Effect of amplification of the Cap b locus on complement-mediated bacteriolysis and opsonization of type b Haemophilus influenzae

Determinants of bacterial survival and proliferation in blood

Bloodstream infection is a major public health concern associated with high mortality and high healthcare costs worldwide. Bacteremia can trigger fatal sepsis whose prevention, diagnosis, and management have been recognized as a global health priority by the World Health Organization. Additionally, infection control is increasingly threatened by antimicrobial resistance, which is the focus of global action plans in the framework of a One Health response. In-depth knowledge of the infection process is needed to develop efficient preventive and therapeutic measures. The pathogenesis of bloodstream infection is a dynamic process resulting from the invasion of the vascular system by bacteria, which finely regulate their metabolic pathways and virulence factors to overcome the blood immune defenses and proliferate. In this review, we highlight our current understanding of determinants of bacterial survival and proliferation in the bloodstream and discuss their interactions with the molecular and cellular components of blood.

Epidemic Trends and Biofilm Formation Mechanisms of Haemophilus influenzae: Insights into Clinical Implications and Prevention Strategies

Haemophilus influenzae (H. influenzae) is a significant pathogen responsible for causing respiratory tract infections and invasive diseases, leading to a considerable disease burden. The Haemophilus influenzae type b (Hib) conjugate vaccine has notably decreased the incidence of severe infections caused by Hib strains, and other non-typable H. influenzae (NTHi) serotypes have emerged as epidemic strains worldwide. As a result, the global epidemic trends and antibiotic resistance characteristics of H. influenzae have been altered. Researches on the virulence factors of H. influenzae, particularly the mechanisms underlying biofilm formation, and the development of anti-biofilm strategies hold significant clinical value. This article provides a summary of the epidemic trends, typing methods, virulence factors, biofilm formation mechanisms, and prevention strategies of H. influenzae. The increasing prevalence of NTHi strains and antibiotic resistance among H. influenzae, especially the high β-lactamase positivity and the emergence of BLNAR strains have increased clinical difficulties. Understanding its virulence factors, especially the formation mechanism of biofilm, and formulating effective anti-biofilm strategies may help to reduce the clinical impact. Therefore, future research efforts should focus on developing new approaches to prevent and control H. influenzae infections.

Genomic insights of international clones of Haemophilus influenzae causing invasive infections in vaccinated and unvaccinated infants

The emergence of invasive Haemophilus influenzae infections in vaccinated patient is a public health concern. We have investigated the genomic basis of invasiveness and possible vaccine failure in H. influenzae causing invasive disease in vaccinated and unvaccinated children in Brazil. Three H. influenzae strains isolated from blood cultures of pediatric patients were sequenced. Serotype, MLST, resistome and virulome were predicted using bioinformatic tools, whereas single nucleotide polymorphisms (SNPs) analysis of cap loci and the presence of the putative virulence-enhancing IS1016-bexA partial deletion were predicted in silico. Infections were caused by H. influenzae type a (Hia), type b (Hib) and nontypeable (NTHi), belonging to international high-risk clones of sequence types ST23, ST6 and ST368, respectively, which have been identified in North American, European and Asian countries. Convergence of ampicillin resistance and virulence in Hib-ST6 was supported by bla_TEM-1B and deletion in the bexA gene, whereas presence of SNPs in the cap-b locus was associated with antigenic modifications of the capsule structure. Hia-ST23 and NTHi-ST368 strains carried galU, lpsA, opsX, rfaF, iga1, lgtC and lic1/lic2 virulence genes, associated with colonization, adaptation and damage to the lung, or invasiveness. In summary, deletion in the bexA gene and presence of SNPs in the cap locus of Hib could be contributing to invasive disease and possible vaccine failure in pediatric patients, whereas serotype replacement of Hib with type "a" and NTHi strains denotes the ability of non-vaccine serotypes to re-colonize vaccinated patients. Finally, the dissemination of international high-risk clones of H. influenzae emphasizes the importance of monitoring changes in the molecular epidemiology of invasive H. influenzae disease.

