Identification of new hmwA alleles from nontypeable Haemophilus influenzae

Distribution and Diversity of hmw1A Among Invasive Nontypeable Haemophilus influenzae Isolates in Iran

BACKGROUND

The pathogenesis of nontypeable Haemophilus influenzae (NTHi) begins with adhesion to the rhinopharyngeal mucosa. Almost 38-80% of NTHi clinical isolates produce proteins that belong to the High Molecular Weight (HMW) family of adhesins, which are believed to facilitate colonization.

METHODS

In the present study, the prevalence of hmwA, which encodes the HMW adhesin, was determined for a collection of 32 NTHi isolates. Restriction Fragment Length Polymorphism (RFLP) was performed to advance our understanding of hmwA binding sequence diversity.

RESULTS

The results demonstrated that hmwA was detected in 61% of NTHi isolates. According to RFLP, isolates were divided into three groups.

CONCLUSION

Based on these observations, it is hypothesized that some strains of nontypeable Haemophilus influenzae infect some specific areas more than other parts.

Prevalence, distribution, and sequence diversity of hmwA among commensal and otitis media non-typeable Haemophilus influenzae

Nontypeable Haemophilus influenzae (NTHi) are Gram-negative coccobacilli that colonize the human pharynx, their only known natural reservoir. Adherence to the host epithelium facilitates NTHi colonization and marks one of the first steps in NTHi pathogenesis. Epithelial cell attachment is mediated, in part, by a pair of high molecular weight (HMW) adhesins that are highly immunogenic, antigenically diverse, and display a wide range of amino acid diversity both within and between isolates. In this study, the prevalence of hmwA, which encodes the HMW adhesin, was determined for a collection of 170 NTHi isolates recovered from the middle ears of children with otitis media (OM isolates) or throats or nasopharynges of healthy children (commensal isolates) from Finland, Israel, and the U.S. Overall, hmwA was detected in 61% of NTHi isolates and was significantly more prevalent (P=0.004) among OM isolates than among commensal isolates; the prevalence ratio comparing hmwA prevalence among ear isolates with that of commensal isolates was 1.47 (95% CI (1.12, 1.92)). Ninety-five percent (98/103) of the hmwA-positive NTHi isolates possessed two hmw loci. To advance our understanding of hmwA binding sequence diversity, we determined the DNA sequence of the hmwA binding region of 33 isolates from this collection. The average amino acid identity across all hmwA sequences was 62%. Phylogenetic analyses of the hmwA binding revealed four distinct sequence clusters, and the majority of hmwA sequences (83%) belonged to one of two dominant sequence clusters. hmwA sequences did not cluster by chromosomal location, geographic region, or disease status.

Phase variation and host immunity against high molecular weight (HMW) adhesins shape population dynamics of nontypeable Haemophilus influenzae within human hosts

Nontypeable Haemophilus influenzae (NTHi) is a bacterium that resides within the human pharynx. Because NTHi is human-restricted, its long-term survival is dependent upon its ability to successfully colonize new hosts. Adherence to host epithelium, mediated by bacterial adhesins, is one of the first steps in NTHi colonization. NTHi express several adhesins, including the high molecular weight (HMW) adhesins that mediate attachment to the respiratory epithelium where they interact with the host immune system to elicit a strong humoral response. hmwA, which encodes the HMW adhesin, undergoes phase variation mediated by 7-base pair tandem repeats located within its promoter region. Repeat number affects both hmwA transcription and HMW-adhesin production such that as the number of repeats increases, adhesin production decreases. Cells expressing large amounts of HMW adhesins may be critical for the establishment and maintenance of NTHi colonization, but they might also incur greater fitness costs when faced with an adhesin-specific antibody-mediated immune response. We hypothesized that the occurrence of large deletion events within the hmwA repeat region allows NTHi cells to maintain adherence in the presence of antibody-mediated immunity. To study this, we developed a mathematical model, incorporating hmwA phase variation and antibody-mediated immunity, to explore the trade-off between bacterial adherence and immune evasion. The model predicts that antibody levels and avidity, catastrophic loss rates, and population carrying capacity all significantly affected numbers of adherent NTHi cells within a host. These results suggest that the occurrence of large, yet rare, deletion events allows for stable maintenance of a small population of adherent cells in spite of HMW adhesin specific antibody-mediated immunity. These adherent subpopulations may be important for sustaining colonization and/or maintaining transmission.

