Limited genetic diversity of recent invasive isolates of non-serotype b encapsulated Haemophilus influenzae

Comparative pangenome analysis of capsulated Haemophilus influenzae serotype f highlights their high genomic stability

Haemophilus influenzae is an opportunistic pathogen adapted to the human respiratory tract. Non-typeable H. influenzae are highly heterogeneous, but few studies have analysed the genomic variability of capsulated strains. This study aims to examine the genetic diversity of 37 serotype f isolates from the Netherlands, Portugal, and Spain, and to compare all capsulated genomes available on public databases. Serotype f isolates belonged to CC124 and shared few single nucleotide polymorphisms (SNPs) (n = 10,999), but a high core genome (> 80%). Three main clades were identified by the presence of 75, 60 and 41 exclusive genes for each clade, respectively. Multi-locus sequence type analysis of all capsulated genomes revealed a reduced number of clonal complexes associated with each serotype. Pangenome analysis showed a large pool of genes (n = 6360), many of which were accessory genome (n = 5323). Phylogenetic analysis revealed that serotypes a, b, and f had greater diversity. The total number of SNPs in serotype f was significantly lower than in serotypes a, b, and e (p < 0.0001), indicating low variability within the serotype f clonal complexes. Capsulated H. influenzae are genetically homogeneous, with few lineages in each serotype. Serotype f has high genetic stability regardless of time and country of isolation.

Analysis of Haemophilus influenzae serotype f isolated from three Japanese children with invasive H. influenzae infection

In Japan, publicly subsidized Haemophilus influenzae serotype b vaccines became available in 2011; consequently, the incidence of invasive H. influenzae infection in paediatric patients of less than 5 years of age decreased dramatically. In 2013, the first case of H. influenzae serotype f (Hif) meningitis in a Japanese infant was reported, and another case of Hif meningitis in a Japanese infant was observed in 2013. We experienced a fatal paediatric case of Hif bacteraemia in 2004; therefore, we conducted an analysis of the three Hif strains isolated from these three Japanese children with invasive Hif infections. All three strains were β-lactamase-non-producing, ampicillin-sensitive strains, with MICs of 1 µg ml(-1) or less. However, one of the three strains showed slightly elevated MICs for ampicillin (1 µg ml(-1)), cefotaxime (0.25 µg ml(-1)) and meropenem (0.13 µg ml(-1)). A molecular analysis by multilocus sequence typing identified all three strains as sequence type (ST) 124, which is a predominant invasive Hif strain in many countries. SmaI-digested PFGE showed variable DNA fragmentation patterns among the strains, suggesting that some highly virulent strains have originated from a single ST124 clone and caused invasive Hif infections in Japan. Additional studies are needed to determine the factors that have led to the clonal expansion of virulent ST124 strains.

Necrotizing fasciitis caused by Haemophilus influenzae serotype f

Haemophilus influenzae is a rare cause of soft tissue infection. In this report, we present a case of multifocal necrotizing fasciitis in a healthy adult patient, secondary to Haemophilus influenzae serotype f infection, and we review literature on this rare cause of necrotizing fasciitis.

Adhesin genes and serum resistance in Haemophilus influenzae type f isolates

The incidence of invasive infections due to Haemophilus influenzae has decreased significantly in developed countries with high rates of vaccination against H. influenzae serotype b (Hib). This vaccine provides no protection against H. influenzae serotype f (Hif), typically associated with invasive infections in adults with chronic disease and/or immunodeficiency, and rarely in otherwise healthy adults and children. The specific properties of Hif associated with virulence remain largely uncharacterized. A panel of 26 Hif strains consisting of both invasive disease-associated and mucosal surface non-invasive disease-associated isolates was surveyed by DNA fingerprinting, biotyping and PCR detection of hmw1, hmw2, hsf, the hif fimbrial locus and the lipo-oligosaccharide (LOS) biosynthetic island, and assessment of β-lactamase expression and determination of resistance to the bactericidal activity of normal adult human serum. Repetitive sequence-based PCR fingerprinting differentiated the 26 strains into three clusters, with the majority of isolates (22/26, 84.6 %) clustered into a single indistinguishable group. Most isolates (24/26, 92.3 %) were of biotype I and two isolates produced β-lactamase with detection of a conjugative plasmid, and the isolates displayed a range of resistances to the bactericidal activity of human serum. All 26 isolates carried the adhesin hsf, 21 carried a partial hif fimbrial operon and 4 had the adhesin genes hmw1/2. A LOS biosynthetic island was detected in 20 isolates consisting of the genes lic2BC. It was concluded that Hif has many recognized virulence properties and comprises a relatively homogeneous group independent of the anatomical source from which it was isolated.

