Cold shock response of the UspA1 outer membrane adhesin of Moraxella catarrhalis

Virulence determinants of Moraxella catarrhalis: distribution and considerations for vaccine development

Moraxella catarrhalis is a human-restricted opportunistic bacterial pathogen of the respiratory mucosa. It frequently colonizes the nasopharynx asymptomatically, but is also an important causative agent of otitis media (OM) in children, and plays a significant role in acute exacerbations of chronic obstructive pulmonary disease (COPD) in adults. As the current treatment options for M. catarrhalis infection in OM and exacerbations of COPD are often ineffective, the development of an efficacious vaccine is warranted. However, no vaccine candidates for M. catarrhalis have progressed to clinical trials, and information regarding the distribution of M. catarrhalis virulence factors and vaccine candidates is inconsistent in the literature. It is largely unknown if virulence is associated with particular strains or subpopulations of M. catarrhalis, or if differences in clinical manifestation can be attributed to the heterogeneous expression of specific M. catarrhalis virulence factors in the circulating population. Further investigation of the distribution of M. catarrhalis virulence factors in the context of carriage and disease is required so that vaccine development may be targeted at relevant antigens that are conserved among disease-causing strains. The challenge of determining which of the proposed M. catarrhalis virulence factors are relevant to human disease is amplified by the lack of a standardized M. catarrhalis typing system to facilitate direct comparisons of worldwide isolates. Here we summarize and evaluate proposed relationships between M. catarrhalis subpopulations and specific virulence factors in the context of colonization and disease, as well as the current methods used to infer these associations.

Potential impact of a Moraxella catarrhalis vaccine in COPD

Moraxella catarrhalis is the second most common cause of exacerbations in adults with COPD, resulting in enormous morbidity and mortality in this clinical setting. Vaccine development for M. catarrhalis has lagged behind the other two important causes of exacerbations in COPD, nontypeable Haemophilus influenzae and Streptococcus pneumoniae. While no licensed vaccine is currently available for M. catarrhalis, several promising candidate vaccine antigens have been identified and characterized and are close to entering clinical trials. Key steps that are required to advance vaccines for M. catarrhalis along the translational pipeline include standardization of assay systems to assess candidate antigens, identification of a reliable correlate of protection and expansion of partnerships between industry, academia and government to overcome regulatory hurdles. A vaccine to prevent M. catarrhalis infections in COPD would have a major impact in reducing morbidity, mortality and healthcare costs in COPD.

Multidrug Efflux Pumps at the Crossroad between Antibiotic Resistance and Bacterial Virulence

Multidrug efflux pumps can be involved in bacterial resistance to antibiotics at different levels. Some efflux pumps are constitutively expressed at low levels and contribute to intrinsic resistance. In addition, their overexpression may allow higher levels of resistance. This overexpression can be transient, in the presence of an effector (phenotypic resistance), or constitutive when mutants in the regulatory elements of the expression of efflux pumps are selected (acquired resistance). Efflux pumps are present in all cells, from human to bacteria and are highly conserved, which indicates that they are ancient elements in the evolution of different organisms. Consequently, it has been suggested that, besides antibiotic resistance, bacterial multidrug efflux pumps would likely contribute to other relevant processes of the microbial physiology. In the current article, we discuss some specific examples of the role that efflux pumps may have in the bacterial virulence of animals’ and plants’ pathogens, including the processes of intercellular communication. Based in these evidences, we propose that efflux pumps are at the crossroad between resistance and virulence of bacterial pathogens. Consequently, the comprehensive study of multidrug efflux pumps requires addressing these functions, which are of relevance for the bacterial–host interactions during infection.

