Limited genetic diversity of Haemophilus influenzae (type b)

The Haemophilus influenzae Type b hcsA and hcsB gene products facilitate transport of capsular polysaccharide across the outer membrane and are essential for virulence

Haemophilus influenzae type b is a common cause of invasive bacterial disease, especially among children in underdeveloped countries. The type b polysaccharide capsule is a polymer of ribose and ribitol-5-phosphate and is a critical determinant of virulence. Expression of the type b capsule is dependent upon the cap b locus, which consists of three functionally distinct regions, designated regions 1 to 3. Region 3 contains the hcsA and hcsB genes, which share significant homology with genes that have been implicated in encapsulation in other pathogenic bacteria but have unclear functions. In this study, we inactivated hcsA alone, hcsB alone, and both hcsA and hcsB together and examined the effects of these mutations on polysaccharide transport and bacterial virulence properties. Inactivation of hcsA alone resulted in accumulation of polysaccharide in the periplasm and a partial decrease in surface-associated polysaccharide, whereas inactivation of hcsB alone or of both hcsA and hcsB together resulted in accumulation of polysaccharide in the periplasm and complete loss of surface-associated polysaccharide. All mutations eliminated serum resistance and abrogated bacteremia and mortality in neonatal rats. These results indicate that the hcsA and hcsB gene products have complementary functions involved in the transport of polysaccharide across the outer membrane and are essential for virulence.

Molecular epidemiology of Haemophilus influenzae type b isolated from children with clinical cases of conjugate vaccine failures

We analyzed eight invasive Haemophilus influenzae strains isolated from individual children vaccinated appropriately for their ages. Five of the strains were type b and three were nontypeable strains. Children infected with nontypeable strains had a protective level of anti-H. influenzae type b antibodies, while only one of the children whose cases represented true vaccine failure did.

The dilution rate affects the outer membrane protein and lipopolysaccharide composition of Haemophilus influenzae type b grown under iron limitation

When Haemophilus influenzae type b was grown under iron limitation in continuous culture, the dilution rate affected the outer membrane protein and lipopolysaccharide composition. Investigations of the effect of the reduced availability of iron or other environmental parameters on these surface components should be controlled for growth rate.

The Haemophilus influenzae capsulation gene cluster: a compound transposon

The population of capsulate Haemophilus influenzae is divided into two phylogenetic divisions. Here we show that in division I strains the capsulation (cap) gene cluster lies between direct repeats of a novel insertion sequence (IS)-like element, IS1016. cap has apparently been mobilized in the chromosome as a compound transposon by IS1016, and the repeats have provided a molecular substrate for reversible cap gene amplification, with augmentation of capsule production, through unequal homologous recombination. Such amplification has occurred in serotype b strains, but in these a large direct repeat of cap genes has become fixed in the population. We have found a 1.2 kb deletion at one end of this duplicated capb locus, removing most of one copy of the polysaccharide export gene bexA. We have shown that this makes capsulation dependent on preservation of the direct repeat structure in order to avoid recombination-mediated loss of the other copy of bexA. Type b strains with this cap configuration are disseminated worldwide and currently cause nearly all invasive Haemophilus infections, leading us to speculate that the 1.2 kb deletion occurred in an ancestral type b strain and conferred significant biological advantage.

Loss of capsule expression by Haemophilus influenzae type b results in enhanced adherence to and invasion of human cells

Haemophilus influenzae type b is a common cause of systemic bacterial disease in children, and the serotype b capsule is a major determinant of virulence. Nevertheless, as a consequence of the genetic configuration of the capb locus, type b strains become capsule deficient at a high frequency. To investigate the potential biological relevance of the predisposition to capsule loss, we compared the adherent and invasive abilities of several strains of H. influenzae type b and their isogenic capsule-deficient mutants by using cultured human epithelial cells. In all cases the capsule-deficient mutant demonstrated significantly greater adherence and invasion than the encapsulated parent. Transformation of one capsule-deficient mutant to restore encapsulation resulted in a marked decrease in adherence and invasion. All strains were capable of adherence and invasion by a pilus-independent mechanism. We conclude that capsule loss by H. influenzae type b results in enhanced in vitro adherence and invasion, properties that may be relevant to colonization of the nasopharynx and persistence within the respiratory tract. These observations suggest an explanation for the evolution of the capb locus as directly repeated segments of DNA with a consequent predisposition to recombination resulting in capsule loss.