Invasive Haemophilus influenzae Disease, Europe, 1996-2006

An international collaboration was established in 1996 to monitor the impact of routine Haemophilus influenzae type b (Hib) vaccination on invasive H. influenzae disease; 14 countries routinely serotype all clinical isolates. Of the 10,081 invasive H. influenzae infections reported during 1996-2006, 4,466 (44%, incidence 0.28 infections/100,000 population) were due to noncapsulated H. influenzae (ncHi); 2,836 (28%, 0.15/100,000), to Hib; and 690 (7%, 0.036/100,000), to non-b encapsulated H. influenzae. Invasive ncHi infections occurred in older persons more often than Hib (median age 58 years vs. 5 years, p<0.0001) and were associated with higher case-fatality ratios (12% vs. 4%, p<0.0001), particularly in infants (17% vs. 3%, p<0.0001). Among non-b encapsulated H. influenzae, types f (72%) and e (21%) were responsible for almost all cases; the overall case-fatality rate was 9%. Thus, the incidence of invasive non-type b H. influenzae is now higher than that of Hib and is associated with higher case fatality.

Epidemiology of Haemophilus influenzae serotype a, North American Arctic, 2000-2005

Serotype a is now the most common seen in the North American Arctic; highest rates occur in indigenous children.

Before the introduction of Haemophilus influenzae type b (Hib) conjugate vaccines, rates of invasive H. influenzae disease among indigenous people of the North American Arctic were among the highest in the world. Routine vaccination reduced rates to low levels; however, serotype replacement with non–type b strains may result in a reemergence of invasive disease in children. We reviewed population-based data on invasive H. influenzae in Alaska and northern Canada from 2000–2005; 138 cases were reported. Among 88 typeable isolates, 42 (48%) were H. influenzae type a (Hia); 35 (83%) occurred in indigenous peoples. Among Hia patients, median age was 1.1 years; 62% were male; 1 adult died. Common clinical manifestations included meningitis, pneumonia, and septic arthritis. Overall annual incidence was 0.9 cases per 100,000 population. Incidence among indigenous children <2 years of age in Alaska and northern Canada was 21 and 102, respectively. Serotype a is now the most common H. influenzae serotype in the North American Arctic; the highest rates are among indigenous children.

Haemophilus influenzae meningitis 5 years after introduction of the Haemophilus influenzae type b conjugate vaccine in Brazil

The long-term impact of Haemophilus influenzae type b (Hib) conjugate vaccine, introduced throughout Latin America in the late 1990s, has not been evaluated. Active surveillance for H. influenzae meningitis was performed from August 9, 1996 to August 8, 2004 in Metropolitan Salvador, Brazil. Five years after the introduction of Hib conjugate vaccine, Hib meningitis incidence decreased from 2.39 to 0.06 cases per 100,000 population (98%) overall, and from 60.9 to 3.1 cases per 100,000 population (95%) in children <1 year of age. A transient serotype replacement phenomenon was observed associated with a small increase of meningitis due to two H. influenzae type a clonal groups. These findings indicate that Hib immunization campaign has led to the virtual elimination of Hib disease in this region.

Reduced susceptibility of Haemophilus influenzae to the peptide deformylase inhibitor LBM415 can result from target protein overexpression due to amplified chromosomal def gene copy number

Previous genetic analysis of Haemophilus influenzae revealed two mechanisms associated with decreased susceptibility to the novel peptide deformylase inhibitor LBM415: AcrAB-TolC-mediated efflux and Fmt bypass, resulting from mutations in the pump repressor gene acrR and in the fmt gene, respectively. We have isolated an additional mutant, CDS23 (LBM415 MIC, 64 microg/ml versus 4 microg/ml against the parent strain NB65044) that lacks mutations in the acrR or fmt structural genes or in the gene encoding Def, the intracellular target of LBM415. Western immunoblot analysis, two-dimensional gel electrophoresis, and tryptic digestion combined with mass spectrometric identification showed that the Def protein was highly overexpressed in the mutant strain. Consistent with this, real-time reverse transcription-PCR revealed a significant increase in def transcript titer. No mutations were found in the region upstream of def that might account for altered expression; however, pulsed-field gel electrophoresis suggested that a genetic rearrangement of the region containing def had occurred. Using a combination of PCR, sequencing, and Southern blot analyses, it was determined that the def gene had undergone copy number amplification, explaining the high level of target protein expression. Inactivation of the AcrAB-TolC efflux pump in this mutant increased susceptibility 16-fold, highlighting the role of efflux in exacerbating the overall reduced susceptibility resulting from target overexpression.