Direct molecular detection of a broad range of bacterial and viral organisms and Streptococcus pneumoniae vaccine serotypes in children with otitis media with effusion

Background

Otitis media with effusion (OME) causes significant morbidity in children, but the causes of OME and methods for prevention are unclear. To look for potential infectious etiologies, we performed a pilot study using multiple-target real-time polymerase chain reaction (qPCR) for 27 infectious agents, including nine bacterial organisms and 18 respiratory viruses in middle ear fluids (MEFs) from children with OME. QPCR was also performed for the 13 Streptococcus pneumoniae serotypes contained in the current vaccine.

Results

Forty-eight MEF samples were obtained and qPCR detected bacterial nucleic acid (NA) in 39/48 (81 %) and viral NA in 7/48 (15 %). Alloiococcus otitidis and S. pneumoniae were both detected in 15/48 (31 %) MEFs, followed by M. catarrhalis in 14/48 (29 %), H. influenzae in 5/48 (10 %) and M. pneumoniae in 4/48 (8 %). Rhinoviruses were most common virus type detected, found in 4/48 (8 %) MEFs. Serotypes included in the current 13-serotype vaccine were detected in only 3/15 (20 %) S. pneumoniae qPCR-positive MEFs.

Conclusions

Bacteria may play an important role in OME, since over 80 % of MEFs contained bacterial NA. Further research into the role of A. otitidis in OME will be helpful. Serotypes of S. pneumoniae not included in the current 13-serotype vaccine may be involved in OME. Larger studies of OME S. pneumoniae serotypes are needed to help determine which additional serotypes should be included in future vaccine formulations in order to try to prevent OME.

Expression of the Oligopeptide Permease Operon of Moraxella catarrhalis Is Regulated by Temperature and Nutrient Availability

Moraxella catarrhalis causes otitis media in children and exacerbations of chronic obstructive pulmonary disease in adults. Together, these two conditions contribute to enormous morbidity and mortality worldwide. The oligopeptide permease (opp) ABC transport system is a nutritional virulence factor important for the utilization of peptides. The substrate binding protein OppA, which binds peptides for uptake, is a potential vaccine antigen, but little was known about the regulation of gene expression. The five opp genes oppB, oppC, oppD, oppF, and oppA are in the same open reading frame. Sequence analysis predicted two promoters, one located upstream of oppB and one within the intergenic region between oppF and oppA. We have characterized the gene cluster as an operon with two functional promoters and show that cold shock at 26°C for ≤ 0.5 h and the presence of a peptide substrate increase gene transcript levels. Additionally, the putative promoter upstream of oppA contributes to the transcription of oppA but is not influenced by the same environmental cues as the promoter upstream of oppB. We conclude that temperature and nutrient availability contribute to the regulation of the Opp system, which is an important nutritional virulence factor in M. catarrhalis.

Moraxella catarrhalis AcrAB-OprM efflux pump contributes to antimicrobial resistance and is enhanced during cold shock response

Moraxella catarrhalis is a common pathogen of the human respiratory tract. Multidrug efflux pumps play a major role in antibiotic resistance and virulence in many Gram-negative organisms. In the present study, the role of the AcrAB-OprM efflux pump in antibiotic resistance was investigated by constructing mutants that lack the acrA, acrB, and oprM genes in M. catarrhalis strain O35E. We observed a moderate (1.5-fold) decrease in the MICs of amoxicillin and cefotaxime and a marked (4.7-fold) decrease in the MICs of clarithromycin for acrA, acrB, and oprM mutants in comparison with the wild-type O35E strain. Exposure of the M. catarrhalis strains O35E and 300 to amoxicillin triggered an increased transcription of all AcrAB-OprM pump genes, and exposure of strains O35E, 300, and 415 to clarithromycin enhanced the expression of acrA and oprM mRNA. Inactivation of the AcrAB-OprM efflux pump genes demonstrated a decreased ability to invade epithelial cells compared to the parental strain, suggesting that acrA, acrB, and oprM are required for efficient invasion of human pharyngeal epithelial cells. Cold shock increases the expression of AcrAB-OprM efflux pump genes in all three M. catarrhalis strains tested. Increased expression of AcrAB-OprM pump genes after cold shock leads to a lower accumulation of Hoechst 33342 (H33342), a substrate of AcrAB-OprM efflux pumps, indicating that cold shock results in increased efflux activity. In conclusion, the AcrAB-OprM efflux pump appears to play a role in the antibiotic resistance and virulence of M. catarrhalis and is involved in the cold shock response.

