Induction of competence in nonencapsulated and encapsulated strains ofHaemophilus influenzae

Serum resistance and phase variation of a nasopharyngeal non-typeable Haemophilus influenzae isolate

Haemophilus influenzae harbours a complex array of factors to resist human complement attack. As non-typeable H. influenzae (NTHi) strains do not possess a capsule, their serum resistance mainly depends on other mechanisms including LOS decoration. In this report, we describe the identification of a highly serum resistant, nasopharyngeal isolate (NTHi23) by screening a collection of 77 clinical isolates. For NTHi23, we defined the MLST sequence type 1133, which matches the profile of a previously published invasive NTHi isolate. A detailed genetic analysis revealed that NTHi23 shares several complement evading mechanisms with invasive disease isolates. These mechanisms include the functional expression of a retrograde phospholipid trafficking system and the presumable decoration of the LOS structure with sialic acid. By screening the NTHi23 population for spontaneous decreased serum resistance, we identified a clone, which was about 103-fold more sensitive to complement-mediated killing. Genome-wide analysis of this isolate revealed a phase variation in the N'-terminal region of lpsA, leading to a truncated version of the glycosyltransferase (LpsA). We further showed that a NTHi23 lpsA mutant exhibits a decreased invasion rate into human alveolar basal epithelial cells. Since only a small proportion of the NTHi23 population expressed the serum sensitive phenotype, resulting from lpsA phase-off, we conclude that the nasopharyngeal environment selected for a population expressing the intact and functional glycosyltransferase.

The haem–haemopexin utilization gene cluster (hxuCBA) as a virulence factor of Haemophilus influenzae

Haemophilus influenzae has an absolute growth requirement for a porphyrin source, which can be supplied in vitro by haem, haemoglobin, or the haemoglobin-haptoglobin, haem-haemopexin and haem-albumin complexes. Utilization of the haem-haemopexin complex is known to be mediated by the products of the hxuCBA gene cluster. It was demonstrated that hxuC, but not hxuA or hxuB, is also essential for the utilization of haem from haem-albumin complexes. Mutants of the type b strain E1a lacking genes in the hxuCBA gene cluster were examined for their ability to cause bacteraemia in rat models of invasive disease. In 5-day-old rats, mutants in the hxuCBA genes yielded a significantly reduced bacteraemic titre compared to the wild-type strain. In addition, 5-day-old rats infected with the hxuCBA mutant strains exhibited significantly improved survival rates compared to those infected with the wild-type strain. Mutations in the haemoglobin/haemoglobin-haptoglobin-binding protein genes (hgps), either alone or in combination with the hxuCBA mutations, had no impact on virulence in 5-day-old rats. In 30-day-old rats infected with either the hxuCBA mutants or the wild-type strains, there was no significant difference in the ability to establish bacteraemia although bacterial titres were lower in rats infected with the hxuCBA mutants than in those infected with the wild-type strain. These age-related differences in the impact of mutations in the hxuCBA gene cluster may be related to changes in levels of host haem-binding proteins during development of the rat.

Complex role of hemoglobin and hemoglobin-haptoglobin binding proteins in Haemophilus influenzae virulence in the infant rat model of invasive infection

Haemophilus influenzae requires an exogenous heme source for aerobic growth in vitro. Hemoglobin or hemoglobin-haptoglobin satisfies this requirement. Heme acquisition from hemoglobin-haptoglobin is mediated by proteins encoded by hgp genes. Both Hgps and additional proteins, including those encoded by the hxu operon, provide independent pathways for hemoglobin utilization. Recently we showed that deletion of the set of three hgp genes from a nontypeable strain (86-028NP) of H. influenzae attenuated virulence in the chinchilla otitis media model of noninvasive disease. The present study was undertaken to investigate the role of the hgp genes in virulence of the wild-type serotype b clinical isolate HI689 in the infant rat model of hematogenous meningitis, an established model of invasive disease requiring aerobic growth. Bacteremia of high titer and long duration (>14 days) and histopathologically confirmed meningitis occurred in >95% of infant rats challenged at 5 days of age with strain HI689. While mutations disrupting either the Hgp- or Hxu-mediated pathway of heme acquisition had no effect on virulence in infant rats, an isogenic mutant deficient for both pathways was unable to sustain bacteremia or produce meningitis. In contrast, mutations disrupting either pathway decreased the limited ability of H. influenzae to initiate and sustain bacteremia in weanling rats. Biochemical and growth studies also indicated that infant rat plasma contains multiple heme sources that change with age. Taken together, these data indicate that both the hgp genes and the hxuC gene are virulence determinants in the rat model of human invasive disease.