Invasive Haemophilus influenzae in Manitoba, Canada, in the postvaccination era

Fifty-two Haemophilus influenzae isolates from patients with invasive disease in the province of Manitoba, Canada, were examined for serotype, biotype, genotype, and antibiotic susceptibility. Half of the 52 isolates were found to be serotype a, and 38.5% (20 isolates) were found to be nonserotypeable (NST). There were only three serotype b strains and one each for serotypes c, d, and f. All 26 serotype a isolates belonged to biotype II and demonstrated identical or highly similar DNA fingerprints by pulsed-field gel electrophoresis. An analysis of these isolates by multilocus sequence typing showed that they belong to the clonal complex ST-23. While 69% (18 of 26) of the serotype a cases were found in males, only 9 (45%) of the 20 patients with NST isolates were males. Twenty (77%) of the 26 serotype a isolates were from patients who were </=24 months old. Twelve (63%) of the NST isolates were from adult or adolescent patients. In contrast to the clonal nature of serotype a isolates, the 20 NST isolates were found to belong to 18 different sequence types. Most of these 18 different sequence types were unrelated to each other, with the exception of 7 sequence types grouped into three clonal groups. Two (6.25%) out of 32 serotypeable isolates (1 serotype a and 1 serotype b) and 6 (30%) of 20 NST isolates were resistant to ampicillin due to beta-lactamase production. These results suggest a change in the epidemiology of H. influenzae disease, with the majority of invasive H. influenzae isolates being associated with serotype a and NST strains.

Nontypeable Haemophilus influenzae strains with the capsule-associated insertion element IS1016 may mimic encapsulated strains

With the elimination of Haemophilus influenzae type b through vaccination, it has been suggested that other types of H. influenzae strains might acquire virulence traits and emerge as important pathogens. The gene sequence IS1016 has been associated with an increased capacity to cause severe infections. It is usually present in encapsulated strains but is sometimes harbored by nontypeable H. influenzae strains. To explore this further, 118 H. influenzae isolates, collected from both patients and healthy carriers, were investigated with PCR with reference to this gene sequence. Isolates positive for the insertion element were bio- and serotyped. The presence of hmw genes for adherence, the genetic profile, and the ability to form biofilm in vitro were investigated. A total of 15 isolates were IS1016-positive, whereof 12 were nontypeable. All 12 nontypeable isolates were obtained from healthy carriers, and 92% of the isolates were biotype I. They cross-reacted to some extent with type-specific antisera or exhibited a restricted genetic diversity like encapsulated strains. Furthermore, they lacked hmw-genes, and their ability to form biofilms was comparable with a capsule-deficient type b strain. Although this subset of strains mimicked traits usually exhibited by encapsulated strains, the isolation frequency did not seem to have been affected by vaccination.

Characteristics of Haemophilus influenzae type b responsible for meningitis in Poland from 1997 to 2004

Two hundred forty-five H. influenzae isolates responsible for meningitis in Poland from 1997 to 2004 were studied. Among these, 233 (95.1%) belonged to serotype b (Hib), 2 belonged to serotype f, and 10 were noncapsulated. The relatedness of all isolates was evaluated by pulsed-field gel electrophoresis (PFGE), and selected representatives were evaluated by multilocus sequence typing. Resistance to ampicillin was identified in 34 (14.6%) of the Hib isolates and was associated with the production of beta-lactamase only. Except for four isolates nonsusceptible to chloramphenicol, all isolates were susceptible to cefotaxime, ciprofloxacin, and rifampin. The PFGE analysis divided the Hib isolates into five PFGE types; however, all of them were possibly related. The most common PFGE type, with 25 subtypes, was characteristic for 97.4% of the isolates. The most prevalent PFGE subtype found in our study was also the most common among the Hib isolates responsible for invasive disease in Italy and the Czech Republic and was found among isolates causing lower respiratory tract infections in Poland. The most prevalent sequence types (STs) in the studied group were ST6 and ST92. Four new STs were found: ST188, ST189, ST190, and ST268. Results of this study support the evidence that the genetic structure of encapsulated H. influenzae is clonal. The continuing high number of meningitis cases due to Hib in Poland underlines the need for mass vaccination against Hib in Poland.

