Transcriptional and posttranscriptional control of cable pilus gene expression in Burkholderia cenocepacia

NaCl-induced modulation of species distribution in a mixed P. aeruginosa / S. aureus /B.cepacia biofilm

Pseudomonas aeruginosa, Staphylococcus aureus, and Burkholderia cepacia are notorious pathogens known for their ability to form resilient biofilms, particularly within the lung environment of cystic fibrosis (CF) patients. The heightened concentration of NaCl, prevalent in the airway liquid of CF patients' lungs, has been identified as a factor that promotes the growth of osmotolerant bacteria like S. aureus and dampens host antibacterial defenses, thereby fostering favorable conditions for infections. In this study, we aimed to investigate how increased NaCl concentrations impact the development of multi-species biofilms in vitro, using both laboratory strains and clinical isolates of P. aeruginosa, S. aureus, and B. cepacia co-cultures. Employing a low-nutrient culture medium that fosters biofilm growth of the selected species, we quantified biofilm formation through a combination of adherent CFU counts, qPCR analysis, and confocal microscopy observations. Our findings reaffirmed the challenges faced by S. aureus in establishing growth within 1:1 mixed biofilms with P. aeruginosa when cultivated in a minimal medium. Intriguingly, at an elevated NaCl concentration of 145 mM, a symbiotic relationship emerged between S. aureus and P. aeruginosa, enabling their co-existence. Notably, this hyperosmotic environment also exerted an influence on the interplay of these two bacteria with B. cepacia. We demonstrated that elevated NaCl concentrations play a pivotal role in orchestrating the distribution of these three species within the biofilm matrix. Furthermore, our study unveiled the beneficial impact of NaCl on the biofilm growth of clinically relevant mucoid P. aeruginosa strains, as well as two strains of methicillin-sensitive and methicillin-resistant S. aureus. This underscores the crucial role of the microenvironment during the colonization and infection processes. The results suggest that hyperosmotic conditions could hold the key to unlocking a deeper understanding of the genesis and behavior of CF multi-species biofilms.

Host-bacteria interaction and adhesin study for development of therapeutics

Host-pathogen interaction is one of the most important areas of study to understand the adhesion of the pathogen to the host organisms. To adhere on the host cell surface, bacteria assemble the diverse adhesive structures on its surface, which play a foremost role in targeting to the host cell. We have highlighted different bacterial adhesins which are either protein mediated or glycan mediated. The present article listed examples of different bacterial adhesin proteins involved in the interactions with their host, types and subtypes of the fimbriae and non-fimbriae bacterial adhesins. Different bacterial surface adhesin subunits interact with host via different host surface biomolecules. We have also discussed the interactome of some of the pathogens with their host. Therefore, the present study will help researchers to have a detailed understanding of different interacting bacterial adhesins and henceforth, develop new therapies, adhesin specific antibodies and vaccines, which can effectively control pathogenicity of the pathogens.

Iron Availability Influences Aggregation, Biofilm, Adhesion and Invasion of Pseudomonas Aeruginosa and Burkholderia Cenocepacia

Pseudomonas aeruginosa and Burkholderia cenocepacia are predominant opportunistic pathogens in cystic fibrosis (CF) patients. In healthy humans the lower respiratory tract as well as all mucosa, contains a very low free iron concentration (10−18 M), while in CF patients' sputum iron concentration is very high, showing a median value of 63×10−6 M. Accumulation of catalytic reactive iron heavily contributes to subsequent clinical complications in the lung disorders by the production of reactive oxygen species and increases bacterial growth and virulence. The data reported in this study indicate that low iron concentration (Fe3+1 μM) induced free-living forms and motility both in P. aeruginosa and B. cenocepacia, while high iron concentrations (Fe3+ 10 and 100 μM) stimulated aggregation and biofilm formation already in the fluid phases, so demonstrating that aggregation and biofilm formation are positively iron-modulated in these bacteria. Moreover, the different morphological forms (free-living, aggregates and biofilm) showed different capabilities of adhering and invading the bronchial cell line A549. P. aeruginosa PAO1 aggregates, and mostly biofilm, exerted the highest adhesion efficiency, while B. cenocepacia PV1 aggregates or biofilm the lowest. A significant reduction in invasion efficiency by P. aeruginosa biofilm and a significant increase in cell internalization by B. cenocepacia biofilm has been reported. Therefore, the iron availability is an important signal to which P. aeruginosa and B. cenocepacia counteract by leaving the motile free-living forms and entering into a new lifestyle, i.e. biofilm. These data could contribute to explain that the iron-overload of the sputum of CF patients, inducing nonmotile forms, aggregates and biofilm, may facilitate penetration of host epithelial barriers contributing to the establishment of infection, colonization, persistence and systemic spread of these opportunistic pathogens.

