Quorum-sensing mutations affect attachment and stability of Burkholderia cenocepacia biofilms

In vitro synthesis and biochemical characterization of acyl-homoserine lactone synthase and its deletion mutant

Quorum sensing can induce density-dependent gene expressions that cause various problems. For quorum-sensing inhibition, fundamental solutions such as gene manipulation are required, and acyl-homoserine lactone synthase (AHL synthase), which synthesizes the universal quorum-sensing signal of gram-negative bacteria, can be used as a target. In this study, researchers synthesized His-tagged AHL synthase and its deletion mutant that lacks the active site and compared their biochemical characteristics. His-YpeI, the 6x His-tagged AHL synthase of Serratia fonticola, and His-ΔYpeI, its deletion mutant, were designed, and their property conservation were examined using in silico projection tools. For in vitro synthesis of enzymes, the His-YpeI CFPS template was synthesized by in vitro gene synthesis, and the His-ΔYpeI CFPS template was obtained by deletion PCR. CFPS was performed and the products were purified with the 6x His-tag. The enzymes' properties were compared using an enzymatic assay. The bioinformatic analysis confirmed the conservation of biochemical properties between 6x His-tagged and untagged enzymes, including helix-turn-helix interactions, hydropathy profiles, and tertiary structure between His-YpeI and YpeI and between His-ΔYpeI and ΔYpeI. His-YpeI and His-ΔYpeI synthesized by CFPS were found to have the expected molecular weights and demonstrated distinct differences in enzyme activity. The analyzed enzymatic constants supported a significant decrease in substrate affinity and reaction rate as a result of YpeI's enzyme active site deletion. This result showed that CFPS could be used for in vitro protein synthesis, and quorum sensing could be inhibited at the enzymatic level due to the enzyme active site's deletion mutation.

Fermentation broth from fruit and vegetable waste works: Reducing the risk of human bacterial pathogens in soil by inhibiting quorum sensing

Fermentation broth from fruit and vegetable waste (FFVW) has demonstrated remarkable ability as a soil amendment and in reducing antibiotic resistance genes (ARGs) pollution. However, the potential of FFVW to mitigate other microbial contamination such as human bacterial pathogens (HBPs) and virulence factor genes (VFGs), which are closely associated with human health, remains unknown. In this study, metagenomic analysis revealed that FFVW reduced the HBPs with high-risk of ARGs and VFGs including Klebsiella pneumoniae (reduced by 40.4 %), Mycobacterium tuberculosis (reduced by 21.4 %) and Streptococcus pneumoniae (reduced by 38.7 %). Correspondingly, VFG abundance in soil decreased from 3.40 copies/cell to 2.99 copies/cell. Further analysis illustrated that these was mainly attributed to the inhibition of quorum sensing (QS). FFVW reduced the abundance of QS signals, QS synthesis genes such as rpaI and luxS, as well as receptor genes such as rpfC and fusK, resulting in a decreased in risk of ARGs and VFGs. The pure culture experiment revealed that the expression of genes related to QS, VFGs, ARGs and mobile genetic elements (MGEs) were downregulated in Pseudomonas aeruginosa, Staphylococcus aureus, Escherichia coli and K. pneumoniae treated by FFVW, consistent with the result of metagenomic analysis. This study suggested an environmentally friendly approach for controlling soil VFGs/ARGs-carrying HBPs, which is crucial for both soil and human health under the framework of "One Health".

Molecular and Phenotypic Investigation on Antibacterial Activities of Limonene Isomers and Its Oxidation Derivative against Xanthomonas oryzae pv. oryzae

Xanthomonas oryzae pv. oryzae (Xoo) causes a devastating bacterial leaf blight in rice. Here, the antimicrobial effects of D-limonene, L-limonene, and its oxidative derivative carveol against Xoo were investigated. We revealed that carveol treatment at ≥ 0.1 mM in liquid culture resulted in significant decrease in Xoo growth rate (> 40%) in a concentration-dependent manner, and over 1 mM, no growth was observed. The treatment with D-limonene and L-limonene also inhibited the Xoo growth but to a lesser extent compared to carveol. These results were further elaborated with the assays of motility, biofilm formation and xanthomonadin production. The carveol treatment over 1 mM caused no motilities, basal level of biofilm formation (< 10%), and significantly reduced xanthomonadin production. The biofilm formation after the treatment with two limonene isomers was decreased in a concentration-dependent manner, but the degree of the effect was not comparable to carveol. In addition, there was negligible effect on the xanthomonadin production mediated by the treatment of two limonene isomers. Field emission-scanning electron microscope (FE-SEM) unveiled that all three compounds used in this study cause severe ultrastructural morphological changes in Xoo cells, showing shrinking, shriveling, and holes on their surface. Moreover, quantitative real-time PCR revealed that carveol and D-limonene treatment significantly down-regulated the expression levels of genes involved in virulence and biofilm formation of Xoo, but not with L-limonene. Together, we suggest that limonenes and carveol will be the candidates of interest in the development of biological pesticides.

Quorum sensing inhibition through site-directed mutation by deletion PCR

Quorum sensing induces biofilms and virulence factors that are adverse industrially and medically. Nowadays, quorum sensing inhibitions focus on signal analogs or signal degradation, but these methods have several downsides, which are temporal and affected by several environmental factors. In this research, we used deletion PCR to perform site-directed mutagenesis on the quorum sensing pathway gene and then analyzed its effects on quorum sensing. Serratia fonticola DSM 4576 strain was utilized as the research strain, and the gram-negative bacteria's universal quorum sensing pathway, which is conducted by acyl-homoserine lactone (AHL), was analyzed. The structure and active site of the AHL synthase enzyme encoded by S. fonticola DSM 4576's luxI-type gene were predicted. The gene's partial section solely encodes the enzyme's active site. By using sequence and ligation-independent cloning, the obtained mutagenic gene was cloned into the suicide vector pEX18Ap. The recombinant vector was used to transform wild-type S. fonticola DSM 4576 strains, and the mutants were determined through two-step selections and PCR genotyping. The gene expression level and biofilm formation were quantitatively analyzed through RT-PCR and biofilm assay, and no significant difference was noted in the gene expression between wild types and mutants. However, when mutants were compared to wildtypes, there was a significant decrease in biofilm formation as a result of quorum sensing induced bioreaction. Thus, we propose a quorum sensing inhibitory technique based on enzyme mutation on the quorum sensing pathway, and we proved the feasibility of enzyme active site's site-directed mutation through deletion PCR.