The bex locus in encapsulated Haemophilus influenzae: a chromosomal region involved in capsule polysaccharide export

The nucleotide sequence of a 5.1 kb region in the Haemophilus influenzae type b capsulation locus has been determined and found to contain four open reading frames: bexD, bexC, bexB, and bexA. Comparison of the deduced products of bexC, bexB, and bexA to known proteins, and TnphoA mutagenesis, suggests that they form components of an ATP-driven polysaccharide export apparatus. Furthermore, close sequence similarity between BexA and BexB and products of the kpsT and kpsM genes at the Escherichia coli K5 capsulation locus (Smith et al., 1990--accompanying paper) suggests that capsulation genes in these organisms may have a common ancestry.

Genetic heterogeneity of Porphyromonas (Bacteroides) gingivalis by genomic DNA fingerprinting

This study describes the use of total genomic DNA fingerprinting with the use of restriction endonucleases to characterize clinical isolates of Porphyromonas gingivalis (Bacteroides gingivalis) obtained from patients with periodontitis or with root-canal infections. The majority of independent isolates had a unique DNA fingerprint, indicating extensive genetic heterogeneity within this species. Twenty-nine distinct DNA fingerprints were found among the 33 isolates investigated. This is in contrast to biotyping and serotyping, where only one type and three types, respectively, have been reported. The observed heterogeneity indicates that DNA fingerprinting is a sensitive measure of genetic dissimilarity between P. gingivalis isolates and is able to characterize individual isolates. These results have ecological implications, indicating that there is considerable natural diversity in the global population of P. gingivalis, and that there are likely to be relatively large numbers of genetically distinct clonal lines. Furthermore, DNA fingerprinting is a sensitive and powerful tool for longitudinal and cross-sectional epidemiological studies. This technique provides far greater discrimination between isolates than either biotyping or serotyping, and will be most helpful in, for example, the analysis of distribution of clonal lines within one periodontal patient, or the analysis of the transmission to and turnover of strain populations within a patient population, since the probability of two strains with the same DNA fingerprint being found by chance is small.

Antigenic relationships among the porin proteins of encapsulated Haemophilus influenzae clones

Monoclonal antibodies (Mabs) specific for Haemophilus influenzae were generated to identify antigenic determinants shared among encapsulated H. influenzae clones. Sixteen MAbs reacted by Western immunoblot with a protein of an approximate molecular size of 40 kilodaltons corresponding to the P2 major outer membrane protein (porin). These MAbs also reacted with purified and recombinant H. influenzae porin. Fourteen of the MAbs recognized cell surface-exposed epitopes, and two of the MAbs, P2-16 and P2-17, identified epitopes that are not present or are not accessible on the cell surface. The reactivity spectrum of the MAb panel was studied by dot immunoassay against 32 serologically nontypeable and 119 encapsulated H. influenzae strains recovered worldwide, representing the major serotype a, b, and d clone families. MAbs P2-4 and P2-6 recognized only serotype b clones assigned to primary phylogenetic division I. These clones account for more than 99% of all invasive episodes worldwide. MAbs P2-3, P2-8, and P2-11 reacted with division I serotype b isolates and also identified all genetically allied strains expressing serotype a and d polysaccharide capsules. In contrast, none of the 16 MAbs reacted with genetically divergent serotype a or b clones assigned to primary phylogenetic division II. These results demonstrate that, in general, the patterns of P2 protein surface epitope exposure are cognate with genetic lineages of encapsulated H. influenzae strains and support the hypothesis that the population structure of encapsulated H. influenzae is predominantly clonal.