Regulation and production of Tcf, a cable-like fimbriae from Salmonella enterica serovar Typhi

tcf (Typhi colonization factor) is one of the 12 putative chaperone/usher fimbrial clusters present in the Salmonella enterica serovar Typhi genome. We investigated the production, expression and regulation of tcf as well as its role during interaction with human cells. The tcf gene cluster was cloned and induced in Escherichia coli and S. Typhi, and the production of intertwined fibres similar to the Cbl (cable) pili of Burkholderia cepacia was observed on the bacterial surface by electron microscopy. In S. Typhi, tcf was expressed more after growth in M63 minimal medium than in standard Luria-Bertani medium. Analysis of the promoter region identified putative binding sites for the global regulators RcsB, ArgR and Fur. The expression of tcf was measured in isogenic strains lacking these global regulators. Under the conditions tested, the results showed that tcf expression was higher in the fur mutant and was regulated by iron concentration. Fur may regulate these fimbriae indirectly via the small RNAs RyhB1 and RyhB2. An isogenic mutant harbouring a deletion of the tcf cluster did not demonstrate any defect in adhesion or invasion of human epithelial cells, or in phagocytosis or survival in macrophages, when compared to the WT serovar Typhi strain. However, the tcf cluster contributed to adherence to human epithelial cells when introduced into E. coli. Thus, tcf genes encode functional fimbriae that can act as an adhesin and may contribute to colonization during typhoid fever.

The pathogenicity island encoded PvrSR/RcsCB regulatory network controls biofilm formation and dispersal in Pseudomonas aeruginosa PA14

Pseudomonas aeruginosa biofilm formation is linked to persistent infections in humans. Biofilm formation is facilitated by extracellular appendages, some of which are assembled by the Chaperone Usher Pathway (Cup). The cupD gene cluster is located on the PAPI-1 pathogenicity island of strain PA14 and has probably been acquired together with four genes encoding two-component signal transduction proteins. We have previously showed that the RcsB response regulator activates expression of the cupD genes, which leads to the production of CupD fimbriae and increased attachment. Here we show that RcsB activity is tightly modulated by two sensors, RcsC and PvrS. While PvrS acts as a kinase that enhances RcsB activity, RcsC has a dual function, first as a phosphorelay, and second as a phosphatase. We found that, under certain growth conditions, overexpression of RcsB readily induces biofilm dispersal. Microarray analysis shows that RcsB positively controls expression of pvrR that encodes the phosphodiesterase required for this dispersal process. Finally, in addition to the PAPI-1 encoded cupD genes, RcsB controls several genes on the core genome, some of which encode orphan response regulators. We thus discovered that RcsB is central to a large regulatory network that fine-tunes the switch between biofilm formation and dispersal.

Differences in strategies to combat osmotic stress in Burkholderia cenocepacia elucidated by NMR-based metabolic profiling

AIMS

To investigate mechanisms of osmotic tolerance in Burkholderia cenocepacia, a member of the B. cepacia complex (Bcc) of closely related strains, which is of clinical as well as environmental importance.

METHODS AND RESULTS

We employed NMR-based metabolic profiling (metabolomics) to elucidate the metabolic consequences of high osmotic stress for five isolates of B. cenocepacia. The strains differed significantly in their levels of osmotic stress tolerance, and we identified three different sets of metabolic responses with the strains least impacted by osmotic stress exhibiting higher levels of the osmo-protective metabolites glycine-betaine and/or trehalose. Strains either increased concentrations or had constitutively high levels of these metabolites.

CONCLUSIONS

Even within the small set of B. cenocepacia isolates, there was a surprising degree of variability in the metabolic responses to osmotic stress.

SIGNIFICANCE AND IMPACT OF THE STUDY

The metabolic responses, and hence osmotic stress tolerance, vary between different B. cenocepacia isolates. This study provides a first look into the potentially highly diverse physiology of closely related strains of one species of the Bcc and illustrates that physiological or clinically relevant phenotypes are unlikely to be inferable from genetic relatedness within this species group.