Invasive Haemophilus influenzae type b (Hib) disease in children in Italy, after 20 years of routine use of conjugate Hib vaccines

Haemophilus influenzae serotype b (Hib) was the leading cause of bacterial meningitis in children before the implementation of infant immunization with conjugate Hib vaccines. Despite the effectiveness of the vaccine, invasive Hib disease cases (i.e. isolation of Hib from a normally sterile site) are still reported in children. All invasive Hib disease cases in children ≤ 15 years reported through the National Surveillance System of Invasive Bacterial Disease, during 2012-2018 in Italy, were analyzed. Hib PCR-confirmed isolates were subjected to MLST and PFGE analysis. The number of copies of the capb locus, a virulence factor potentially contributing to true vaccine failures (TVFs), was determined by Southern blot analysis. Vaccine effectiveness (VE) was determined using a multiple Poisson regression model. 31 cases of invasive Hib disease in children were reported. Fourteen children were vaccinated (TVFs), 14 were unvaccinated and 2 partially vaccinated (vaccination status was unknown for 1 case). The median age of children was 12 months (range 3 months-15 years). A decrease in vaccination coverage was observed in 2014-2016 (source Ministry of Health), and a rise in incidence was documented from 2016 until 2018, especially in children < 5 years. Vaccine effectiveness was estimated to be 83% (95% CI:45-95). 24 isolates were available. The predominant ST was ST6 (70.8%). Cluster analysis of ST6 isolates by PFGE identified five variants. Six isolates (25%) contained multiple copies of the capb locus distributed among TVFs (30%) and unvaccinated children (16.7%). Our data show that both failures to vaccinate and TVFs are associated with invasive Hib disease in children in Italy, during the vaccination era. Most cases in children ≤ 2 years were vaccine-preventable, since they occurred in unvaccinated subjects (13/21 cases, 62%). No host predisposing factors for TVF were recognized. TVFs were not significantly associated with either specific genotypes or amplification status of the capb locus.

"Gene accordions" cause genotypic and phenotypic heterogeneity in clonal populations of Staphylococcus aureus

Gene tandem amplifications are thought to drive bacterial evolution, but they are transient in the absence of selection, making their investigation challenging. Here, we analyze genomic sequences of Staphylococcus aureus USA300 isolates from the same geographical area to identify variations in gene copy number, which we confirm by long-read sequencing. We find several hotspots of variation, including the csa1 cluster encoding lipoproteins known to be immunogenic. We also show that the csa1 locus expands and contracts during bacterial growth in vitro and during systemic infection of mice, and recombination creates rapid heterogeneity in initially clonal cultures. Furthermore, csa1 copy number variants differ in their immunostimulatory capacity, revealing a mechanism by which gene copy number variation can modulate the host immune response.

Gene tandem amplifications can drive bacterial evolution. Here, Belikova et al. identify copy number variations of lipoprotein-encoding genes in Staphylococcus aureus clinical isolates, and show that the loci expand and contract during bacterial growth in vitro and in mice, leading to changes in immunostimulatory capacity.

Complement evasion by the human respiratory tract pathogens Haemophilus influenzae and Moraxella catarrhalis

All infective bacterial species need to conquer the innate immune system in order to colonize and survive in their hosts. The human respiratory pathogens Haemophilus influenzae and Moraxella catarrhalis are no exceptions and have developed sophisticated mechanisms to evade complement-mediated killing. Both bacterial species carry lipooligosaccharides preventing complement attacks and attract and utilize host complement regulators C4b binding protein and factor H to inhibit the classical and alternative pathways of complement activation, respectively. In addition, the regulator of the terminal pathway of complement activation, vitronectin, is hijacked by both bacteria. An array of different outer membrane proteins (OMP) in H. influenzae and M. catarrhalis simultaneously binds complement regulators, but also plasminogen. Several of the bacterial complement-binding proteins are important adhesins and contain highly conserved regions for interactions with the host. Thus, some of the OMP are viable targets for new therapeutics, including vaccines aimed at preventing respiratory tract diseases such as otitis media in children and exacerbations in patients suffering from chronic obstructive pulmonary disease.