Comparative genomic analyses of 17 clinical isolates of Gardnerella vaginalis provide evidence of multiple genetically isolated clades consistent with subspeciation into genovars

Gardnerella vaginalis is associated with a spectrum of clinical conditions, suggesting high degrees of genetic heterogeneity among stains. Seventeen G. vaginalis isolates were subjected to a battery of comparative genomic analyses to determine their level of relatedness. For each measure, the degree of difference among the G. vaginalis strains was the highest observed among 23 pathogenic bacterial species for which at least eight genomes are available. Genome sizes ranged from 1.491 to 1.716 Mb; GC contents ranged from 41.18% to 43.40%; and the core genome, consisting of only 746 genes, makes up only 51.6% of each strain's genome on average and accounts for only 27% of the species supragenome. Neighbor-grouping analyses, using both distributed gene possession data and core gene allelic data, each identified two major sets of strains, each of which is composed of two groups. Each of the four groups has its own characteristic genome size, GC ratio, and greatly expanded core gene content, making the genomic diversity of each group within the range for other bacterial species. To test whether these 4 groups corresponded to genetically isolated clades, we inferred the phylogeny of each distributed gene that was present in at least two strains and absent in at least two strains; this analysis identified frequent homologous recombination within groups but not between groups or sets. G. vaginalis appears to include four nonrecombining groups/clades of organisms with distinct gene pools and genomic properties, which may confer distinct ecological properties. Consequently, it may be appropriate to treat these four groups as separate species.

The distributed genome hypothesis as a rubric for understanding evolution in situ during chronic bacterial biofilm infectious processes

Most chronic infectious disease processes associated with bacteria are characterized by the formation of a biofilm that provides for bacterial attachment to the host tissue or the implanted medical device. The biofilm protects the bacteria from the host's adaptive immune response as well as predation by phagocytic cells. However, the most insidious aspect of biofilm biology from the host's point of view is that the biofilm provides an ideal setting for bacterial horizontal gene transfer (HGT). HGT provides for large-scale genome content changes in situ during the chronic infectious process. Obviously, for HGT processes to result in the reassortment of alleles and genes among bacterial strains, the infection must be polyclonal (polymicrobial) in nature. In this review, we marshal the evidence that all of the factors are present in biofilm infections to support HGT that results in the ongoing production of novel strains with unique combinations of genic characteristics and that the continual production of large numbers of novel, but related bacterial strains leads to persistence. This concept of an infecting population of bacteria undergoing mutagenesis to produce a 'cloud' of similar strains to confuse and overwhelm the host's immune system parallels genetic diversity strategies used by viral and parasitic pathogens.

Variation in expression of HMW1 and HMW2 adhesins in invasive nontypeable Haemophilus influenzae isolates

Background

Among surface antigens of nontypeable Haemophilus influenzae (NTHi), the HMW1 and HMW2 proteins are the major adhesins promoting colonization of the upper respiratory tract. Since they are potential vaccine candidates, knowledge concerning variation in HMW proteins expression among clinical isolates is of great interest. In this study, expression of hmw1A and hmw2A genes was evaluated by quantitative real-time reverse transcription-PCR in 3 NTHi invasive isolates (strains 56, 72, 91) and in the prototype strain 12. Number of 7-bp repeats within the hmwA promoters and presence of HMW proteins by Western blotting were also determined.

Results

Results showed that gene transcription varied not only among different isolates but also between the hmw1A and hmw2A genes from the same isolate. Compared to that found in prototype strain 12, up-regulation of the hmw1A gene expression was found in strain 56, down-regulation of both hmw1A and hmw2A genes transcripts was observed in strain 72 whereas the two hmwA genes appeared differentially expressed in strain 91 with the hmw1A transcript enhanced but the hmw2A transcript reduced.

Conclusion

Increasing numbers of 7-bp repeats within the hmwA promoters generally correlated with decreased amounts of mRNA transcript, however additional control mechanisms contributing to modulation of hmw1A gene seem to be present.

Histidine auxotrophy in commensal and disease-causing nontypeable Haemophilus influenzae

Histidine biosynthesis is one of the best studied metabolic pathways in bacteria. Although this pathway is thought to be highly conserved within and between bacterial species, a previous study identified a genetic region within the histidine operon (his) of nontypeable strains of Haemophilus influenzae (NTHI) that was more prevalent among otitis media strains than among throat commensal NTHI strains. In the present study, we further characterized this region and showed that genes in the complete his operon (hisG, -D, -C, -NB, -H, -A, -F, and -IE) are >99% conserved among four fully sequenced NTHI strains, are present in the same location in these four genomes, and are situated in the same gene order. Using PCR and dot blot hybridization, we determined that the his operon was significantly more prevalent in otitis media NTHI strains (106/121; 87.7%) than in throat strains (74/137; 54%) (prevalence ratio, 1.62; P<0.0001), suggesting a possible role in middle ear survival and/or acute otitis media. NTHI strains lacking the his operon showed attenuated growth in histidine-restricted media, confirming them as his-negative auxotrophs. Our results suggest that the ability to make histidine is an important factor in bacterial growth and survival in the middle ear, where nutrients such as histidine may be found in limited amounts. Those isolates lacking the histidine pathway were still able to survive well in the throat, which suggests that histidine is readily available in the throat environment.