Identification of an outer membrane lipoprotein involved in nasopharyngeal colonization by Moraxella catarrhalis in an animal model

Colonization of the human nasopharynx by Moraxella catarrhalis is presumed to involve attachment of this bacterium to the mucosa. DNA microarray analysis was used to determine whether attachment of M. catarrhalis to human bronchial epithelial (HBE) cells in vitro affected gene expression in this bacterium. Attachment affected expression of at least 454 different genes, with 163 being upregulated and 291 being downregulated. Among the upregulated genes was one (ORF113) previously annotated as encoding a protein with some similarity to outer membrane protein A (OmpA). The protein encoded by ORF113 was predicted to have a signal peptidase II cleavage site, and globomycin inhibition experiments confirmed that this protein was indeed a lipoprotein. The ORF113 protein also contained a predicted peptidoglycan-binding domain in its C-terminal half. The use of mutant and recombinant M. catarrhalis strains confirmed that the ORF113 protein was present in outer membrane preparations, and this protein was also shown to be at least partially exposed on the bacterial cell surface. A mutant unable to produce the ORF113 protein showed little or no change in its growth rate in vitro, in its ability to attach to HBE cells in vitro, or in its autoagglutination characteristics, but it did exhibit a reduced ability to survive in the chinchilla nasopharynx. This is the first report of a lipoprotein essential to the ability of M. catarrhalis to persist in an animal model.

Molecular mechanisms of moraxella catarrhalis-induced otitis media

Moraxella catarrhalis is a Gram-negative bacterium, exclusively present in humans and a leading causative agent of otitis media (OM) in children. Most children (80 %) experience at least one episode of OM by their third birthday and half suffer multiple episodes of infection. Over the last 10 years, increased evidence suggests that M. cat possesses multiple virulence factors which can be carried through biologically active outer membrane vesicles (OMVs) that are themselves able to activate host-immune responses. It has also been noted that multiple toll-like receptors are responsible for M. cat recognition. This review is intended to summarize the key findings and progress in recent years of the molecular mechanisms of M. cat-induced otitis media with particular emphasis on adhesion, invasion, and activation of the host immune system, biofilm formation, and vaccine development.

RNA-Seq-based analysis of the physiologic cold shock-induced changes in Moraxella catarrhalis gene expression

BACKGROUND

Moraxella catarrhalis, a major nasopharyngeal pathogen of the human respiratory tract, is exposed to rapid downshifts of environmental temperature when humans breathe cold air. The prevalence of pharyngeal colonization and respiratory tract infections caused by M. catarrhalis is greatest in winter. We investigated how M. catarrhalis uses the physiologic exposure to cold air to regulate pivotal survival systems that may contribute to M. catarrhalis virulence.

RESULTS

In this study we used the RNA-seq techniques to quantitatively catalogue the transcriptome of M. catarrhalis exposed to a 26 °C cold shock or to continuous growth at 37 °C. Validation of RNA-seq data using quantitative RT-PCR analysis demonstrated the RNA-seq results to be highly reliable. We observed that a 26 °C cold shock induces the expression of genes that in other bacteria have been related to virulence a strong induction was observed for genes involved in high affinity phosphate transport and iron acquisition, indicating that M. catarrhalis makes a better use of both phosphate and iron resources after exposure to cold shock. We detected the induction of genes involved in nitrogen metabolism, as well as several outer membrane proteins, including ompA, m35-like porin and multidrug efflux pump (acrAB) indicating that M. catarrhalis remodels its membrane components in response to downshift of temperature. Furthermore, we demonstrate that a 26 °C cold shock enhances the induction of genes encoding the type IV pili that are essential for natural transformation, and increases the genetic competence of M. catarrhalis, which may facilitate the rapid spread and acquisition of novel virulence-associated genes.