Kingella kingae Surface Polysaccharides Promote Resistance to Human Serum and Virulence in a Juvenile Rat Model

Kingella kingae is a Gram-negative coccobacillus that is increasingly being recognized as an important cause of invasive disease in young children. The pathogenesis of K. kingae disease begins with colonization of the oropharynx, followed by invasion of the bloodstream, survival in the intravascular space, and dissemination to distant sites. Recent studies have revealed that K. kingae produces a number of surface factors that may contribute to the pathogenic process, including a polysaccharide capsule and an exopolysaccharide. In this study, we observed that K. kingae was highly resistant to the bactericidal effects of human serum complement. Using mutant strains deficient in expression of capsule, exopolysaccharide, or both in assays with human serum, we found that elimination of both capsule and exopolysaccharide was required for efficient binding of IgG, IgM, C4b, and C3b to the bacterial surface and for complement-mediated killing. Abrogation of the classical complement pathway using EGTA-treated human serum restored survival to wild-type levels by the mutant lacking both capsule and exopolysaccharide, demonstrating that capsule and exopolysaccharide promote resistance to the classical complement pathway. Consistent with these results, loss of both capsule and exopolysaccharide eliminated invasive disease in juvenile rats with an intact complement system but not in rats lacking complement. Based on these observations, we conclude that the capsule and the exopolysaccharide have important redundant roles in promoting survival of K. kingae in human serum. Each of these surface factors is sufficient by itself to fully prevent serum opsonin deposition and complement-mediated killing of K. kingae, ultimately facilitating intravascular survival and promoting K. kingae invasive disease.

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.

Competing for Iron: Duplication and Amplification of the isd Locus in Staphylococcus lugdunensis HKU09-01 Provides a Competitive Advantage to Overcome Nutritional Limitation

Staphylococcus lugdunensis is a coagulase negative bacterial pathogen that is particularly associated with severe cases of infectious endocarditis. Unique amongst the coagulase-negative staphylococci, S. lugdunensis harbors an iron regulated surface determinant locus (isd). This locus facilitates the acquisition of heme as a source of nutrient iron during infection and allows iron limitation caused by “nutritional immunity” to be overcome. The isd locus is duplicated in S. lugdunensis HKU09-01 and we show here that the duplication is intrinsically unstable and undergoes accordion-like amplification and segregation leading to extensive isd copy number variation. Amplification of the locus increased the level of expression of Isd proteins and improved binding of hemoglobin to the cell surface of S. lugdunensis. Furthermore, Isd overexpression provided an advantage when strains were competing for a limited amount of hemoglobin as the sole source of iron. Gene duplications and amplifications (GDA) are events of fundamental importance for bacterial evolution and are frequently associated with antibiotic resistance in many species. As such, GDAs are regarded as evolutionary adaptions to novel selective pressures in hostile environments pointing towards a special importance of isd for S. lugdunensis. For the first time we show an example of a GDA that involves a virulence factor of a Gram-positive pathogen and link the GDA directly to a competitive advantage when the bacteria were struggling with selective pressures mimicking “nutritional immunity”.

Sometimes changing environmental conditions force bacteria to boost protein expression above the level that can be achieved by transcriptional or translational control. Gene duplication and amplification (GDA) represents a simple and effective means to augment protein expression and is frequently associated with the development of resistance against antibacterial agents. As such GDAs can be seen as recent evolutionary adaptions towards novel selective pressures within the environment. We describe herein the GDA of the isd locus in a clinical isolate of Staphylococcus lugdunensis a bacterial species frequently associated with infectious endocarditis. This amplification made individual lineages significantly more successful when competing for the trace element iron supplied in the form of hemoglobin (hb). Human body fluids are actively depleted of iron to prevent bacterial proliferation and hb represents the most abundant iron source during infection. This deepens our understanding about adaption processes of pathogens and demonstrates how selective pressures drive the evolution of pathogens to become more successful. This study suggests that investigating GDAs in clinical isolates can help to identify chromosomal regions that are of special importance for bacterial fitness under changing environmental conditions. The gene products can therefore represent attractive targets for novel antibiotics supporting the natural immune defenses by targeting the vulnerable spots of the bacterial anti-immunity strategies.