Major subtypes of invasive Haemophilus influenzae from 1983 to 1985 in Atlanta, Ga

We compared outer membrane protein (OMP) patterns of Haemophilus influenzae isolated in metropolitan Atlanta, Ga., from July 1983 to June 1985. Of 74 randomly selected H. influenzae serotype b, biotype I, isolates (24% of the total number of H. influenzae, and 32% of the total number of H. influenzae serotype b, biotype I, isolates), 66 (89.2%) had the same OMP pattern. Of the remaining eight, five (6.7%) had an identical OMP pattern. The other three isolates had separate and distinct patterns. A greater diversity of OMP patterns was found with H. influenzae serotype b, biotype II, and nonserotypeable H. influenzae. Of the 18 H. influenzae serotype b, biotype II, isolates (5.8% of the total number of H. influenzae isolates), 1 had an OMP pattern similar to that of the predominate biotype I OMP type, 6 (33% of the biotype II) had the same pattern, and 11 had heterogeneous patterns. Of the 19 recoverable, nonserotypeable biotype II isolates (6.8% of the total number of H. influenzae), 18 had different OMP patterns, and no pattern was similar to those observed with serotype b. These findings indicate that most H. influenzae strains isolated during this 2-year period were indistinguishable by serotype, biotype, or OMP patterns.

Molecular genetic analysis of Actinobacillus actinomycetemcomitans epidemiology

Research over the past decade has identified many of the microorganisms involved in the etiology of human periodontitis such as Actinobacillus actinomycetemcomitans. Efforts are now directed toward defining these species' role in the pathogenic process. Since microbial colonization of host tissues is a key first step in developing a bacterial infection, determining the source of the periodontal pathogens and their route of transmission is likely to be crucial in formulating preventive strategies. Recently, a technique from molecular biology, restriction endonuclease analysis, has been used to track bacterial infections. In the present study, this method was used to investigate the epidemiology of A. actinomycetemcomitans infection. One hundred twenty-four human subgingival plaque isolates of A. actinomycetemcomitans were examined including bacterial strains from the United States, Korea, and Norway as well as 15 strains from cynomolgus (Macaca fascicularis) and spider monkeys (Macaca iris) and 4 reference strains. The genomic DNA from each strain was purified, digested with each of 16 restriction endonucleases, and the DNA digests were resolved by electrophoresis. The resulting patterns of DNA fragments were compared and also correlated with the A. actinomycetemcomitans serotype determined using serotype-specific antisera in immunofluorescence. Human isolates of A. actinomycetemcomitans even from disparate geographic sources showed little diversity by restriction endonuclease analysis. Three major restriction patterns were found. Restriction pattern I was common to all 20 of the serotype a isolates, restriction pattern II was associated with 58% of the 73 serotype b isolates examined, while restriction pattern III was associated with the remaining serotype b strains and with all 15 of the serotype c strains.(ABSTRACT TRUNCATED AT 250 WORDS)

Genetic and phenotypic variation in the capsulation and virulence of culture collection strains of Haemophilus influenzae type b

Haemophilus influenzae type b strains deposited in the National Reference Collections of the United States, United Kingdom and Sweden are derived from strains isolated in the United States during the 1940s. With respect to both the genetic locus for capsulation and to virulence, assessed in the infant rat bacteremia model, these strains are no longer representative of clinical isolates which currently cause meningitis and other serious infections in children all over the world. Alternative well-characterized strains resembling current common clinical isolates may often be more suitable for the study of Haemophilus influenzae pathogenicity.

Identification and characterization of a serotype b-specific segment of the Haemophilus influenzae genome

Within the region of the Haemophilus influenzae serotype b genome required for capsule expression, a 4.6-kilobase segment was cloned, used as a hybridization probe, and found to be specific for serotype b strains. Deletion mutagenesis within this segment defined a locus necessary for serotype b capsule production and another involved in maintenance of wild-type colony appearance.

Common organization of chromosomal loci for production of different capsular polysaccharides in Haemophilus influenzae

Cloned Haemophilus influenzae type b capsulation genes were used as hybridization probes to isolate DNA from the capsulation loci (cap) of other serotypes of H. influenzae. Mapping of the resulting clones and Southern hybridization analysis of chromosomal DNAs from type a, b, c, and d strains showed that in each strain cap was organized in the same way: a central DNA segment specific to each serotype flanked by DNA segments of common structure. We infer that enzymes necessary for the synthesis of specific capsular polysaccharide are encoded in the central segment of cap, while proteins involved in a more general way in the process of capsulation are encoded in the flanking segments. Studies of the function of the DNA in one of these non-serotype-specific flanking segments (J. S. Kroll, I. Hopkins, and E. R. Moxon, Cell 53:347-356, 1988) have previously identified a gene encoding a protein necessary for polysaccharide export, an event now deduced to proceed by a mechanism independent of the nature of the disaccharide subunit in the polysaccharide. The near-total duplication of cap that has been found in most type b strains was not found at the analogous locus in the other serotypes. This reinforces our previous hypothesis, based on study of type b strains alone, that while such a duplication is unnecessary for capsulation, it confers some unexplained survival advantage on the widely prevalent strains with this clinically important serotype.