CONCLUSION

Cold shock at a physiologically relevant temperature of 26 °C induces in M. catarrhalis a complex of adaptive mechanisms that could convey novel pathogenic functions and may contribute to enhanced colonization and virulence.

Moraxella catarrhalis: from interactions with the host immune system to vaccine development

Moraxella catarrhalis is a human-restricted commensal that over the last two decades has developed into an emerging respiratory tract pathogen. The bacterial species is equipped with various adhesins to facilitate its colonization. Successful evasion of the human immune system is a prerequisite for Moraxella infection. This strategy involves induction of an excessive proinflammatory response, intervention of granulocyte recruitment to the infection site, activation of selected pattern recognition receptors and cellular adhesion molecules to counteract the host bacteriolytic attack, as well as, finally, reprogramming of antigen presenting cells. Host immunomodulator molecules are also exploited by Moraxella to aid in resistance against complement killing and host bactericidal molecules. Thus, breaking the basis of Moraxella immune evasion mechanisms is fundamental for future invention of effective therapy in controlling Moraxella infection.

Development of a LacZ-based transcriptional reporter system for use with Moraxella catarrhalis

The lack of a transcriptional reporter system for use in Moraxella catarrhalis has hindered studies of gene regulation in this pathogen. PCR and recombinant DNA methods were used to insert a multicloning site (MCS) and promoterless full-length Escherichia coli lacZ gene, flanked by transcriptional terminators both immediately upstream and downstream, into the M. catarrhalis recombinant plasmid pWW115. Insertion into the MCS in the newly constructed plasmid pASE222 of M. catarrhalis promoter regions controlled by either a repressor (i.e., NsrR) or activator (i.e., PhoB) yielded transcriptional fusion constructs that were appropriately responsive to signal inputs dependent on the host strain genotype, as measured quantitatively by means of a Miller β-galactosidase assay. The transcriptional reporter plasmid pASE222 should prove to be a useful tool for rapid screening of factors affecting gene expression in M. catarrhalis.

A novel group of Moraxella catarrhalis UspA proteins mediates cellular adhesion via CEACAMs and vitronectin

Moraxella catarrhalis (Mx) is a common cause of otitis media and exacerbation of chronic obstructive pulmonary disease, an increasing worldwide problem. Surface proteins UspA1 and UspA2 of Mx bind to a number of human receptors and may function in pathogenesis. Genetic recombination events in the pathogen can generate hybrid proteins termed UspA2H. However, whether certain key functions (e.g. UspA1-specific CEACAM binding) can be exchanged between these adhesin families remains unknown. In this study, we have shown that Mx can incorporate the UspA1 CEACAM1-binding region not only into rare UspA1 proteins devoid of CEACAM-binding ability, but also into UspA2 which normally lack this capacity. Further, a screen of Mx isolates revealed the presence of novel UspA2 Variant proteins (UspA2V) in ∼14% of the CEACAM-binding population. We demonstrate that the expression of UspA2/2V with the CEACAM-binding domain enable Mx to bind both to cell surface CEACAMs and to integrins, the latter via vitronectin. Such properties of UspA2/2V have not been reported to date. The studies demonstrate that the UspA family is much more heterogeneous than previously believed and illustrate the in vivo potential for exchange of functional regions between UspA proteins which could convey novel adhesive functions whilst enhancing immune evasion.

Physiologic cold shock of Moraxella catarrhalis affects the expression of genes involved in the iron acquisition, serum resistance and immune evasion

Background

Moraxella catarrhalis, a major nasopharyngeal pathogen of the human respiratory tract, is exposed to rapid downshifts of environmental temperature when humans breathe cold air. It was previously shown that the prevalence of pharyngeal colonization and respiratory tract infections caused by M. catarrhalis are greatest in winter. The aim of this study was to investigate how M. catarrhalis uses the physiologic exposure to cold air to upregulate pivotal survival systems in the pharynx that may contribute to M. catarrhalis virulence.