Current Understanding on Adhesion and Biofilm Development in Actinobacteria

Biofilm formation and microbial adhesion are two related and complex phenomena. These phenomena are known to play an important role in microbial life and various functions with positive and negative aspects. Actinobacteria have wide distribution in aquatic and terrestrial ecosystems. This phylum is very large and diverse and contains two important genera Streptomyces and Mycobacteria. The genus Streptomyces is the most biotechnologically important, while the genus Mycobacteria contains the pathogenic species of Mycobacteriaceae. According to the literature, the majority of studies carried out on actinomycetes are focused on the detection of new molecules. Despite the well-known diversity and metabolic activities, less attention has been paid to this phylum. Research on adhesion and biofilm formation is not well developed. In the present review, an attempt has been made to review the literature available on the different aspects on biofilm formation and adhesion of Actinobacteria. We focus especially on the genus Streptomyces. Furthermore, a brief overview about the molecules and structures involved in the adhesion phenomenon in the most relevant genus is summarized. We mention the mechanisms of quorum sensing and quorum quenching because of their direct association with biofilm formation.

Effect of essential oils on pathogenic and biofilm-forming Vibrio parahaemolyticus strains

In this study, the effect of three essential oils (EOs) - clove oil (CO), thyme oil (TO), and garlic oil (GO), which are generally recognized as safe - on the planktonic growth, minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), motility, biofilm formation, and quorum sensing (QS) of Vibrio parahaemolyticus was investigated. All three EOs showed bacteriostatic activity, with MICs in the range 0.02%-0.09% (v/v). CO and TO completely controlled planktonic growth at 0.28% and 0.08% (v/v), which is four times their MIC (4 × MIC), after 10 min, whereas GO completely controlled growth at 0.36% (v/v) (4 × MIC) after treatment for 20 min. V. parahaemolyticus motility was significantly reduced by all three EOs at 4 × MIC (0.28% for CO, 0.08% for TO, and 0.36% for GO), whereas QS was controlled and biofilm formation reduced by all three EOs at 8 × MIC (0.56% for CO, 0.16% for TO, and 0.72% for GO) after 30 min of treatment. These results suggest that CO, TO, and GO have a significant inhibitory effect on V. parahaemolyticus cells in biofilm sand thus represent a promising strategy for improving food safety. These results provide the evidence required to encourage further research into the practical use of the proposed EOs in food preparation processes.

Loss of O-Linked Protein Glycosylation in Burkholderia cenocepacia Impairs Biofilm Formation and Siderophore Activity and Alters Transcriptional Regulators

O-linked protein glycosylation is a conserved feature of the Burkholderia genus. The addition of the trisaccharide β-Gal-(1,3)-α-GalNAc-(1,3)-β-GalNAc to membrane exported proteins in Burkholderia cenocepacia is required for bacterial fitness and resistance to environmental stress. However, the underlying causes of the defects observed in the absence of glycosylation are unclear. Using proteomics, luciferase reporter assays, and DNA cross-linking, we demonstrate the loss of glycosylation leads to changes in transcriptional regulation of multiple proteins, including the repression of the master quorum CepR/I. These proteomic and transcriptional alterations lead to the abolition of biofilm formation and defects in siderophore activity. Surprisingly, the abundance of most of the known glycosylated proteins did not significantly change in the glycosylation-defective mutants, except for BCAL1086 and BCAL2974, which were found in reduced amounts, suggesting they could be degraded. However, the loss of these two proteins was not responsible for driving the proteomic alterations, biofilm formation, or siderophore activity. Together, our results show that loss of glycosylation in B. cenocepacia results in a global cell reprogramming via alteration of the transcriptional regulatory systems, which cannot be explained by the abundance changes in known B. cenocepacia glycoproteins.IMPORTANCE Protein glycosylation is increasingly recognized as a common posttranslational protein modification in bacterial species. Despite this commonality, our understanding of the role of most glycosylation systems in bacterial physiology and pathogenesis is incomplete. In this work, we investigated the effect of the disruption of O-linked glycosylation in the opportunistic pathogen Burkholderia cenocepacia using a combination of proteomic, molecular, and phenotypic assays. We find that in contrast to recent findings on the N-linked glycosylation systems of Campylobacter jejuni, O-linked glycosylation does not appear to play a role in proteome stabilization of most glycoproteins. Our results reveal that loss of glycosylation in B. cenocepacia strains leads to global proteome and transcriptional changes, including the repression of the quorum-sensing regulator cepR (BCAM1868) gene. These alterations lead to dramatic phenotypic changes in glycosylation-null strains, which are paralleled by both global proteomic and transcriptional alterations, which do not appear to directly result from the loss of glycosylation per se. This research unravels the pleiotropic effects of O-linked glycosylation in B. cenocepacia, demonstrating that its loss does not simply affect the stability of the glycoproteome, but also interferes with transcription and the broader proteome.

N-Acylhomoserine lactone-mediated quorum sensing regulates biofilm structure in Methylobacterium populi P-1M, an isolate from a pink-pigmented household biofilm

Numerous gram-negative bacteria have quorum-sensing systems and produce AHL as a quorum-sensing signal molecule. In this study, we demonstrated that Methylobacterium populi P-1M, an isolate from a pink-pigmented household biofilm, produced two AHLs, C14:1-HSL as a predominant product and 3OHC14-HSL as a minor product. The complete genome sequence of M. populi P-1M revealed the presence of genes that are predicted to encode an AHL synthase (mpoI) and AHL receptor (mpoR). M. populi P-1M formed a pellicle-like biofilm, which had a flat surface and was easily removable. In contrast, biofilms formed by mpoI and/or mpoR deletion mutants had a wavy surface structure and strongly adhered to the glass tube. When C14:1-HSL was added to the mpoI mutant culture, the biofilm structure resembled that of the wild-type strain. These results demonstrated that the structure and adhesion strength of M. populi P-1M biofilms are determined in part by AHL-mediated quorum sensing.