Evolutionary genetics of the encapsulated strains of Haemophilus influenzae

Genetic relationships among 2209 isolates of Haemophilus influenzae of polysaccharide capsule serotypes a, b, c, d, e, and f were determined by analyzing electrophoretically demonstrable allelic variation at 17 chromosomal enzyme loci. We distinguished 280 electrophoretic types (ETs), representing distinctive multilocus genotypes. Genetic diversity among ETs of isolates of the same serotype was, on average, only 67% of that in the total sample, and no ETs were shared among isolates of different serotypes. Cluster analysis of the ETs revealed 2 primary phylogenetic divisions at a genetic distance of 0.66 and 12 major lineages diverging from one another at distances greater than 0.42. In general, strains of different phylogenetic lines or groups of allied lineages have characteristic cap region restriction fragment length polymorphism patterns obtained by digestion of genomic DNA with EcoRI. Strains producing serotype c, e, and f capsules have no close relationships to those of other encapsulated strains. Lineages of both serotype a and b strains occur in each primary phylogenetic division, most probably as a result of the transfer of serotype-specific sequences of the cap region between clonal lineages. Serotype a strains allied in division I with a group of abundant serotype b clones and the serotype d strains apparently are more virulent than the serotype a strains in division II, which are related to serotype b and f strains that rarely cause invasive disease.

Clonal population structure of encapsulated Haemophilus influenzae

Chromosomal genotypes of 2,209 isolates of the six polysaccharide capsule types of Haemophilus influenzae recovered from human hosts worldwide were characterized by an analysis of electrophoretically demonstrable allelic profiles at 17 metabolic enzyme loci. For 222 representative isolates, restriction fragment length polymorphism patterns produced by digestion of cap region DNA were also determined. With few exceptions, isolates belonging to individual phylogenetic lines or groups of allied lineages identified by multilocus enzyme electrophoresis had characteristic cap region restriction fragment length polymorphism patterns and characteristic combinations of cap region patterns and outer membrane protein types. The occurrence of strong associations of characters and the recovery of isolates with identical genetic properties in widely separated geographic regions and over a 40-year period indicated that the population structure of encapsulated H. influenzae is clonal. Recombination of chromosomal genes, including those mediating capsule synthesis, apparently is not a major factor in the short-term evolution of these pathogenic organisms and, therefore, may be of minor clinical significance.

Capsule loss in H. influenzae type b occurs by recombination-mediated disruption of a gene essential for polysaccharide export

The capsulation locus cap in H. influenzae type b contains directly repeated segments of DNA flanking a bridge region. Here we show that this bridge region contains a gene, bexA, encoding a 24.7 kd protein essential for export of capsular polysaccharide. bexA is disrupted, with loss of part of its coding sequence, in the spontaneous reduction of the duplicated cap locus to single-copy that accompanies loss of capsule expression. The predicted amino acid sequence of BexA aligns significantly with that of MalK from E. coli and with HisP and OppD of S. typhimurium. Thus, polysaccharide export might occur via an energy-dependent transporter with similarities to those identified for the import of various substrates into Gram-negative bacteria, BexA being the "energizer" of the transporter.

Structural and serological relatedness of Haemophilus influenzae type b pili

The structural and serological relatedness of the pilus proteins of several isolates of Haemophilus influenzae type b cultured from patients with invasive disease and from different anatomic sites within the same patient was examined. Epithelial cell-adherent variants of 25 nonadherent parent isolates were obtained by selection for organisms that adhered to human erythrocytes. Outer membrane protein analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed the presence of an additional 24- to 24.5-kilodalton protein among all adherent variants but absent from all nonadherent parent isolates. Polyclonal rabbit antiserum against the intact native pilus protein of H. influenzae M43 cross-reacted with 20 of 25 adherent H. influenzae in both immunodot and slide-agglutination assays. No differences in reactivity among isolates cultured from more than one anatomic site in the same patient were noted. Anti-M43 pilus antiserum had bactericidal activity against both the homologous strain and a heterologous strain that demonstrated serologic identity in the immunodot and slide agglutination assays. The adherence of these strains to human epithelial cells in vitro was inhibited by Fab fragments purified from the antipilus antiserum. These data indicate that a remarkable degree of homogeneity in pilin subunit size exists among the pili of H. influenzae type b and that major antigenic determinants are shared among most of these pili. Also, antibodies directed against H. influenzae pilus proteins may be able to contribute to host defenses through serum bactericidal activity and by blocking the adherence of this bacterium to host epithelial cells.