Genome-wide analysis of DNA repeats in Burkholderia cenocepacia J2315 identifies a novel adhesin-like gene unique to epidemic-associated strains of the ET-12 lineage

Members of the Burkholderia cepacia complex (Bcc) are respiratory pathogens in patients with cystic fibrosis (CF). Close repetitive DNA sequences often associate with surface antigens to promote genetic variability in pathogenic bacteria. The genome of Burkholderia cenocepacia J2315, a CF isolate belonging to the epidemic lineage Edinburgh–Toronto (ET-12), was analysed for the presence of close repetitive DNA sequences. Among the 422 DNA close repeats, 45 genes potentially involved in virulence were identified and grouped into 12 classes; of these, 13 genes were included in the antigens class. Two trimeric autotransporter adhesins (TAA) among the 13 putative antigens are absent from the other Burkholderia genomes and are clustered downstream of the cci island that is a marker for transmissible B. cenocepacia strains. This cluster contains four adhesins, one outer-membrane protein, one sensor histidine kinase and two transcriptional regulators. By using PCR, we analysed three genes among 47 Bcc isolates to determine whether the cluster was conserved. These three genes were present in the isolates of the ET-12 lineage but absent in all the other members. Furthermore, the BCAM0224 gene was exclusively detected in this epidemic lineage and may serve as a valuable new addition to the field of Bcc diagnostics. The BCAM0224 gene encodes a putative TAA that demonstrates adhesive properties to the extracellular matrix protein collagen type I. Quantitative real-time PCR analysis indicated that BCAM0224 gene expression occurred preferentially for cells grown under high osmolarity, oxygen-limited conditions and oxidative stress. Inactivation of BCAM0224 in B. cenocepacia attenuates the ability of the mutant to promote cell adherence in vitro and impairs the overall bacterial virulence against Galleria mellonella as a model of infection. Together, our data show that BCAM0224 from B. cenocepacia J2315 represents a new collagen-binding TAA with no bacterial orthologues which has an important role in cellular adhesion and virulence.

Identification of potential therapeutic targets for Burkholderia cenocepacia by comparative transcriptomics

Background

Burkholderia cenocepacia is an endemic soil dweller and emerging opportunistic pathogen in patients with cystic fibrosis (CF). The identification of virulence factors and potential therapeutic targets has been hampered by the genomic diversity within the species as many factors are not shared among the pathogenic members of the species.

Methodology/Principal Findings

In this study, global identification of putative virulence factors was performed by analyzing the transcriptome of two related strains of B. cenocepacia (one clinical, one environmental) under conditions mimicking cystic fibrosis sputum versus soil. Soil is a natural reservoir for this species; hence, genes induced under CF conditions relative to soil may represent adaptations that have occurred in clinical strains. Under CF conditions, several genes encoding proteins thought to be involved in virulence were induced and many new ones were identified. Our analysis, in combination with previous studies, reveals 458 strain-specific genes, 126 clinical-isolate-specific, and at least four species-specific genes that are induced under CF conditions. The chromosomal distribution of the induced genes was disproportionate to the size of the chromosome as genes expressed under soil conditions by both strains were more frequent on the second chromosome and those differentially regulated between strains were more frequent on the third chromosome. Conservation of these induced genes was established using the 11 available Bcc genome sequences to indicate whether potential therapeutic targets would be species-wide.

Conclusions/Significance

Comparative transcriptomics is a useful way to identify new potential virulence factors and therapeutic targets for pathogenic bacteria. We identified eight genes induced under CF conditions that were also conserved in the Bcc and may constitute particularly attractive therapeutic targets due to their signal sequence, predicted cellular location, and homology to known therapeutic targets.

Transcriptional regulation of the tad locus in Aggregatibacter actinomycetemcomitans: a termination cascade

The tad (tight adherence) locus of Aggregatibacter actinomycetemcomitans includes genes for the biogenesis of Flp pili, which are necessary for bacterial adhesion to surfaces, biofilm formation, and pathogenesis. Although studies have elucidated the functions of some of the Tad proteins, little is known about the regulation of the tad locus in A. actinomycetemcomitans. A promoter upstream of the tad locus was previously identified and shown to function in Escherichia coli. Using a specially constructed reporter plasmid, we show here that this promoter (tadp) functions in A. actinomycetemcomitans. To study expression of the pilin gene (flp-1) relative to that of tad secretion complex genes, we used Northern hybridization analysis and a lacZ reporter assay. We identified three terminators, two of which (T1 and T2) can explain flp-1 mRNA abundance, while the third (T3) is at the end of the locus. T1 and T3 have the appearance and behavior of intrinsic terminators, while T2 has a different structure and is inhibited by bicyclomycin, indicating that T2 is probably Rho dependent. To help achieve the appropriate stoichiometry of the Tad proteins, we show that a transcriptional-termination cascade is important to the proper expression of the tad genes. These data indicate a previously unreported mechanism of regulation in A. actinomycetemcomitans and lead to a more complete understanding of its Flp pilus biogenesis.