Abbreviations: AHL: N-acyl-l-homoserine lactone; C14:1-HSL: N-tetradecenoyl-l-homoserine lactone; 3OHC14-HSL: N-(3-hydroxytetradecanoyl)-l-homoserine lactone; SAM: S-adenosyl-l-methionine; ACP: acyl-acyl carrier protein; EPS: extracellular polysaccharide; DMSO: dimethyl sulfoxide

Competitive Fitness of Essential Gene Knockdowns Reveals a Broad-Spectrum Antibacterial Inhibitor of the Cell Division Protein FtsZ

To streamline the elucidation of antibacterial compounds' mechanism of action, comprehensive high-throughput assays interrogating multiple putative targets are necessary. However, current chemogenomic approaches for antibiotic target identification have not fully utilized the multiplexing potential of next-generation sequencing. Here, we used Illumina sequencing of transposon insertions to track the competitive fitness of a Burkholderia cenocepacia library containing essential gene knockdowns. Using this method, we characterized a novel benzothiadiazole derivative, 10126109 (C109), with antibacterial activity against B. cenocepacia, for which whole-genome sequencing of low-frequency spontaneous drug-resistant mutants had failed to identify the drug target. By combining the identification of hypersusceptible mutants and morphology screening, we show that C109 targets cell division. Furthermore, fluorescence microscopy of bacteria harboring green fluorescent protein (GFP) cell division protein fusions revealed that C109 prevents divisome formation by altering the localization of the essential cell division protein FtsZ. In agreement with this, C109 inhibited both the GTPase and polymerization activities of purified B. cenocepacia FtsZ. C109 displayed antibacterial activity against Gram-positive and Gram-negative cystic fibrosis pathogens, including Mycobacterium abscessus C109 effectively cleared B. cenocepacia infection in the Caenorhabditis elegans model and exhibited additive interactions with clinically relevant antibiotics. Hence, C109 is an enticing candidate for further drug development.

Laser-induced vapour nanobubbles improve drug diffusion and efficiency in bacterial biofilms

Hindered penetration of antibiotics through biofilms is one of the reasons for the alarming increase in bacterial tolerance to antibiotics. Here, we investigate the potential of laser-induced vapour nanobubbles (VNBs) formed around plasmonic nanoparticles to locally disturb biofilm integrity and improve antibiotics diffusion. Our results show that biofilms of both Gram-negative (Burkholderia multivorans, Pseudomonas aeruginosa) and Gram-positive (Staphylococcus aureus) bacteria can be loaded with cationic 70-nm gold nanoparticles and that subsequent laser illumination results in VNB formation inside the biofilms. In all types of biofilms tested, VNB formation leads to substantial local biofilm disruption, increasing tobramycin efficacy up to 1-3 orders of magnitude depending on the organism and treatment conditions. Altogether, our results support the potential of laser-induced VNBs as a new approach to disrupt biofilms of a broad range of organisms, resulting in improved antibiotic diffusion and more effective biofilm eradication.

Control of Biofilm Formation in Healthcare: Recent Advances Exploiting Quorum-Sensing Interference Strategies and Multidrug Efflux Pump Inhibitors

Biofilm formation in healthcare is an issue of considerable concern, as it results in increased morbidity and mortality, imposing a significant financial burden on the healthcare system. Biofilms are highly resistant to conventional antimicrobial therapies and lead to persistent infections. Hence, there is a high demand for novel strategies other than conventional antibiotic therapies to control biofilm-based infections. There are two approaches which have been employed so far to control biofilm formation in healthcare settings: one is the development of biofilm inhibitors based on the understanding of the molecular mechanism of biofilm formation, and the other is to modify the biomaterials which are used in medical devices to prevent biofilm formation. This review will focus on the recent advances in anti-biofilm approaches by interrupting the quorum-sensing cellular communication system and the multidrug efflux pumps which play an important role in biofilm formation. Research efforts directed towards these promising strategies could eventually lead to the development of better anti-biofilm therapies than the conventional treatments.

Investigating the Mechanism of Action of Diketopiperazines Inhibitors of the Burkholderia cenocepacia Quorum Sensing Synthase CepI: A Site-Directed Mutagenesis Study

Quorum sensing (QS) is a bacterial intercellular communication process which controls the production of major virulence factors, such as proteases, siderophores, and toxins, as well as biofilm formation. Since the inhibition of this pathway reduces bacterial virulence, QS is considered a valuable candidate drug target, particularly for the treatment of opportunistic infections, such as those caused by Burkholderia cenocepacia in cystic fibrosis patients. Diketopiperazine inhibitors of the acyl homoserine lactone synthase CepI have been recently described. These compounds are able to impair the ability of B. cenocepacia to produce proteases, siderophores, and to form biofilm, being also active in a Caenorhabditis elegans infection model. However, the precise mechanism of action of the compounds, as well as their effect on the cell metabolism, fundamental for candidate drug optimization, are still not completely defined. Here, we performed a proteomic analysis of B. cenocepacia cells treated with one of these inhibitors, and compared it with a cepI deleted strain. Our results demonstrate that the effects of the compound are similar to the deletion of cepI, clearly confirming that these molecules function as inhibitors of the acyl homoserine lactone synthase. Moreover, to deepen our knowledge about the binding mechanisms of the compound to CepI, we exploited previously published in silico structural insights about this enzyme structure and validated different candidate binding pockets on the enzyme surface using site-directed mutagenesis and biochemical analyses. Our experiments identified a region near the predicted S-adenosylmethionine binding site critically involved in interactions with the inhibitor. These results could be useful for future structure-based optimization of these CepI inhibitors.