Type b capsular polysaccharide as a virulence factor of Haemophilus influenzae

The capsular polysaccharide of Haemophilus influenzae serotype b [(3)-beta-D-ribose-(1-1)-ribitol-5-phosphate] is a major virulence factor and a target for serum antibodies which protect individuals against invasive infections. Studies in an experimental rat model of meningitis, using genetically defined H. influenzae transformants, provide evidence that chromosomal genes within or limited to a region (cap b) containing genes necessary for type b capsule are critical for efficient intravascular survival of H. influenzae. Within cap b there is a duplication of a 17 kb region organized as direct repeats separated by a smaller (1-2 kb) region of non-repeated DNA. Homologous recombination between the direct repeats is rec dependent and results in high-frequency loss of capsule expression and virulence.

Limited diversity of the immunoglobulin A1 protease gene (iga) among Haemophilus influenzae serotype b strains

Immunoglobulin A1 (IgA1) proteases are thought to be important virulence factors in certain bacterial infections, including meningitis, and may have potential usage in vaccines. In this study, we compared the locations of EcoRI, BamHI, and PstI restriction endonuclease sites in the IgA1 protease gene (iga) region of whole-cell DNA from 76 Haemophilus influenzae strains. The analysis was performed by using isolated fragments of the cloned iga gene, which encodes the IgA1 protease originating from a H. influenzae serotype d strain, as probes in Southern blot experiments. All strains, including three without detectable IgA1 protease activity, had DNA sequences with a high degree of homology to the iga probes. The numbers and sizes of the DNA fragments hybridizing with the probes indicated that only three strains, none of which was of serotype b, had more than one iga gene. The iga restriction fragment length patterns of 60 clinical isolates of serotype b were of only four distinct types, which correlated with previously observed clusters of multilocus genotypes (electrophoretic types). This correlation supports the concept of the clonal population structure of H. influenzae. Three of the iga gene restriction types, which appear to represent 98% of the H. influenzae serotype b population, encode IgA1 proteases that were inhibited by antisera to any one of these types and therefore could form the basis for the development of a vaccine against H. influenzae meningitis.

Capsulation and gene copy number at the cap locus of Haemophilus influenzae type b

Although more than 98% of natural isolates of Haemophilus influenzae type b carry a duplication of 17 kilobases (kb) of DNA at the chromosomal capsulation locus, only one copy is required for capsulation. In one laboratory-derived and two clinical type b strains, the capsulation locus had a single copy of this 17-kb segment, together with 1.3 kb of DNA identified as lying between the repeats of the duplicated locus. This 1.3 kb appears to be crucial for capsule production, since strains lacking it, although retaining a 17-kb segment, were capsule deficient. On comparing capsule polysaccharide production by these three type b strains with that by a prototypic type b strain with a duplicated locus, a gene dosage effect was demonstrated, with a halving of detectable polysaccharide in the single-copy strains. Despite this reduction in polysaccharide, these strains retained virulence potential as evidenced by bacteremia and meningitis in infant rats. As well as subserving augmented capsule polysaccharide production, a duplicated configuration of the type b cap locus endows strains with genetic instability not found in capsulate single-copy variants. We speculate that a survival advantage might be conferred on strains carrying a duplication at this locus as a result of gene dosage, the genetic instability of the locus, or both.

Haemophilus influenzae serotype a: outer membrane protein classification and correlation with DNA polymorphism at the cap locus

Fifty-four temporally and geographically distinct isolates of Haemophilus influenzae serotype a were characterized by (i) outer membrane protein composition and (ii) DNA polymorphism at the cap locus. In 53 of 54 cases, knowledge of the outer membrane protein type was accurately predictive of the cap locus polymorphism, suggesting that the majority of H. influenzae (type a) isolates have evolved from a limited number of clones.