Results

A 26°C cold shock induces the expression of genes involved in transferrin and lactoferrin acquisition, and enhances binding of these proteins on the surface of M. catarrhalis. Exposure of M. catarrhalis to 26°C upregulates the expression of UspA2, a major outer membrane protein involved in serum resistance, leading to improved binding of vitronectin which neutralizes the lethal effect of human complement. In contrast, cold shock decreases the expression of Hemagglutinin, a major adhesin, which mediates B cell response, and reduces immunoglobulin D-binding on the surface of M. catarrhalis.

Conclusion

Cold shock of M. catarrhalis induces the expression of genes involved in iron acquisition, serum resistance and immune evasion. Thus, cold shock at a physiologically relevant temperature of 26°C induces in M. catarrhalis a complex of adaptive mechanisms that enables the bacterium to target their host cellular receptors or soluble effectors and may contribute to enhanced growth, colonization and virulence.

Down-regulation of porin M35 in Moraxella catarrhalis by aminopenicillins and environmental factors and its potential contribution to the mechanism of resistance to aminopenicillins

Objectives

The outer membrane protein M35 of Moraxella catarrhalis is an antigenically conserved porin. Knocking out M35 significantly increases the MICs of aminopenicillins. The aim of this study was to determine the biological mechanism of this potentially new antimicrobial resistance mechanism of M. catarrhalis and the behaviour of M35 in general stress situations.

Methods

PCR using m35-specific primers was used to detect the m35 gene in clinical isolates. The m35 mRNA expression of strains 300, O35E and 415 after exposure to amoxicillin and different stress conditions was measured by real-time PCR and normalized in relation to their 16S rRNA expression. The expression of M35 protein was analysed by SDS-PAGE and western blotting.

Results

Screening of 52 middle ear isolates resulted in positive PCR products for all tested strains. The analysis of m35 mRNA expression after amoxicillin treatment showed 24%–85% down-regulation compared with the respective amoxicillin-free controls in all three strains tested. Also, analysis of protein concentrations revealed lower M35 expression after growth with amoxicillin. Investigation of M35 during general stress responses showed down-regulation of the porin with growth at 26°C and 42°C, under hyperosmolar stress and under iron restriction.

Conclusions

The reduced expression of M35 after aminopenicillin exposure indicates a novel resistance mechanism against aminopenicillins in M. catarrhalis, which may be relevant in vivo. The differences in expression after different stress treatments demonstrate that M35 is involved in general stress responses.

Molecular aspects of Moraxella catarrhalis pathogenesis

In recent years, Moraxella catarrhalis has established its position as an important human mucosal pathogen, no longer being regarded as just a commensal bacterium. Further, current research in the field has led to a better understanding of the molecular mechanisms involved in M. catarrhalis pathogenesis, including mechanisms associated with cellular adherence, target cell invasion, modulation of the host's immune response, and metabolism. Additionally, in order to be successful in the host, M. catarrhalis has to be able to interact and compete with the commensal flora and overcome stressful environmental conditions, such as nutrient limitation. In this review, we provide a timely overview of the current understanding of the molecular mechanisms associated with M. catarrhalis virulence and pathogenesis.

Outer membrane porin M35 of Moraxella catarrhalis mediates susceptibility to aminopenicillins

Background

The outer membrane protein M35 is a conserved porin of type 1 strains of the respiratory pathogen Moraxella catarrhalis. It was previously shown that M35 is involved in the uptake of essential nutrients required for bacterial growth and for nasal colonization in mice. The aim of this study was (i) to characterize the potential roles of M35 in the host-pathogen interactions considering the known multifunctionality of porins and (ii) to characterize the degree of conservation in the phylogenetic older subpopulation (type 2) of M. catarrhalis.