Thyme Oil Reduces Biofilm Formation and Impairs Virulence of Xanthomonas oryzae

Xanthomonas oryzae pv. oryzae (Xoo), a common bacterial plant pathogen regulates its virulence and biofilm formation attribute via a chemical method of communication. Disabling this mechanism offers a promising alternative to reduce the virulence and pathogencity of the microorganism. In this study, the effect of thyme (THY) oil on Quorum Sensing mediated synthesis of various virulence factors and biofilm formation was analyzed. Treatment of Xoo with 500 ppm THY oil displayed a significant diminution in swimming, swarming, exopolysaccharide and xanthomonadin secretion. However, no effect was observed on bacterial growth kinetics and metabolic activity of the cells. Results were further authenticated by RT-qPCR as significant reduction in motA, motB, and flgE genes was observed upon THY oil treatment. Similarly, the expression of some extracellular enzyme genes such as endoglucanase, xylanase, cellobiosidase, and polygalacturonase was also found to be significantly reduced. However, biochemical plate assays revealed insignificant effect of 500 ppm THY oil on secretion of protease, cellulase, and lipase enzymes. The rpfF gene known to play a crucial role in the virulence of the phytopathogenic bacteria was also significantly reduced in the THY oil treated Xoo cells. HPTLC analysis further revealed significant reduction in DSF and BDSF signaling molecules when Xoo cells were treated with 500 ppm THY oil. Disease reduction was observed in in vitro agar plate assay as lesion length was reduced in THY oil treated Xoo cells when compared with the alone treatment. GC-MS result revealed thymol as the active and major component of THY oil which showed potential binding with rpfF gene. Application of 75 μM thymol resulted in downregulation of gumC, motA, estA, virulence acvB and pglA along with rpfF. The other genes such as cheD, flgA, cheY, and pilA, were not found to be significantly affected. Overall, the results clearly indicated THY oil and its active component Thymol to be a potential candidate for the development of anti-virulence agent which in future when applied in combination with conventional bactericides might not only help in lowering the dose of bactericides but also be successful in curbing the disease progression in rice.

Immune Recognition of the Epidemic Cystic Fibrosis Pathogen Burkholderia dolosa

Burkholderia dolosa caused an outbreak in the cystic fibrosis (CF) clinic at Boston Children's Hospital from 1998 to 2005 and led to the infection of over 40 patients, many of whom died due to complications from infection by this organism. To assess whether B. dolosa significantly contributes to disease or is recognized by the host immune response, mice were infected with a sequenced outbreak B. dolosa strain, AU0158, and responses were compared to those to the well-studied CF pathogen Pseudomonas aeruginosa In parallel, mice were also infected with a polar flagellin mutant of B. dolosa to examine the role of flagella in B. dolosa lung colonization. The results showed a higher persistence in the host by B. dolosa strains, and yet, neutrophil recruitment and cytokine production were lower than those with P. aeruginosa The ability of host immune cells to recognize B. dolosa was then assessed, B. dolosa induced a robust cytokine response in cultured cells, and this effect was dependent on the flagella only when bacteria were dead. Together, these results suggest that B. dolosa can be recognized by host cells in vitro but may avoid or suppress the host immune response in vivo through unknown mechanisms. B. dolosa was then compared to other Burkholderia species and found to induce similar levels of cytokine production despite being internalized by macrophages more than Burkholderia cenocepacia strains. These data suggest that B. dolosa AU0158 may act differently with host cells and is recognized differently by immune systems than are other Burkholderia strains or species.

Prevalence and Outcomes of Achromobacter Species Infections in Adults with Cystic Fibrosis: a North American Cohort Study

Achromobacter species are increasingly being detected in cystic fibrosis (CF) patients, with an unclear epidemiology and impact. We studied a cohort of patients attending a Canadian adult CF clinic who had positive sputum cultures for Achromobacter species in the period from 1984 to 2013. Infection was categorized as transient or persistent (≥50% positive cultures for 1 year). Those with persistent infection were matched 2:1 with age-, sex-, and time-matched controls without a history of Achromobacter infection, and mixed-effects models were used to assess pulmonary exacerbation (PEx) frequency and lung function decline. Isolates from a biobank were retrospectively assessed, identified to the species level by nrdA sequencing, and genotyped using pulsed-field gel electrophoresis (PFGE). Thirty-four patients (11% of those in our clinic), with a median age of 24 years (interquartile range [IQR], 20.3 to 29.8 years), developed Achromobacter infection. Ten patients (29%) developed persistent infection. Persistence did not denote permanence, as most patients ultimately cleared infection, often after years. Patients were more likely to experience PEx at incident isolation than at prior or subsequent visits (odds ratio [OR], 2.7 [95% confidence interval {CI}, 1.2 to 6.7]; P = 0.03). Following persistent infection, there was no difference in annual lung function decline (-1.08% [95% CI, -2.73 to 0.57%] versus -2.74% [95% CI, -4.02 to 1.46%]; P = 0.12) or the odds of PEx (OR, 1.21 [95% CI, 0.45 to 3.28]; P = 0.70). Differential virulence among Achromobacter species was not observed, and no cases of transmission occurred. We demonstrated that incident Achromobacter infection was associated with a greater risk of PEx; however, neither transient nor chronic infection was associated with a worsened long-term prognosis. Large, multicenter studies are needed to clarify the clinical impact, natural history, and transmissibility of Achromobacter.

Control of Attachment of Pseudomonas aeruginosa and Burkholderia cepacia to Surfaces by Shear Force

  The effect of physical shearing on the attachment of six Pseudomonas aeruginosa strains and six Burkholderia cepacia strains to glass, stainless steel, polystyrene and Teflon® was determined. A significant (p < 0.05) decrease in hydrophobicity was apparent for all P. aeruginosa strains (17-36%) and B. cepacia, MS 5 (20%) after shearing. A significant (p < 0.05) decrease in attachment of some P. aeruginosa (0.2-0.5 log CFU/cm2) and B. cepacia (0.2-0.4 log CFU/cm2) strains to some surface types was apparent after shearing. Significant (p < 0.05) correlation was observed for both numbers of flagellated cells and hydrophobicity against attachment to glass, stainless steel and polystyrene for P. aeruginosa while only hydrophobicity showed significant correlation against the same surfaces for B. cepacia. Scanning electron microscopy and protein analysis showed that shearing removed surface proteins from the cells and may have led to the observed changes in hydrophobicity and attachment to abiotic surfaces.

Activity of Cysteamine against the Cystic Fibrosis Pathogen Burkholderia cepacia Complex

There are no wholly successful chemotherapeutic strategies against Burkholderia cepacia complex (BCC) colonization in cystic fibrosis (CF). We assessed the impact of cysteamine (Lynovex) in combination with standard-of-care CF antibiotics in vitro against BCC CF isolates by the concentration at which 100% of bacteria were killed (MIC₁₀₀) and checkerboard assays under CLSI standard conditions. Cysteamine facilitated the aminoglycoside-, fluoroquinolone- and folate pathway inhibitor-mediated killing of BCC organisms that were otherwise resistant or intermediately sensitive to these antibiotic classes. Slow-growing BCC strains are often recalcitrant to treatment and form biofilms. In assessing the impact of cysteamine on biofilms, we demonstrated inhibition of BCC biofilm formation at sub-MIC₁₀₀s of cysteamine.