Identification of sodium chloride-regulated genes in Burkholderia cenocepacia

Previous studies have suggested that the airways of cystic fibrosis (CF) patients have elevated sodium chloride (NaCl) levels due to the malfunctioning of the CF transmembrane conductance regulator protein. For bacteria to survive in this high-salt environment, they must adjust by altering the regulation of gene expression. Among the different bacteria inhabiting the airways of CF patients is the opportunistic pathogen Burkholderia cenocepacia. Previous studies have indicated that B. cenocepacia produces a toxin and cable pili under high osmolar conditions. We used transposon mutagenesis to identify NaCl-regulated genes in the clinical strain B. cenocepacia K56-2. Six transconjugants were induced with increasing NaCl concentration. The DNA flanking the transposon was sequenced and five distinct open reading frames were identified encoding the following putative proteins: an integrase, an NAD-dependent deacetylase, TolB, an oxidoreductase, and a novel hypothetical protein. The collective results of this study provide important information about the physiology of B. cenocepacia when faced with osmotic stress and suggest the identity of significant virulence mechanisms in this opportunistic pathogen.

Evolution of the chaperone/usher assembly pathway: fimbrial classification goes Greek

Many Proteobacteria use the chaperone/usher pathway to assemble proteinaceous filaments on the bacterial surface. These filaments can curl into fimbrial or nonfimbrial surface structures (e.g., a capsule or spore coat). This article reviews the phylogeny of operons belonging to the chaperone/usher assembly class to explore the utility of establishing a scheme for subdividing them into clades of phylogenetically related gene clusters. Based on usher amino acid sequence comparisons, our analysis shows that the chaperone/usher assembly class is subdivided into six major phylogenetic clades, which we have termed alpha-, beta-, gamma-, kappa-, pi-, and sigma-fimbriae. Members of each clade share related operon structures and encode fimbrial subunits with similar protein domains. The proposed classification system offers a simple and convenient method for assigning newly discovered chaperone/usher systems to one of the six major phylogenetic groups.

Modulation of virulence factors in Francisella tularensis determines human macrophage responses

Francisella tularensis, the causative agent of tularemia and Category A biodefense agent, is known to replicate within host macrophages, though the pathogenesis of this organism is incompletely understood. We have isolated a variant of F. tularensis live vaccine strain (LVS) based on colony morphology and its effect on macrophages. Human monocyte-derived macrophages produced more tumor necrosis factor alpha (TNFalpha), interleukin (IL)-1beta, IL-6, and IL-12 p40 following exposure to the variant, designated the activating variant (ACV). The immunoreactivity of the lipopolysaccharide (LPS) from both LVS and ACV was comparable to the previously described blue variant and was distinct from the gray variant of LVS. We found, however, the soluble protein fractions of LVS and ACV differed. Further investigation using two-dimensional gel electrophoresis demonstrated higher levels of several proteins in the parental LVS isolate. The differentially expressed proteins featured several associated with virulence in F. tularensis and other pathogens, including intracellular growth locus C (IglC), a sigma(54)-modulation protein family member (YhbH), and aconitase. ACV reverted to the LVS phenotype, indicated by low cytokine induction and high IglC expression, after growth in a chemically defined medium. These data provide evidence that the levels of virulence factors in F. tularensis are modulated based on culture conditions and that this modulation impacts host responses. This work provides a basis for investigation of Francisella virulence factor regulation and the identification of additional factors, co-regulated with IglC, that affect macrophage responses.