Results

Isogenic m35 mutants of the type 1 strains O35E, 300 and 415 were tested for their antimicrobial susceptibility against 15 different agents. Differences in the MIC (Minimum Inhibitory Concentration) between wild-type and mutant strains were found for eight antibiotics. For ampicillin and amoxicillin, we observed a statistically significant 2.5 to 2.9-fold MIC increase (p < 0.03) in the m35 mutants. Immunoblot analysis demonstrated that human saliva contains anti-M35 IgA. Wild-type strains and their respective m35 mutants were indistinguishable with respect to the phenotypes of autoagglutination, serum resistance, iron acquisition from human lactoferrin, adherence to and invasion of respiratory tract epithelial cells, and proinflammatory stimulation of human monocytes. DNA sequencing of m35 from the phylogenetic subpopulation type 2 strain 287 revealed 94.2% and 92.8% identity on the DNA and amino acid levels, respectively, in comparison with type 1 strains.

Conclusion

The increase in MIC for ampicillin and amoxicillin, respectively, in the M35-deficient mutants indicates that this porin affects the outer membrane permeability for aminopenicillins in a clinically relevant manner. The presence of IgA antibodies in healthy human donors indicates that M35 is expressed in vivo and recognized as a mucosal antigen by the human host. However, immunoblot analysis of human saliva suggests the possibility of antigenic variation of immunoreactive epitopes, which warrants further analysis before M35 can be considered a potential vaccine candidate.

The evaluation of putative endogenous control housekeeping genes for real-time polymerase chain reaction expression studies in Moraxella catarrhalis

Comparisons of endogenous control genes in real-time polymerase chain reaction gene expression studies involving Moraxella catarrhalis are rare. This study shows that a combination of the iron sequestering gene copB and 16S rRNA genes would be useful for lineage 1 (16S rRNA type 1) isolates, but not lineage 2 (16S rRNA types 2 and 3) isolates.

Moraxella catarrhalis expresses an unusual Hfq protein

The Hfq protein is recognized as a global regulatory molecule that facilitates certain RNA-RNA interactions in bacteria. BLAST analysis identified a 630-nucleotide open reading frame in the genome of Moraxella catarrhalis ATCC 43617 that was highly conserved among M. catarrhalis strains and which encoded a predicted protein with significant homology to the Hfq protein of Escherichia coli. This protein, containing 210 amino acids, was more than twice as large as the Hfq proteins previously described for other bacteria. The C-terminal half of the M. catarrhalis Hfq protein was very hydrophilic and contained two different types of amino acid repeats. A mutation in the M. catarrhalis hfq gene affected both the growth rate of this organism and its sensitivity to at least two different types of stress in vitro. Provision of the wild-type M. catarrhalis hfq gene in trans eliminated these phenotypic differences in the hfq mutant. This M. catarrhalis hfq mutant exhibited altered expression of some cell envelope proteins relative to the wild-type parent strain and also had a growth advantage in a continuous flow biofilm system. The presence of the wild-type M. catarrhalis hfq gene in trans in an E. coli hfq mutant fully reversed the modest growth deficiency of this E. coli mutant and partially reversed the stress sensitivity of this E. coli mutant to methyl viologen. The use of an electrophoretic mobility shift assay showed that this M. catarrhalis Hfq protein could bind RNA derived from a gene whose expression was altered in the M. catarrhalis hfq mutant.