Prevalence and serotype distribution of encapsulated Haemophilus influenzae isolates from patients with lower respiratory tract infections in Poland

Among 1,272 H. influenzae isolates obtained from patients with lower respiratory tract infections, 5.6% were found to be encapsulated. The most common were serotypes b (40.3%) and e (38.9%) followed by serotypes f (16.7%) and d (4.1%). All encapsulated isolates within a particular serotype showed high genetic homogeneity and did not produce beta-lactamases.

Epidemiology of invasive Haemophilus influenzae type A disease among Navajo and White Mountain Apache children, 1988-2003

BACKGROUND

Before the introduction of Haemophilus influenzae type b (Hib) conjugate vaccines, rates of H. influenzae disease among Navajo and White Mountain Apache (WMA) children were among the highest reported worldwide. Routine Hib vaccination has significantly reduced rates of Hib disease in these populations. As Hib disease rates decrease to very low levels, there are concerns that non-type b strains of H. influenzae may emerge as more prevalent causes of invasive disease in children.

METHODS

We reviewed population-based, active laboratory surveillance data from the period of 1988-2003 for invasive H. influenzae type a (Hia) disease among Navajo and WMA children aged <5 years. Clinical information on cases was collected by chart review. A sample of Hia isolates from Navajo children was typed by pulsed-field gel electrophoresis (PFGE).

RESULTS

During 1988-2003, a total of 76 reported cases of invasive Hia disease occurred among Navajo and WMA children. The overall annual incidence was 20.2 cases per 100,000 population aged <5 years. There was no increase in Hia disease rates after Hib vaccination was introduced. The median age of patients was 12 months. Meningitis (50% of cases) was the most common presentation, followed by pneumonia (27.6%). Two children with Hia disease died. PFGE analysis revealed a limited genetic diversity of Hia strains in this population.

CONCLUSIONS

Active surveillance data showed high rates of invasive Hia disease among Navajo and WMA children but no increase in the incidence after Hib vaccination was introduced. The presentation of Hia disease is similar to that of Hib disease in the prevaccine era.

Emergence of nonencapsulated and encapsulated non-b-type invasive Haemophilus influenzae isolates in Portugal (1989-2001)

Phenotypes and genetic relatedness of invasive Haemophilus influenzae strains were evaluated from 1989 through 2001. Among 119 isolates, multidrug resistance decreased (from 50 to 0%), the level of H. influenzae serotype b (Hib) strains declined (from 81 to 16%), the level of noncapsulated strains rose (from 19 to 80%), and the first invasive H. influenzae serotype f strain was described. This study documents changes in invasive H. influenzae infections in Portugal, i.e., the emergence of non-type-b strains that are genetically diverse and unrelated to Hib.

Comparison of usefulness of randomly amplified polymorphic DNA and amplified-fragment length polymorphism techniques in epidemiological studies on nasopharyngeal carriage of non-typable Haemophilus influenzae

Randomly amplified polymorphic DNA (RAPD) and automated amplified-fragment length polymorphism (AFLP) techniques with fluorescently labelled primers were used to type non-serotypable Haemophilus influenzae (NTHI) isolates. Eighty-seven isolates from healthy children attending day-care centres or living at orphanages in southern Poland were investigated. Through comparison of the AFLP data with RAPD analysis, it has been concluded that the discriminatory power of AFLP for NTHI typing is higher than RAPD. Generally, the NTHI isolates analysed were highly heterogeneous, as detected with a HindIII/TaqI AFLP genotyping scheme on intra/inter similarity levels of 94 and 96 % using Pearson's correlation coefficient. The range of similarity values found for isolates from children permanently residing at a particular day-care centre was much wider than that for isolates from orphanages. AFLP can efficiently access NTHI strain diversity and can monitor their turn-over for comparative typing in local and inter-local epidemiological investigations.

