G-12), developed against the unknown variable region 2 on the class 1 protein of one of these strains, bound to 19 of the 15:P1.12 strains, 4 strains bound the subtype P1.13 reference monoclonal antibody MN24H10.75, and the remaining strain showed no reaction. Sequencing of porA genes demonstrated a series of nine threonine residues in the deduced variable region 2 of the latter strain, while four and five threonine residues were found in the corresponding regions of strains reacting with the monoclonal antibodies 202,G-12 and MN24H10.75, respectively. Epitope mapping with synthetic peptides showed that 202,G-12 bound to a sequence of 11 amino acids which included the four threonine residues specific for subtype P1.13a. Immunoglobulin G antibodies against the P1.7,16 subtype protein, induced in volunteers after vaccination with the Norwegian meningococcal vaccine, did not cross-react on immunoblots with the subtype protein of clone ET-5c. Thus, postvaccination class 1 protein antibodies, assumed to be protective, may not be effective against infection with the new clone.

Intergeneric transfer and recombination of the 6-phosphogluconate dehydrogenase gene (gnd) in enteric bacteria

The gnd gene, encoding 6-phosphogluconate dehydrogenase (EC 1.1.1.44), was sequenced in 87 strains of 15 species assigned to five nominal genera of the Enterobacteriaceae, including 36 isolates of Salmonella enterica and 32 strains of Escherichia coli. In S. enterica, the effective (realized) rate of recombination of horizontally transferred gnd sequences is only moderately higher than the rates for other chromosomal housekeeping genes. In contrast, recombination at gnd has occurred with such high frequency in Escherichia coli that the indicated evolutionary relationships among strains are not congruent with those estimated by sequence analysis of other genes and by multilocus enzyme electrophoresis. E. coli and S. enterica apparently have not exchanged gnd sequences, but those of several strains of E. coli have been imported from species of Citrobacter and Klebsiella. The relatively frequent exchange of gnd within and among taxonomic groups of the Enterobacteriaceae, compared with other housekeeping genes, apparently results from its close linkage with genes that are subject to diversifying selection, including those of the rfb region determining the structure of the O antigen polysaccharide.

The opaH locus of Neisseria gonorrhoeae MS11A is involved in epithelial cell invasion

In order to produce a successful infection, Neisseria gonorrhoeae (GC) must attach to and invade mucosal epithelial cells. To identify GC gene products involved in this early interaction with host cells we constructed a gene bank derived from a clinical isolate of GC, and isolated a clone which had the capacity to adhere to the human endometrial adenocarcinoma tissue-culture line HEC-1-B. The cloned sequence was identified as a member of the opa gene family whose protein products have been associated with virulence. The GC chromosome contains numerous variant opa genes which, in MS11, are designated opaA-K. Previous work showed that expression of opaC confers a highly invasive phenotype upon strain MS11. When our cloned opa gene was mutated and returned to the GC MS11A chromosome by transformation and homologous recombination, we isolated one transformant that was significantly reduced in its invasive capacity. The locus mutated in this transformant was identified as opaH. Our results indicate that invasiveness of GC for human epithelial cells can be determined by more than one opa gene in strain MS11A.

Microevolution within a clonal population of pathogenic bacteria: recombination, gene duplication and horizontal genetic exchange in the opa gene family of Neisseria meningitidis

Opacity (Opa) proteins are a family of antigenically variable outer-membrane proteins of Neisseria meningitidis. Even among clonally related epidemic meningococcal isolates, there is greater variation of Opa protein expression than can be accounted for by the opa gene repertoire of any individual strain. We characterized the opa genes of eight closely related isolates of serogroup A N. meningitidis (subgroup IV-1) from a recent meningitis epidemic in West Africa. DNA sequence analysis and Southern blot experiments indicated that changes occurred in the opa genes of these bacteria as they spread through the human population, over a relatively short period of time. Such changes in one or a few loci within a clonal population are referred to as microevolution. The distribution of sequences present in hypervariable (HV) regions of the opa genes suggests that duplication of all or part of opa genes into other opa loci changed the repertoire of Opa proteins that could be expressed. Additional variability in this gene family appears to have been introduced by horizontal exchange of opa sequences from other meningococcal strains and from Neisseria gonorrhoeae. These results indicate that processes of recombination and genetic exchange contributed to variability in major surface antigens of this clonal population of pathogenic bacteria.

The porA gene in serogroup A meningococci: evolutionary stability and mechanism of genetic variation

Molecular analyses were applied to the genes encoding variants of the serosubtyping antigen, the class 1 outer membrane protein (PorA), from 55 serogroup A Neisseria meningitidis strains. These genes were evolutionarily stable and exhibited a limited range of genetic variation, primarily generated by recombination. Translation of the gene sequences revealed a total of 19 distinct amino acid sequences in the variable regions of the protein, 6 of which were not recognized by currently available serosubtyping monoclonal antibodies. Knowledge of these amino acid sequences permitted a rational re-assignment of serosubtype names. Comparison of the complete genes with porA gene sequences from serogroup B and C meningococci showed that serogroup A possessed a limited number of the possible porA genes from a globally distributed gene pool. Each serogroup A subgroup was characterized by one of four porA gene types, probably acquired upon subgroup divergence, which was stable over periods of decades and during epidemiological spread. Comparison with other variable genes (pil and iga) indicated that the three alleles were independently assorted within the subgroup, suggesting that their gene types were older than the subgroups in which they occurred.