Allelic Variation of the Capsule Promoter Diversifies Encapsulation and Virulence In Streptococcus pneumoniae

The polysaccharide capsule is the major virulence factor of Streptococcus pneumoniae (pneumococcus), a major human pathogen. The sequences in the promoter and coding regions of the capsule gene locus undergo extensive variations through the natural transformation-mediated horizontal gene transfer. The sequence variations in the coding region have led to at least 97 capsular serotypes. However, it remains unclear whether the sequence polymorphisms in the promoter region have any biological significance. In this study, we determined the sequences of the cps promoter region from 225 invasive pneumococcal isolates, and identified modular composition and remarkable inter-strain sequence variations in this region. The strain-to strain variations in the cps promoter are characterized by diversity in sequence and size, mosaic combinations of nucleotide polymorphisms and sequence modules, selective preservation of the sequence combinations, and promiscuous assortments of the sequences between the promoter and coding regions. Isogenic pneumococci carrying allelic variants of the cps promoter displayed significant differences in the transcription of the capsule genes, capsule production, adhesion to host epithelial cells, anti-phagocytosis and virulence in mouse bacteremia model. This study has thus indicated that the sequence polymorphisms in the cps promoter represent a novel mechanism for fine-tuning the level of encapsulation and virulence among S. pneumoniae strains.

Children with Haemophilus influenzae type b (Hib) vaccine failure have long-term bactericidal antibodies against virulent Hib strains with multiple capsular loci

Children who develop invasive Haemophilus influenzae serotype b (Hib) disease after immunisation with a highly-effective conjugate vaccine are more likely to have been infected with Hib strains possessing multiple copies of the capsulation locus. Using a recently-validated serum bactericidal antibody (SBA) assay, we tested convalescent sera from 127 Hib vaccine failure cases against clinical Hib strains expressing 1-5 copies of the capsulation locus. SBA titres correlated weakly with anti-capsular IgG antibody concentrations and there was no association between SBA geometric mean titres and number of capsulation locus copies. After infection, children with Hib vaccine failure were equally protected against Hib strains with 1-5 copies of the capsulation locus.

Haemophilus influenzae serotype a as a cause of serious invasive infections

Haemophilus influenzae, particularly H influenzae serotype b (Hib), is an important pathogen that causes serious diseases like meningitis and septicaemia. Since the introduction of Hib conjugate vaccines in the 1990s, the epidemiology of invasive H influenzae disease has changed substantially, with most infections now caused by non-Hib strains. We discuss the importance of H influenzae serotype a (Hia) as a cause of serious morbidity and mortality and its global epidemiology, clinical presentation, microbiology, immunology, prevention, and control. Much like Hib, the capsule of Hia is an important virulence factor contributing to the development of invasive disease. Molecular typing of Hia has identified distinct clonal groups, with some linked to severe disease and high case-fatality rates. Similarities between Hia and Hib capsules, their clinical presentation, and immunology of infection suggest that a bivalent Hia-Hib capsular polysaccharide-protein conjugate vaccine could offer protection against these two important serotypes of H influenzae.

Evasion of killing by human antibody and complement through multiple variations in the surface oligosaccharide of Haemophilus influenzae

The lipopolysaccharide (LPS) of H. influenzae is highly variable. Much of the structural diversity is derived from phase variation, or high frequency on-off switching, of molecules attached during LPS biosynthesis. In this study, we examined the dynamics of LPS phase variation following exposure to human serum as a source of antibody and complement in multiple H. influenzae isolates. We show that lic2A, lgtC and lex2A switch from phase-off to phase-on following serial passage in human serum. These genes, which control attachment of a galα1-4gal di-galactoside structure (lic2A and lgtC phase-on) or an alternative glucose extension (lex2A phase-on) from the same hexose moiety, reduce binding of bactericidal antibody to conserved inner core LPS structures. The effects of the di-galactoside and alternative glucose extension were also examined in the context of the additional LPS phase variable structures phosphorylcholine (ChoP) and sialic acid. We found that di-galactoside, the alternative glucose extension, ChoP, and sialic acid each contribute independently to bacterial survival in the presence of human complement, and have an additive effect in combination. We propose that LPS phase variable extensions serve to shield conserved inner core structures from recognition by host immune components encountered during infection.

Haemophilus influenzae and the complement system

The respiratory tract pathogen Haemophilus influenzae is responsible for a variety of infections in humans including septicemia, bronchitis, pneumonia, and acute otitis media. The pathogenesis of H. influenzae relies on its capacity to resist human host defenses including the complement system, and thus H. influenzae has developed several efficient strategies to circumvent complement attack. In addition to attracting specific host complement regulators directly to the bacterial surface, the capsule, lipooligosaccharides, and several outer membrane proteins contribute to resistance against complement-mediated attacks and hence increased bacterial survival. Insights into the mechanisms of complement evasion by H. influenzae are important for understanding pathogenesis and for developing vaccines and new therapies aimed at patients with, for example, chronic obstructive pulmonary disease. Here we overview current knowledge on the different mechanisms by which H. influenzae evades attack by the host complement system.