Quorum Sensing Influences Burkholderia thailandensis Biofilm Development and Matrix Production

Members of the genus Burkholderia are known to be adept at biofilm formation, which presumably assists in the survival of these organisms in the environment and the host. Biofilm formation has been linked to quorum sensing (QS) in several bacterial species. In this study, we characterized Burkholderia thailandensis biofilm development under flow conditions and sought to determine whether QS contributes to this process. B. thailandensis biofilm formation exhibited an unusual pattern: the cells formed small aggregates and then proceeded to produce mature biofilms characterized by "dome" structures filled with biofilm matrix material. We showed that this process was dependent on QS. B. thailandensis has three acyl-homoserine lactone (AHL) QS systems (QS-1, QS-2, and QS-3). An AHL-negative strain produced biofilms consisting of cell aggregates but lacking the matrix-filled dome structures. This phenotype was rescued via exogenous addition of the three AHL signals. Of the three B. thailandensis QS systems, we show that QS-1 is required for proper biofilm development, since a btaR1 mutant, which is defective in QS-1 regulation, forms biofilms without these dome structures. Furthermore, our data show that the wild-type biofilm biomass, as well as the material inside the domes, stains with a fucose-binding lectin. The btaR1 mutant biofilms, however, are negative for fucose staining. This suggests that the QS-1 system regulates the production of a fucose-containing exopolysaccharide in wild-type biofilms. Finally, we present data showing that QS ability during biofilm development produces a biofilm that is resistant to dispersion under stress conditions.

IMPORTANCE

The saprophyte Burkholderia thailandensis is a close relative of the pathogenic bacterium Burkholderia pseudomallei, the causative agent of melioidosis, which is contracted from its environmental reservoir. Since most bacteria in the environment reside in biofilms, B. thailandensis is an ideal model organism for investigating questions in Burkholderia physiology. In this study, we characterized B. thailandensis biofilm development and sought to determine if quorum sensing (QS) contributes to this process. Our work shows that B. thailandensis produces biofilms with unusual dome structures under flow conditions. Our findings suggest that these dome structures are filled with a QS-regulated, fucose-containing exopolysaccharide that may be involved in the resilience of B. thailandensis biofilms against changes in the nutritional environment.

Discovery of new diketopiperazines inhibiting Burkholderia cenocepacia quorum sensing in vitro and in vivo

Burkholderia cenocepacia, an opportunistic respiratory pathogen particularly relevant for cystic fibrosis patients, is difficult to eradicate due to its high level of resistance to most clinically relevant antimicrobials. Consequently, the discovery of new antimicrobials as well as molecules capable of inhibiting its virulence is mandatory. In this regard quorum sensing (QS) represents a good target for anti-virulence therapies, as it has been linked to biofilm formation and is important for the production of several virulence factors, including proteases and siderophores. Here, we report the discovery of new diketopiperazine inhibitors of the B. cenocepacia acyl homoserine lactone synthase CepI, and report their anti-virulence properties. Out of ten different compounds assayed against recombinant CepI, four were effective inhibitors, with IC₅₀ values in the micromolar range. The best compounds interfered with protease and siderophore production, as well as with biofilm formation, and showed good in vivo activity in a Caenorhabditis elegans infection model. These molecules were also tested in human cells and showed very low toxicity. Therefore, they could be considered for in vivo combined treatments with established or novel antimicrobials, to improve the current therapeutic strategies against B. cenocepacia.

Burkholderia cepacia Complex Vaccines: Where Do We Go from here?

Burkholderia comprises a wide variety of environmental Gram-negative bacteria. Burkholderia cepacia complex (Bcc) includes several Burkholderia species that pose a health hazard as they are able to cause respiratory infections in patients with chronic granulomatous disease and cystic fibrosis. Due to the intrinsic resistance to a wide array of antibiotics and naturally occurring immune evasion strategies, treatment of Bcc infections often proves to be unsuccessful. To date, limited work related to vaccine development has been performed for Bcc pathogens. In this review, we have gathered key aspects of Bcc research that have been reported in recent years related to vaccine efforts, virulence, immune responses, and animal models, and use this information to inform the research community of areas of opportunity toward development of a viable Bcc vaccine.

A Comparative Analysis of Acyl-Homoserine Lactone Synthase Assays

Quorum sensing is cell-to-cell communication that allows bacteria to coordinate attacks on their hosts by inducing virulent gene expression, biofilm production, and other cellular functions, including antibiotic resistance. AHL synthase enzymes synthesize N-acyl-l-homoserine lactones, commonly referred to as autoinducers, to facilitate quorum sensing in Gram-negative bacteria. Studying the synthases, however, has proven to be a difficult road. Two assays, including a radiolabeled assay and a colorimetric (DCPIP) assay are well-documented in literature to study AHL synthases. In this paper, we describe additional methods that include an HPLC-based, C-S bond cleavage and coupled assays to investigate this class of enzymes. In addition, we compare and contrast each assay for both acyl-CoA- and acyl-ACP-utilizing synthases. The expanded toolkit described in this study should facilitate mechanistic studies on quorum sensing signal synthases and expedite discovery of antivirulent compounds.

Regulation of biofilm formation in Pseudomonas and Burkholderia species

In the present review, we describe and compare the molecular mechanisms that are involved in the regulation of biofilm formation by Pseudomonas putida, Pseudomonas fluorescens, Pseudomonas aeruginosa and Burkholderia cenocepacia. Our current knowledge suggests that biofilm formation is regulated by cyclic diguanosine-5'-monophosphate (c-di-GMP), small RNAs (sRNA) and quorum sensing (QS) in all these bacterial species. The systems that employ c-di-GMP as a second messenger regulate the production of exopolysaccharides and surface proteins which function as extracellular matrix components in the biofilms formed by the bacteria. The systems that make use of sRNAs appear to regulate the production of exopolysaccharide biofilm matrix material in all these species. In the pseudomonads, QS regulates the production of extracellular DNA, lectins and biosurfactants which all play a role in biofilm formation. In B.cenocepacia QS regulates the expression of a large surface protein, lectins and extracellular DNA that all function as biofilm matrix components. Although the three regulatory systems all regulate the production of factors used for biofilm formation, the molecular mechanisms involved in transducing the signals into expression of the biofilm matrix components differ between the species. Under the conditions tested, exopolysaccharides appears to be the most important biofilm matrix components for P.aeruginosa, whereas large surface proteins appear to be the most important biofilm matrix components for P.putida, P.fluorescens, and B.cenocepacia.