Antibodies specific for the high-molecular-weight adhesion proteins of nontypeable Haemophilus influenzae are opsonophagocytic for both homologous and heterologous strains

The HMW1/HMW2-like adhesion proteins of nontypeable Haemophilus influenzae (NTHI) are expressed by 75% of NTHI strains. Antibodies directed against these proteins are opsonophagocytic in vitro and are protective in an animal model of infection. The objective of the present study was to determine the opsonophagocytic activity of high-titer anti-HMW1/HMW2 immune sera against both homologous and heterologous NTHI strains. Chinchillas were immunized with purified HMW1/HMW2-like proteins from five prototype NTHI strains. Serum opsonophagocytic activity was monitored in an assay that uses a human promyelocytic cell line, HL-60, as the source of phagocytic cells. Preimmune sera did not demonstrate opsonophagocytic killing of any strains. In contrast, the immune sera demonstrated killing of the five homologous NTHI strains at titers ranging from 1:320 to 1:640. The immune sera also demonstrated killing of eight heterologous NTHI strains that express HMW1/HMW2-like proteins at titers ranging from 0 to 1:640. Killing of heterologous strains sometimes demonstrated a prozone phenomenon. None of the immune sera killed NTHI strains that did not express HMW1/HMW2-like proteins. Adsorption of immune sera with HMW1/HMW2-like proteins purified from either homologous or heterologous NTHI strains eliminated opsonophagocytic killing of homologous strains in most cases. These data demonstrate that antibodies produced following immunization with the HMW1/HMW2-like proteins are opsonophagocytic for both homologous and heterologous NTHI and strongly suggest that common epitopes recognized by functionally active antibodies exist on the HMW1/HMW2-like proteins of unrelated NTHI strains. The results argue for the continued investigation of the HMW1/HMW2-like proteins as potential vaccine candidates for the prevention of NTHI disease.

Surface anchoring of a bacterial adhesin secreted by the two-partner secretion pathway

In Gram-negative bacteria, most surface-associated proteins are present as integral outer-membrane proteins. Exceptions include the Haemophilus influenzae HMW1 and HMW2 adhesins and a subset of other proteins secreted by the two-partner secretion system. In the present study we sought to determine the mechanism by which HMW1 is anchored to the bacterial surface. In initial experiments we found that HMW1 forms hair-like fibres on the bacterial surface and is usually present as pairs that appear to be joined together at one end. Further analysis established that HMW1 is anchored to the multimeric HMW1B outer membrane translocator, resulting in a direct correlation between the level of surface-associated HMW1 and the quantity of HMW1B in the outer membrane. Mutagenesis and polyethylene glycol maleimide labelling revealed that anchoring of HMW1 requires the C-terminal 20 amino acids of the protein and is dependent upon disulphide bond formation between two conserved cysteine residues in this region. Immunolabelling studies demonstrated that the immediate C-terminus of HMW1 is inaccessible to surface labelling, suggesting that it remains in the periplasm or is buried in HMW1B. Coexpression of HMW1 lacking the C-terminal 20 amino acids and wild-type HMW1 supported the conclusion that the C-terminus of HMW1 occupies the HMW1B pore. These observations may have broad relevance to proteins secreted by the two-partner secretion system, especially given the conservation of C-terminal cysteine residues among surface-associated proteins in this family.

Conservation and diversity of HMW1 and HMW2 adhesin binding domains among invasive nontypeable Haemophilus influenzae isolates

The pathogenesis of nontypeable Haemophilus influenzae (NTHi) begins with adhesion to the rhinopharyngeal mucosa. In almost 80% of NTHi clinical isolates, the HMW proteins are the major adhesins. The prototype HMW1 and HMW2 proteins, identified in NTHi strain 12, exhibit different binding specificities. The two binding domains have been localized in regions of maximal sequence dissimilarity (40% identity, 58% similarity). Two areas within these binding domains have been found essential for full level adhesive activity (designated the core-binding domains). To investigate the conservation and diversity of the HMW1 and HMW2 core-binding domains among isolates, PCR and DNA sequencing were used. First, we separately amplified the hmw1A-like and hmw2A-like structural genes in nine invasive NTHi isolates, discovering two new hmwA alleles, whose sequences are herein reported. Then, the hmw1A-like and hmw2A-like PCR products were used as the template in nested PCR to produce amplicons encompassing the encoding sequences of the two core-binding domains. In-depth sequence analysis was then performed among sequences of each group, with the support of specific computer programs. Overall, extensive sequence diversity among isolates was highlighted. However, similarity plots showed patterns consisting of peaks of relatively high similarity alternating with strongly divergent regions. The phylogenetic tree clearly indicated the HMW1-like and HMW2-like core-binding domain sequences as two clusters. Distinct sets of conserved amino acid motifs were identified within each group of sequences using the MEME/MOTIFSEARCH tool. Since HMW adhesins could represent candidates for future vaccines, identification of specific patterns of conserved motifs in otherwise highly variable regions is of great interest.