Poor clinical outcome for meningitis caused by Haemophilus influenzae serotype A strains containing the IS1016-bexA deletion

Since the introduction of Haemophilus influenzae type b (Hib) conjugate vaccines, meningitis caused by serotypes other than Hib has gained in importance. We conducted active hospital-based surveillance for meningitis over an 11-year period in Salvador, Brazil. H. influenzae isolates were serotyped and analyzed by polymerase chain reaction, pulsed-field gel electrophoresis, and DNA sequencing to identify strains with a specific deletion (IS1016) in the bexA gene (IS1016-bexA). We identified 43 meningitis cases caused by non-type b H. influenzae: 28 (65%) were caused by type a (Hia), 9 (21%) were caused by noncapsulated strains, and 3 (7%) each were caused by types e and f. Hia isolates clustered in 2 clonal groups; clonal group A strains (n = 9) had the IS1016-bexA deletion. Among children <5 years of age, meningitis caused by Hia from clonal group A had higher case-fatality than meningitis caused by clonal group B. Despite small numbers, these results indicate that the presence of the IS1016-bexA deletion is associated with enhanced virulence in non-type b H. influenzae.

Gene amplification and adaptive evolution in bacteria

Gene duplication-amplification (GDA) processes are highly relevant biologically because they generate extensive and reversible genetic variation on which adaptive evolution can act. Whenever cellular growth is restricted, escape from these growth restrictions often occurs by GDA events that resolve the selective problem. In addition, GDA may facilitate subsequent genetic change by allowing a population to grow and increase in number, thereby increasing the probability for subsequent adaptive mutations to occur in the amplified genes or in unrelated genes. Mathematical modeling of the effect of GDA on the rate of adaptive evolution shows that GDA will facilitate adaptation, especially when the supply of mutations in the population is rate-limiting. GDA can form via several mechanisms, both RecA-dependent and RecA-independent, including rolling-circle amplification and nonequal crossing over between sister chromatids. Due to the high intrinsic instability and fitness costs associated with GDAs, they are generally transient in nature, and consequently their evolutionary and medical importance is often underestimated.

Two variants among Haemophilus influenzae serotype b strains with distinct bcs4, hcsA and hcsB genes display differences in expression of the polysaccharide capsule

Background

Despite nearly complete vaccine coverage, a small number of fully vaccinated children in the Netherlands have experienced invasive disease caused by Haemophilus influenzae serotype b (Hib). This increase started in 2002, nine years after the introduction of nationwide vaccination in the Netherlands. The capsular polysaccharide of Hib is used as a conjugate vaccine to protect against Hib disease. To evaluate the possible rise of escape variants, explaining the increased number of vaccine failures we analyzed the composition of the capsular genes and the expressed polysaccharide of Dutch Hib strains collected before and after the introduction of Hib vaccination.

Results

The DNA sequences of the complete capsular gene clusters of 9 Dutch Hib strains were assessed and two variants, designated type I and type II were found. The two variants displayed considerable sequence divergence in the hcsA and hcsB genes, involved in transport of capsular polysaccharide to the cell surface. Application of hcsA type specific PCRs on 670 Hib strains collected from Dutch patients with invasive Hib disease showed that 5% of the strains collected before 1996 were type II. No endogenous type II Hib strains were isolated after 1995 and all type II strains were isolated from 0–4 year old, non-vaccinated children only. Analysis of a worldwide collection of Hib strains from the pre-vaccination era revealed considerable geographic differences in the distribution of the type I and type II strains with up to 73% of type II strains in the USA. NMR analysis of type I and type II capsule polysaccharides did not reveal structural differences. However, type I strains were shown to produce twice as much surface bound capsular polysaccharide.

Conclusion

Type II strains were only isolated during the pre-vaccination era from young, non-vaccinated individuals and displayed a lower expression of capsular polysaccharide than type I strains. The higher polysaccharide expression may have provided a selective advantage for type I strains resulting in the rapid elimination of type II from the Dutch Hib population after introduction of nationwide Hib vaccination. However, this phenomenon does not explain the increase in the number of Hib vaccine failures in the Netherlands.

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.