Influence of the spxB gene on competence in Streptococcus pneumoniae

In Streptococcus pneumoniae expression of pyruvate oxidase (SpxB) peaks during the early growth phase, coincident with the time of natural competence. This study investigated whether SpxB influences parameters of competence, such as spontaneous transformation frequency, expression of competence genes, and DNA release. Knockout of the spxB gene in strain D39 abolished spontaneous transformation (compared to a frequency of 6.3 x 10(-6) in the parent strain [P < 0.01]). It also reduced expression levels of comC and recA as well as DNA release from bacterial cells significantly during the early growth phase, coincident with the time of spontaneous competence in the parent strain. In the spxB mutant, supplementation with competence-stimulating peptide 1 (CSP-1) restored transformation (rate, 1.8 x 10(-2)). This speaks against the role of SpxB as a necessary source of energy for competence. Neither supplementation with CSP-1 nor supplementation with the SpxB products H2O2 and acetate altered DNA release. Supplementation of the parent strain with catalase did not reduce DNA release significantly. In conclusion, the pneumococcal spxB gene influences competence; however, the mechanism remains elusive.

Metabolic analysis of Moraxella catarrhalis and the effect of selected in vitro growth conditions on global gene expression

The nucleotide sequence from the genome of Moraxella catarrhalis ATCC 43617 was annotated and used both to assess the metabolic capabilities and limitations of this bacterium and to design probes for a DNA microarray. An absence of gene products for utilization of exogenous carbohydrates was noteworthy and could be correlated with published phenotypic data. Gene products necessary for aerobic energy generation were present, as were a few gene products generally ascribed to anaerobic systems. Enzymes for synthesis of all amino acids except proline and arginine were present. M. catarrhalis DNA microarrays containing 70-mer oligonucleotide probes were designed from the genome-derived nucleotide sequence data. Analysis of total RNA extracted from M. catarrhalis ATCC 43617 cells grown under iron-replete and iron-restricted conditions was used to establish the utility of these DNA microarrays. These DNA microarrays were then used to analyze total RNA from M. catarrhalis cells grown in a continuous-flow biofilm system and in the planktonic state. The genes whose expression was most dramatically increased by growth in the biofilm state included those encoding a nitrate reductase, a nitrite reductase, and a nitric oxide reductase. Real-time reverse transcriptase PCR analysis was used to validate these DNA microarray results. These results indicate that growth of M. catarrhalis in a biofilm results in increased expression of gene products which can function not only in energy generation but also in resisting certain elements of the innate immune response.

Identification of gene products involved in biofilm production by Moraxella catarrhalis ETSU-9 in vitro

Moraxella catarrhalis ETSU-9 was subjected to random transposon insertion mutagenesis to identify genes encoding products involved in the ability of the organism to form biofilms in vitro. Screening of approximately 3,000 transposon insertion mutants in the crystal violet-based biofilm assay system yielded six mutants that exhibited greatly reduced abilities to form biofilms. Three of these mutants had transposon insertions in the uspA2H gene, which encodes a surface protein previously shown to be involved in the ability of M. catarrhalis to both attach to human cell lines in vitro and resist killing by normal human serum. Random insertion mutagenesis of the uspA2H gene, involving the introduction of a 15-nucleotide fragment encoding 5 amino acids, was used to attempt to identify the domain(s) necessary for biofilm formation. Most of these insertions adversely affected biofilm formation, whereas the abilities of these same mutants to attach to Chang conjunctival epithelial cells in vitro were usually not reduced. Gain-of-function experiments showed that introduction of the M. catarrhalis ETSU-9 uspA2H gene into Escherichia coli conferred biofilm formation ability on this recombinant strain. Two of the other three M. catarrhalis ETSU-9 transposon insertion mutants that had greatly reduced abilities to form biofilms were shown to have insertions in genes encoding products predicted to be directly or indirectly involved in cell wall metabolism.

Trimeric autotransporter adhesins: variable structure, common function

Trimeric autotransporter adhesins (TAAs) are important virulence factors in gram-negative pathogens. Despite the variety of hosts ranging from plants to mammals and the specialized regulation of TAAs, their molecular organization follows surprisingly simple rules: they form trimeric surface structures with a head-stalk-anchor architecture. The head and stalk are composed of a small set of domains, building blocks that are frequently arranged repetitively. We propose that this repetitive arrangement facilitates recombination of domains to modulate the specificity of the common function: adhesion to the host.