Identification of an RTX determinant of Burkholderia cenocepacia J2315 by subtractive hybridization

This study utilized suppressive subtractive hybridization between the clinical isolate Burkholderia cenocepacia J2315 and the closely related environmental isolate Burkholderia cepacia ATCC 25416T to isolate DNA fragments specific to B. cenocepacia J2315. Analysis of the resulting pools of B. cenocepacia-specific DNAs identified several fragments that may be part of putative virulence factors. Further in silico analysis of a single fragment indicated that it was internal to a gene of which the predicted product had characteristics of repeat in toxin (RTX)-like proteins and high similarity to proteins in other human or plant pathogens. In conjunction with this finding, phenotypic traits associated with known RTX proteins were assessed. A haemagglutinating activity of B. cenocepacia J2315 was identified that was absent in B. cepacia ATCC 25416T. The expression of this activity appeared to be growth phase-dependent. Analysis of the gene presence and haemagglutinating activity across the species of the B. cepacia complex showed that both were common to the ET12 lineage of B. cenocepacia, but were absent in the other species examined. Haemagglutinating activity was limited to isolates with the RTX-like gene. Expression studies utilizing quantitative PCR demonstrated an association between onset of haemagglutinating activity and increased expression of the gene, which suggests that the putative RTX determinant encodes a haemagglutinating activity.

The multifarious, multireplicon Burkholderia cepacia complex

The Burkholderia cepacia complex (Bcc) is a collection of genetically distinct but phenotypically similar bacteria that are divided into at least nine species. Bcc bacteria are found throughout the environment, where they can have both beneficial and detrimental effects on plants and some members can also degrade natural and man-made pollutants. Bcc bacteria are now recognized as important opportunistic pathogens that can cause variable lung infections in cystic fibrosis patients, which result in asymptomatic carriage, chronic infection or 'cepacia syndrome', which is characterized by a rapid decline in lung function that can include invasive disease. Here we highlight the unique characteristics of the Bcc, focusing on the factors that determine virulence.

Expression of type IV pili by Moraxella catarrhalis is essential for natural competence and is affected by iron limitation

Type IV pili, filamentous surface appendages primarily composed of a single protein subunit termed pilin, play a crucial role in the initiation of disease by a wide range of pathogenic bacteria. Although previous electron microscopic studies suggested that pili might be present on the surface of Moraxella catarrhalis isolates, detailed molecular and phenotypic analyses of these structures have not been reported to date. We identified and cloned the M. catarrhalis genes encoding PilA, the major pilin subunit, PilQ, the outer membrane secretin through which the pilus filament is extruded, and PilT, the NTPase that mediates pilin disassembly and retraction. To initiate investigation of the role of this surface organelle in pathogenesis, isogenic pilA, pilT, and pilQ mutants were constructed in M. catarrhalis strain 7169. Comparative analyses of the wild-type 7169 strain and three isogenic pil mutants demonstrated that M. catarrhalis expresses type IV pili that are essential for natural genetic transformation. Our studies suggest type IV pilus production by M. catarrhalis is constitutive and ubiquitous, although pilin expression was demonstrated to be iron responsive and Fur regulated. These data indicate that additional studies aimed at elucidating the prevalence and role of type IV pili in the pathogenesis and host response to M. catarrhalis infections are warranted.

Genetic characterization of a multicomponent signal transduction system controlling the expression of cable pili in Burkholderia cenocepacia

Cable pili are peritrichous organelles expressed by certain strains of Burkholderia cenocepacia, believed to facilitate colonization of the lower respiratory tract in cystic fibrosis patients. The B. cenocepacia cblBACDS operon encodes the structural and accessory proteins required for the assembly of cable pili, as well as a gene designated cblS, predicted to encode a hybrid sensor kinase protein of bacterial two-component signal transduction systems. In this study we report the identification of two additional genes, designated cblT and cblR, predicted to encode a second hybrid sensor kinase and a response regulator, respectively. Analyses of the deduced amino acid sequences of the cblS and cblT gene products revealed that both putative sensor kinases have transmitter and receiver domains and that the cblT gene product has an additional C-terminal HPt domain. Mutagenesis of the cblS, cblT, or cblR gene led to a block in expression of CblA, the major pilin subunit, and a severe decrease in cblA transcript abundance. Using transcriptional fusion analyses, the decrease in the abundance of the cblA transcript in the cblS, cblT, and cblR mutants was shown to be due to a block in transcription from the cblB-proximal promoter, located upstream of the cblBACDS operon. Furthermore, ectopic expression of either cblS or cblR in wild-type B. cenocepacia strain BC7 led to a significant increase, while ectopic expression of cblT resulted in a dramatic decrease, in abundance of the CblA major pilin and the cblA transcript. Our results demonstrate that the B. cenocepacia cblS, cblT, and cblR genes are essential for cable pilus expression and that their effect is exerted at the level of transcription of the cblBACDS operon. These findings are consistent with the proposed function of the cblSTR gene products as a multicomponent signal transduction pathway controlling the expression of cable pilus biosynthetic genes in B. cenocepacia.