Detection of six copies of the capsulation b locus in a Haemophilus influenzae type b strain isolated from a splenectomized patient with fulminant septic shock

We report on the first detection of six copies of the capsulation b locus in a Haemophilus influenzae type b strain isolated from a splenectomized patient with fulminant septic shock associated with disseminated intravascular coagulation and death. The unusual amplification of the locus might have contributed to the rare and severe clinical presentation.

Molecular characterisation of Haemophilus influenzae type a and untypeable strains isolated simultaneously from cerebrospinal fluid and blood: novel use of quantitative real-time PCR based on the cap copy number to determine virulence

This study investigated the genetic structure of the cap region of an isolate of Haemophilus influenzae serotype a (Hia) from the cerebrospinal fluid (CSF) of a child with meningitis. In addition, the genetic structure of the cap region of a non-serotypeable H. influenzae isolate, obtained simultaneously from the blood of the same patient, was determined. According to restriction fragment length polymorphism analysis, the CSF and blood isolates were identical, with the exception of a single band shift of c. 35 kb. PCR analyses suggested that the CSF isolate possessed the IS1016-bexA gene and cap region II, whereas the blood isolate only had the IS1016 element. Furthermore, Southern analysis of DNA from both isolates showed that the CSF isolate carried the cap gene(s), while the blood isolate did not. Using a novel quantitative real-time PCR approach for determining the cap copy number, it was demonstrated that the CSF isolate had two intact tandem repeats of the cap gene containing three copies of IS1016, whereas the blood isolate had only one copy of IS1016. This study provided evidence that H. influenzae serotypes other than serotype b can cause serious disease, and that the virulence of these non-serotype b strains relates primarily to the cap gene copy number and the structure of the cap locus. Therefore, the quantitative real-time PCR assay described in this study should be useful for the rapid and definitive identification of strains of H. influenzae type a that represent a risk for serious disease.

Resistant Haemophilus influenzae in community-acquired respiratory tract infections: a role for cefixime

An increase in Haemophilus influenzae resistance has been documented around the world during the last 30 years. Resistance is due to the production of beta-lactamases, and/or changes to penicillin-binding protein (PBP) targets. The resistance problem has led to the need for new therapeutic strategies aimed at maintaining effective management of both upper respiratory tract infections (URTIs) and lower respiratory tract infections (LRTIs). Among antimicrobial agents tested, third-generation cephalosporins have been shown to possess excellent in vitro activity against beta-lactamase-positive and -negative isolates, corresponding with proven clinical efficacy in a wide range of RTIs. The role of H. influenzae in RTIs is outlined, changing trends in epidemiological surveillance studies monitored and implications for therapy, based upon results of clinical trials discussed.

BrkA protein of Bordetella pertussis inhibits the classical pathway of complement after C1 deposition

Bordetella pertussis produces a 73-kDa protein, BrkA (Bordetella resistance to killing), which inhibits the bactericidal activity of complement. In this study we characterized the step in the complement cascade where BrkA acts, using three strains: a wild-type strain, a strain containing an insertional disruption of brkA, and a strain containing two copies of the brkA locus. Following incubation with 10% human serum, killing was greatest for the BrkA mutant, followed by that for the wild-type strain, while the strain with two copies of brkA was the most resistant. Complement activation was monitored by enzyme-linked immunosorbent assay (ELISA) or Western blotting. ELISAs for SC5b-9, the soluble membrane attack complex, showed that production of SC5b-9 was greatest with the brkA mutant, less with the wild type, and least with the strain containing two copies of brkA. Deposition of complement proteins on the bacteria was monitored by Western blotting. A decrease in deposition on the bacteria of C4, C3, and C9 corresponded with decreased complement sensitivity. Deposition of C1, however, was not affected by the presence of BrkA. These studies show that BrkA inhibits the classical pathway of complement activation and prevents accumulation of deposited C4.

Complement-resistance mechanisms of bacteria

Despite more than a century of parallel research on bacteria and the complement system, relatively little is known of the mechanisms whereby pathogenic bacteria can escape complement-related opsonophagocytosis and direct killing. It is likely that pathogenicity in bacteria has arisen more accidentally than in viruses, and on the basis of selection from natural mutants rather than by outright stealing or copying of genetic codes from the host. In this review we will discuss complement resistance as one of the features that makes a bacterium a pathogen.