Two quorum sensing systems control biofilm formation and virulence in members of the Burkholderia cepacia complex

The Burkholderia cepacia complex (Bcc) consists of 17 closely related species that are problematic opportunistic bacterial pathogens for cystic fibrosis patients and immunocompromised individuals. These bacteria are capable of utilizing two different chemical languages: N-acyl homoserine lactones (AHLs) and cis-2-unsaturated fatty acids. Here we summarize the current knowledge of the underlying molecular architectures of these communication systems, showing how they are interlinked and discussing how they regulate overlapping as well as specific sets of genes. A particular focus is laid on the role of these signaling systems in the formation of biofilms, which are believed to be highly important for chronic infections. We review genes that have been implicated in the sessile lifestyle of this group of bacteria. The new emerging role of the intracellular second messenger cyclic dimeric guanosine monophosphate (c-di-GMP) as a downstream regulator of the fatty acid signaling cascade and as a key factor in biofilm formation is also discussed.

Ferrous iron is a significant component of bioavailable iron in cystic fibrosis airways

Chronic, biofilm-like infections by the opportunistic pathogen Pseudomonas aeruginosa are a major cause of mortality in cystic fibrosis (CF) patients. While much is known about P. aeruginosa from laboratory studies, far less is understood about what it experiences in vivo. Iron is an important environmental parameter thought to play a central role in the development and maintenance of P. aeruginosa infections, for both anabolic and signaling purposes. Previous studies have focused on ferric iron [Fe(III)] as a target for antimicrobial therapies; however, here we show that ferrous iron [Fe(II)] is abundant in the CF lung (~39 µM on average for severely sick patients) and significantly correlates with disease severity (ρ = −0.56, P = 0.004), whereas ferric iron does not (ρ = −0.28, P = 0.179). Expression of the P. aeruginosa genes bqsRS, whose transcription is upregulated in response to Fe(II), was high in the majority of patients tested, suggesting that increased Fe(II) is bioavailable to the infectious bacterial population. Because limiting Fe(III) acquisition inhibits biofilm formation by P. aeruginosa in various oxic in vitro systems, we also tested whether interfering with Fe(II) acquisition would improve biofilm control under anoxic conditions; concurrent sequestration of both iron oxidation states resulted in a 58% reduction in biofilm accumulation and 28% increase in biofilm dissolution, a significant improvement over Fe(III) chelation treatment alone. This study demonstrates that the chemistry of infected host environments coevolves with the microbial community as infections progress, which should be considered in the design of effective treatment strategies at different stages of disease.

Iron is an important environmental parameter that helps pathogens thrive in sites of infection, including those of cystic fibrosis (CF) patients. Ferric iron chelation therapy has been proposed as a novel therapeutic strategy for CF lung infections, yet until now, the iron oxidation state has not been measured in the host. In studying mucus from the infected lungs of multiple CF patients from Europe and the United States, we found that ferric and ferrous iron change in concentration and relative proportion as infections progress; over time, ferrous iron comes to dominate the iron pool. This information is relevant to the design of novel CF therapeutics and, more broadly, to developing accurate models of chronic CF infections.

Quorum sensing systems influence Burkholderia cenocepacia virulence

Burkholderia cepacia complex strains communicate using N-acyl homoserine lactones and BDSF-dependent quorum sensing (QS) systems. Burkholderia cenocepacia QS systems include CepIR, CciIR, CepR2 and BDSF. Analysis of CepR, CciIR, CepR2 and RpfF (BDSF synthase) QS regulons revealed that these QS systems both independently regulate and coregulate many target genes, often in an opposing manner. The role of QS and several QS-regulated genes in virulence has been determined using vertebrate, invertebrate and plant infection models. Virulence phenotypes are strain and model dependent, suggesting that different QS-regulated genes are important depending on the strain and type of infection. QS inhibitors in combination with antibiotics can reduce biofilm formation and virulence in infection models.

Phenotypic and genotypic characterisation of Burkholderia cenocepacia J2315 mutants affected in homoserine lactone and diffusible signal factor-based quorum sensing systems suggests interplay between both types of systems

Many putative virulence factors of Burkholderia cenocepacia are controlled by various quorum sensing (QS) circuits. These QS systems either use N-acyl homoserine lactones (AHL) or cis-2-dodecenoic acid ("Burkholderia diffusible signal factor", BDSF) as signalling molecules. Previous work suggested that there is little cross-talk between both types of systems. We constructed mutants in B. cenocepacia strain J2315, in which genes encoding CepI (BCAM1870), CciI (BCAM0239a) and the BDSF synthase (BCAM0581) were inactivated, and also constructed double (ΔcepIΔBCAM0581, ΔcciIΔBCAM0581 and ΔcepIΔcciI) mutants and a triple (ΔcepIΔcciIΔBCAM0581) mutant. Subsequently we investigated phenotypic properties (antibiotic susceptibility, biofilm formation, production of AHL and BDSF, protease activity and virulence in Caenorhabditis elegans) and measured gene expression in these mutants, and this in the presence and absence of added BDSF, AHL or both. The triple mutant was significantly more affected in biofilm formation, antimicrobial susceptibility, virulence in C. elegans, and protease production than either the single or double mutants. The ΔBCAM0581 mutant and the ΔcepIΔBCAM0581 and ΔcciIΔBCAM0581 double mutants produced significantly less AHL compared to the WT strain and the ΔcepI and ΔcciI single mutant, respectively. The expression of cepI and cciI in ΔBCAM0581, was approximately 3-fold and 7-fold (p<0.05) lower than in the WT, respectively. The observed differences in AHL production, expression of cepI and cciI and QS-controlled phenotypes in the ΔBCAM0581 mutant could (at least partially) be restored by addition of BDSF. Our data suggest that, in B. cenocepacia J2315, AHL and BDSF-based QS systems co-regulate the same set of genes, regulate different sets of genes that are involved in the same phenotypes and/or that the BDSF system controls the AHL-based QS system. As the expression of the gene encoding the C6-HSL synthase CciI (and to a lesser extent the C8-HSL synthase CepI) is partially controlled by BDSF, it seems likely that the BDSF QS systems controls AHL production through this system.