Characterization of three new competence-regulated operons in Haemophilus influenzae

Haemophilus influenzae is one of a growing number of bacteria in which the natural ability to uptake exogenous DNA for potential genomic transformation has been recognized. To date, several operons involved in transformation in this organism have been described. These operons are characterized by a conserved 22-bp regulatory element upstream of the first gene and are induced coincident with transfer from rich to nutrient-depleted media. The previously identified operons comprised genes encoding proteins that include members of the type II secretion system and type IV pili, shown to be essential for transformation in other bacteria, and other proteins previously identified as required for transformation in H. influenzae. In the present study, three novel competence operons were identified by comparative genomics and transcriptional analysis. These operons have been further characterized by construction of null mutants and examination of the resulting transformation phenotypes. The putative protein encoded by the HI0366 gene was shown to be essential for DNA uptake, but not binding, and is homologous to a protein shown to be required for pilus biogenesis and twitching motility in Pseudomonas aeruginosa. An insertion in HI0939 abolished both DNA binding and uptake. The predicted product of this gene shares characteristics with PulJ, a pseudopilin involved in pullulanase export in Klebsiella oxytoca.

Reduced severity of middle ear infection caused by nontypeable Haemophilus influenzae lacking the hemoglobin/hemoglobin–haptoglobin binding proteins (Hgp) in a chinchilla model of otitis media

Since Haemophilus influenzae lacks enzymes necessary for synthesis of the porphyrin ring, it has an absolute growth requirement for a porphyrin source. This requirement can be satisfied in vitro by hemoglobin and hemoglobin complexed to haptoglobin. The products of the hgp genes mediate the utilization of heme from hemoglobin-haptoglobin. These genes are also involved in the use of heme from hemoglobin, although additional gene products independently mediate the acquisition of heme from this substrate. Different strains of H. influenzae possess one to four hgp genes. A nontypeable H. influenzae mutant lacking all the hgp genes was constructed and compared to the wild-type strain in a chinchilla (Chinchilla lanigera) model of otitis media. Compared to the wild-type strain, the hgp-deficient mutant exhibited a significantly delayed onset of detectable middle ear infection and significantly reduced duration of infection as assessed by both video otoscopy and tympanometry and as evidenced by viable bacterial counts in middle ear effusions. In addition, the maximum bacterial load in the middle ears of chinchillas infected with the mutant strain was significantly reduced when compared to the parent. These data indicate that the hemoglobin/hemoglobin-haptoglobin binding proteins are required for bacterial proliferation during H. influenzae-induced otitis media in chinchillas.

Sialic acid metabolism's dual function in Haemophilus influenzae

Many bacterial commensals and pathogens use the sialic acids as carbon and nitrogen sources. In Escherichia coli, the breakdown of these sugars is catalysed by gene products of the nan (Nacylneuraminate) operon; other microorganisms may use a similar catabolic strategy. Despite the known ligand and antirecognition functions of the sialic acids, the contribution of their catabolism to infection or host colonization has never been directly investigated. We addressed these questions with Haemophilus influenzae type b, which metabolizes relatively few carbohydrates, using the infant-rat infection model. The predicted H. influenzae homologue (HI0142) of the E. coli sialic acid aldolase structural gene, nanA, was subcloned and mutagenized by insertion of a kanamycin resistance cassette. Phenotypic investigation of the resulting H. influenzae aldolase mutants showed that: (i) HI0142 is essential for sialic acid degradation; (ii) the products of the open reading frames (ORFs) flanking HI0142 (HI0140, 41, 44 and 45) are likely to have the same functions as those of their counterparts in E. coli; (iii) sialylation of the lipooligosaccharide (LOS) epitope recognized by monoclonal antibody 3F11 is dependent on an environmental source of sialic acid; (iv) a nanA mutant hypersialylates its LOS sialyl acceptor, corresponding to an apparent increased fitness of the mutant in the infant-rat model; and (v) expression of the LOS sialyl acceptor is altered in cells grown without exogenous sialic acid, indicating the direct or indirect effect of sialic acid metabolism on LOS antigenicity. Taken together the data show the dual role of sialic acid catabolism in nutrition and cell surface modulation.

Creation of an isogenic P1-deficient mutant of Haemophilus influenzae biogroup aegyptius

Haemophilus influenzae biogroup aegyptius, the causative agent of Brazilian purpuric fever (BPF), expresses a heat-modifiable 48 kDa outer membrane protein, P1, which is conserved in most Brazilian case-clone isolates. To study the role of P1 in pathogenesis of BPF we constructed via homologous recombination an isogenic P1-deficient mutant of H. influenzae biogroup aegyptius. The procedure involved a modification of Hererot's method for development of competence. Modifications included variations in the growth conditions, use of cAMP, specific characteristics of the donor DNA, and antibiotic selection. P1-deficient mutants were confirmed by SDS-PAGE, loss of reactivity with a specific monoclonal antibody on Western blot, restriction analysis and Southern blot. Our results establish the first successful transformation of homologous DNA into H. influenzae biogroup aegyptius.