Infections due to Haemophilus influenzae serotype E: microbiological, clinical, and epidemiological features

Surveillance after introduction of Haemophilus influenzae serotype b vaccination in Spain identified 26 H. influenzae serotype e (HiE) isolates. Of these, 16 (61.5%) were recovered from patients aged >16 years and 10 (38.5%) from children <16 years of age. HiE caused respiratory infections in 14 patients (9 with pneumonia), conjunctivitis in 4, vaginitis in 2, abscess in 2, and cellulitis, peritoneal infection, sepsis and meningitis in 1 patient each. HiE was strongly clonal and highly resistant to ampicillin and cotrimoxazole, and the incidence of HiE infection did not increase over time.

Invasive disease due to nontypeable Haemophilus influenzae among children in Arkansas

In this study, we reviewed cases of invasive disease due to nontypeable Haemophilus influenzae among children hospitalized at Arkansas Children's Hospital from 1993 to 2001. A total of 28 cases were examined, including 21 associated with bacteremia and 4 associated with meningitis. Of the patients examined, 86% were Collapse

Key Words Collapse

MESH Headings

• Adolescent
• Arkansas/epidemiology
• Bacteremia/epidemiology
• Bacteremia/microbiology
• Bacterial Proteins/genetics
• Blotting, Southern
• Child
• Child, Preschool
• Electrophoresis, Gel, Pulsed-Field
• Female
• Genetic Variation
• Genotype
• Haemophilus Infections/epidemiology
• Haemophilus Infections/microbiology
• Haemophilus influenzae/classification
• Haemophilus influenzae/genetics
• Hospitalization
• Humans
• Infant
• Infant, Newborn
• Male
• Sequence Analysis, DNA

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Grants

• R01 AI049322 NIAID NIH HHS
• CA023944-20 NCI NIH HHS
• R01-AI44167 NIAID NIH HHS
• R25 CA023944 NCI NIH HHS
• R01 DC002873 NIDCD NIH HHS
• P30 CA21765 NCI NIH HHS
• R01-AI49322 NIAID NIH HHS
• P30 CA021765 NCI NIH HHS
• R01 AI044167 NIAID NIH HHS
• R01-DC02873 NIDCD NIH HHS

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Affiliation(s)

Joshua M O'Neill Department of Pediatrics, University of Arkansas for Medical Sciences/Arkansas Children's Hospital, Little Rock, Arkansas, USA.
Joseph W St Geme
David Cutter
Elisabeth E Adderson
Juliana Anyanwu
Richard F Jacobs
Gordon E Schutze

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Pgi genotyping is a surrogate for serotyping of encapsulated Haemophilus influenzae

Study of the epidemiology of invasive infections caused by encapsulated Haemophilus influenzae has been complicated by the poor sensitivity and specificity of the serologic assays used to identify specific capsular polysaccharides. The population structure of these bacteria is highly clonal, however, and serotype is highly correlated with other genetic characteristics. We sought to determine if alleles of the highly conserved phosphoglucose isomerase (pgi) gene correspond to the serotypes of encapsulated H. influenzae strains. pgi alleles of 52 well-characterized encapsulated H. influenzae isolates were amplified by PCR, sequenced, and compared to one another and to additional previously reported H. influenzae pgi alleles. Overall, 83% of the strains possessed pgi alleles associated with the major serotype a, b, e, and f clonotypes that cause the most invasive disease in the United States. Six strains (four type a and two type f) had unusual pgi alleles, which suggested that these strains belonged to less common clonotypes of encapsulated bacteria or were actually nontypeable strains. pgi genotyping may provide a simple and stable surrogate for capsular serotyping. Further studies correlating pgi typing with the expression of capsule are likely to increase our understanding of the epidemiology and pathogenesis of these infections.

Prevalence and distribution of adhesins in invasive non-type b encapsulated Haemophilus influenzae