Tangled bank of experimentally evolved Burkholderia biofilms reflects selection during chronic infections

How diversity evolves and persists in biofilms is essential for understanding much of microbial life, including the uncertain dynamics of chronic infections. We developed a biofilm model enabling long-term selection for daily adherence to and dispersal from a plastic bead in a test tube. Focusing on a pathogen of the cystic fibrosis lung, Burkholderia cenocepacia, we sequenced clones and metagenomes to unravel the mutations and evolutionary forces responsible for adaptation and diversification of a single biofilm community during 1,050 generations of selection. The mutational patterns revealed recurrent evolution of biofilm specialists from generalist types and multiple adaptive alleles at relatively few loci. Fitness assays also demonstrated strong interference competition among contending mutants that preserved genetic diversity. Metagenomes from five other independently evolved biofilm lineages revealed extraordinary mutational parallelism that outlined common routes of adaptation, a subset of which was found, surprisingly, in a planktonic population. These mutations in turn were surprisingly well represented among mutations that evolved in cystic fibrosis isolates of both Burkholderia and Pseudomonas. These convergent pathways included altered metabolism of cyclic diguanosine monophosphate, polysaccharide production, tricarboxylic acid cycle enzymes, global transcription, and iron scavenging. Evolution in chronic infections therefore may be driven by mutations in relatively few pathways also favored during laboratory selection, creating hope that experimental evolution may illuminate the ecology and selective dynamics of chronic infections and improve treatment strategies.

Dependence of structure stability and integrity of aerobic granules on ATP and cell communication

Aerobic granules are dense and compact microbial aggregates with various bacterial species. Recently, aerobic granulation technology has been extensively explored for treatment of municipal and industrial wastewaters. However, little information is currently available with regard to their structure stability and integrity at levels of energy metabolism and cell communication. In the present study, a typical chemical uncoupler, 3,3',4',5-tetrachlorosalicylanilide with the power to dissipate proton motive force and subsequently inhibit adenosine triphosphate (ATP) generation, was used to investigate possible roles of ATP and cell communication in maintaining the structure stability and integrity of aerobic granules. It was found that inhibited ATP synthesis resulted in the reduced production of autoinducer-2 and N-acylhomoserine lactones essential for cell communication, while lowered extracellular polymeric substance (EPS) production was also observed. As a consequence, aerobic granules appeared to break up. This study showed that ATP-dependent quorum sensing and EPS were essential for sustaining the structure stability and integrity of aerobic granules.

The Burkholderia cenocepacia sensor kinase hybrid AtsR is a global regulator modulating quorum-sensing signalling

Burkholderia cenocepacia is commonly found in the environment and also as an important opportunistic pathogen infecting patients with cystic fibrosis. Successful infection by this bacterium requires coordinated expression of virulence factors, which is achieved through different quorum sensing (QS) regulatory systems. Biofilm formation and Type 6 secretion system (T6SS) expression in B. cenocepacia K56-2 are positively regulated by QS and negatively regulated by the sensor kinase hybrid AtsR. This study reveals that in addition to affecting biofilm and T6SS activity, the deletion of atsR in B. cenocepacia leads to overproduction of other QS-regulated virulence determinants including proteases and swarming motility. Expression of the QS genes, cepIR and cciIR, was upregulated in the ΔatsR mutant and resulted in early and increased N-acylhomoserine lactone (AHL) production, suggesting that AtsR plays a role in controlling the timing and fine-tuning of virulence gene expression by modulating QS signalling. Furthermore, a ΔatsRΔcepIΔcciI mutant could partially upregulate the same virulence determinants indicating that AtsR also modulates the expression of virulence genes by a second mechanism, independently of any AHL production. Together, our results strongly suggest that AtsR is a global virulence regulator in B. cenocepacia.

Synthesis and evaluation of the quorum sensing inhibitory effect of substituted triazolyldihydrofuranones

Acylhomoserine lactone (AHL) analogues in which the amide function is replaced by a triazole group were synthesized and evaluated for their effect on quorum sensing (QS) and biofilm formation in Burkholderia cenocepacia and Pseudomonas aeruginosa. In addition, the influence of the length of the acyl-mimicking chain was investigated. The compounds showed selectivity between two different AHL QS systems. 3-(1H-1,2,3-Triazol-1-yl)dihydrofuran-2(3H)-ones, in which the 4-substituent best resembled the acyl chain of the native AHL molecule exhibited significant QS agonistic and antagonistic activities. Replacing this aliphatic substituent by a phenyl-containing moiety resulted in active inhibitors of QS. The most active compounds showed biofilm inhibitory as well as biofilm eradicating activities in both test organisms.

A unique regulator contributes to quorum sensing and virulence in Burkholderia cenocepacia

Burkholderia cenocepacia causes chronic and life-threatening respiratory infections in immunocompromized people. The B. cenocepacia N-acyl-homoserine lactone (AHL)-dependent quorum sensing system relies on the production of AHLs by the synthases CepI and CciI while CepR, CciR and CepR2 control expression of many genes important for pathogenesis. Downstream from, and co-transcribed with cepI, lies BCAM1871 encoding a hypothetical protein that was uncharacterized prior to this study. Orthologs of B. cenocepacia BCAM1871 are uniquely found in Burkholderia spp and are conserved in their genomic locations in pathogenic Burkholderia. We observed significant effects on AHL activity upon mutation or overexpression of BCAM1871, although these effects were more subtle than those observed for CepI indicating BCAM1871 acts as an enhancer of AHL activity. Transcription of cepI, cepR and cciIR was significantly reduced in the BCAM1871 mutant. Swimming and swarming motilities as well as transcription of fliC, encoding flagellin, were significantly reduced in the BCAM1871 mutant. Protease activity and transcription of zmpA and zmpB, encoding extracellular zinc metalloproteases, were undetectable in the BCAM1871 mutant indicating a more significant effect of mutating BCAM1871 than cepI. Exogenous addition of OHL restored cepI, cepR and fliC transcription but had no effect on motility, protease activity or zmpA or zmpB transcription suggesting AHL-independent effects. The BCAM1871 mutant exhibited significantly reduced virulence in rat chronic respiratory and nematode infection models. Gene expression and phenotypic assays as well as vertebrate and invertebrate infection models showed that BCAM1871 significantly contributes to pathogenesis in B. cenocepacia.