Adhesion to the respiratory epithelium plays an important role in Haemophilus influenzae infection. The distribution of H. influenzae adhesins in type b and nontypeable strains has been characterized, but little is known about the prevalence of these factors in non-type b encapsulated strains. We analyzed 53 invasive type a, type e, and type f strains for the presence of hap, hia, hmw, and hif genes; Hap, Hia, and HMW1/2 adhesins; and hemagglutinating pili. The hap gene was ubiquitous, and homologs of hmw and hia were present in 7 of 53 (13.2%) and 45 of 53 (84.9%) strains, respectively. Hap was detected in 28 of 45 (62.2%) hap(+) strains, HMW1/2 was detected in 5 of 7 (71.4%) hmw(+) strains, and Hia was detected in 31 of 45 (68.8%) hia(+) strains. The hif gene cluster was present in 26 of 53 strains (49.1%), and 21 of 26 hif(+) strains (80.8%) agglutinated (HA) red blood cells. Nine isolates exhibited HA but lacked the hif gene cluster. The distribution of adhesin genes correlated with the genetic relatedness of the strains. Strains belonging to one type a clonotype and the major type e clonotype possessed hia but lacked the hif cluster. Strains belonging to the second type a clonotype possessed both hia and hif genes. All type f strains belonging to the major type f clonotype possessed hia and lacked hifB. Although the specific complement of adhesin genes in non-type b encapsulated H. influenzae varies, most invasive strains express Hap and Hia, suggesting these adhesins may be especially important to the virulence of these organisms.

Characterization of encapsulated and noncapsulated Haemophilus influenzae and determination of phylogenetic relationships by multilocus sequence typing

A multilocus sequence typing (MLST) scheme has been developed for the unambiguous characterization of encapsulated and noncapsulated Haemophilus influenzae isolates. The sequences of internal fragments of seven housekeeping genes were determined for 131 isolates, comprising a diverse set of 104 serotype a, b, c, d, e, and f isolates and 27 noncapsulated isolates. Many of the encapsulated isolates had previously been characterized by multilocus enzyme electrophoresis (MLEE), and the validity of the MLST scheme was established by the very similar clustering of isolates obtained by these methods. Isolates of serotypes c, d, e, and f formed monophyletic groups on a dendrogram constructed from the differences in the allelic profiles of the isolates, whereas there were highly divergent lineages of both serotype a and b isolates. Noncapsulated isolates were distinct from encapsulated isolates and, with one exception, were within two highly divergent clusters. The relationships between the major lineages of encapsulated H. influenzae inferred from MLEE data could not be discerned on a dendrogram constructed from differences in the allelic profiles, but were apparent on a tree reconstructed from the concatenated nucleotide sequences. Recombination has not therefore completely eliminated phylogenetic signal, and in support of this, for encapsulated isolates, there was significant congruence between many of the trees reconstructed from the sequences of the seven individual loci. Congruence was less apparent for noncapsulated isolates, suggesting that the impact of recombination is greater among noncapsulated than encapsulated isolates. The H. influenzae MLST scheme is available at www.mlst.net, it allows any isolate to be compared with those in the MLST database, and (for encapsulated isolates) it assigns isolates to their phylogenetic lineage, via the Internet.

Position-based scanning for comparative genomics and identification of genetic islands in Haemophilus influenzae type b

Bacteria exhibit extensive genetic heterogeneity within species. In many cases, these differences account for virulence properties unique to specific strains. Several such loci have been discovered in the genome of the type b serotype of Haemophilus influenzae, a human pathogen able to cause meningitis, pneumonia, and septicemia. Here we report application of a PCR-based scanning procedure to compare the genome of a virulent type b (Hib) strain with that of the laboratory-passaged Rd KW20 strain for which a complete genome sequence is available. We have identified seven DNA segments or H. influenzae genetic islands (HiGIs) present in the type b genome and absent from the Rd genome. These segments vary in size and content and show signs of horizontal gene transfer in that their percent G+C content differs from that of the rest of the H. influenzae genome, they contain genes similar to those found on phages or other mobile elements, or they are flanked by DNA repeats. Several of these loci represent potential pathogenicity islands, because they contain genes likely to mediate interactions with the host. These newly identified genetic islands provide areas of investigation into both the evolution and pathogenesis of H. influenzae. In addition, the genome scanning approach developed to identify these islands provides a rapid means to compare the genomes of phenotypically diverse bacterial strains once the genome sequence of one representative strain has been determined.

Challenges for the development of vaccines against Haemophilus influenzae and Neisseria meningitidis

The development of protein-polysaccharide conjugate vaccines has had a major impact on Haemophilus influenzae type b disease. The application of this technology to Neisseria meningitidis is also striking, particularly for serogroup C. However, significant challenges exist for the development of vaccines against non-typeable H. influenzae and against N. meningitidis serogroup B. Issues such as non-vaccine-strain replacement and correlates of protection need to be addressed as well as the longer-term implications of vaccination against what are essentially 'normal' microflora.