Sticky situations: key components that control bacterial surface attachment

The formation of bacterial biofilms is initiated by cells transitioning from the free-swimming mode of growth to a surface. This review is aimed at highlighting the common themes that have emerged in recent research regarding the key components, signals, and cues that aid in the transition and those involved in establishing a more permanent surface association during initial attachment.

Effect of periodontal pathogens on the metatranscriptome of a healthy multispecies biofilm model

Oral bacterial biofilms are highly complex microbial communities with up to 700 different bacterial taxa. We report here the use of metatranscriptomic analysis to study patterns of community gene expression in a multispecies biofilm model composed of species found in healthy oral biofilms (Actinomyces naeslundii, Lactobacillus casei, Streptococcus mitis, Veillonella parvula, and Fusobacterium nucleatum) and the same biofilm plus the periodontopathogens Porphyromonas gingivalis and Aggregatibacter actinomycetemcomitans. The presence of the periodontopathogens altered patterns in gene expression, and data indicate that transcription of protein-encoding genes and small noncoding RNAs is stimulated. In the healthy biofilm hypothetical proteins, transporters and transcriptional regulators were upregulated while chaperones and cell division proteins were downregulated. However, when the pathogens were present, chaperones were highly upregulated, probably due to increased levels of stress. We also observed a significant upregulation of ABC transport systems and putative transposases. Changes in Clusters of Orthologous Groups functional categories as well as gene set enrichment analysis based on Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways showed that in the absence of pathogens, only sets of proteins related to transport and secondary metabolism were upregulated, while in the presence of pathogens, proteins related to growth and division as well as a large portion of transcription factors were upregulated. Finally, we identified several small noncoding RNAs whose predicted targets were genes differentially expressed in the open reading frame libraries. These results show the importance of pathogens controlling gene expression of a healthy oral community and the usefulness of metatranscriptomic techniques to study gene expression profiles in complex microbial community models.

Quorum sensing inhibitors increase the susceptibility of bacterial biofilms to antibiotics in vitro and in vivo

Although the exact role of quorum sensing (QS) in various stages of biofilm formation, maturation, and dispersal and in biofilm resistance is not entirely clear, the use of QS inhibitors (QSI) has been proposed as a potential antibiofilm strategy. We have investigated whether QSI enhance the susceptibility of bacterial biofilms to treatment with conventional antimicrobial agents. The QSI used in our study target the acyl-homoserine lactone-based QS system present in Pseudomonas aeruginosa and Burkholderia cepacia complex organisms (baicalin hydrate, cinnamaldehyde) or the peptide-based system present in Staphylococcus aureus (hamamelitannin). The effect of tobramycin (P. aeruginosa, B. cepacia complex) and clindamycin or vancomycin (S. aureus), alone or in combination with QSI, was evaluated in various in vitro and in vivo biofilm model systems, including two invertebrate models and one mouse pulmonary infection model. In vitro the combined use of an antibiotic and a QSI generally resulted in increased killing compared to killing by an antibiotic alone, although reductions were strain and model dependent. A significantly higher fraction of infected Galleria mellonella larvae and Caenorhabditis elegans survived infection following combined treatment, compared to treatment with an antibiotic alone. Finally, the combined use of tobramycin and baicalin hydrate reduced the microbial load in the lungs of BALB/c mice infected with Burkholderia cenocepacia more than tobramycin treatment alone. Our data suggest that QSI may increase the success of antibiotic treatment by increasing the susceptibility of bacterial biofilms and/or by increasing host survival following infection.

The Burkholderia cenocepacia LysR-type transcriptional regulator ShvR influences expression of quorum-sensing, protease, type II secretion, and afc genes

Burkholderia cenocepacia is a significant opportunistic pathogen in individuals with cystic fibrosis. ShvR, a LysR-type transcriptional regulator, has previously been shown to influence colony morphology, biofilm formation, virulence in plant and animal infection models, and some quorum-sensing-dependent phenotypes. In the present study, it was shown that ShvR negatively regulates its own expression, as is typical for LysR-type regulators. The production of quorum-sensing signal molecules was detected earlier in growth in the shvR mutant than in the wild type, and ShvR repressed expression of the quorum-sensing regulatory genes cepIR and cciIR. Microarray analysis and transcriptional fusions revealed that ShvR regulated over 1,000 genes, including the zinc metalloproteases zmpA and zmpB. The shvR mutant displayed increased gene expression of the type II secretion system and significantly increased protease and lipase activities. Both ShvR and CepR influence expression of a 24-kb genomic region adjacent to shvR that includes the afcA and afcC operons, required for the production of an antifungal agent; however, the reduction in expression was substantially greater in the shvR mutant than in the cepR mutant. Only the shvR mutation resulted in reduced antifungal activity against Rhizoctonia solani. ShvR, but not CepR, was shown to directly regulate expression of the afcA and afcC promoters. In summary, ShvR was determined to have a significant influence on the expression of quorum-sensing, protease, lipase, type II secretion, and afc genes.

A decade of Burkholderia cenocepacia virulence determinant research

The Burkholderia cepacia complex (Bcc) is a group of genetically related environmental bacteria that can cause chronic opportunistic infections in patients with cystic fibrosis (CF) and other underlying diseases. These infections are difficult to treat due to the inherent resistance of the bacteria to antibiotics. Bacteria can spread between CF patients through social contact and sometimes cause cepacia syndrome, a fatal pneumonia accompanied by septicemia. Burkholderia cenocepacia has been the focus of attention because initially it was the most common Bcc species isolated from patients with CF in North America and Europe. Today, B. cenocepacia, along with Burkholderia multivorans, is the most prevalent Bcc species in patients with CF. Given the progress that has been made in our understanding of B. cenocepacia over the past decade, we thought that it was an appropriate time to review our knowledge of the pathogenesis of B. cenocepacia, paying particular attention to the characterization of virulence determinants and the new tools that have been developed to study them. A common theme emerging from these studies is that B. cenocepacia establishes chronic infections in immunocompromised patients, which depend more on determinants mediating host niche adaptation than those involved directly in host cells and tissue damage.

Social interactions in the Burkholderia cepacia complex: biofilms and quorum sensing

Burkholderia cepacia complex bacteria are opportunistic pathogens that cause respiratory tract infections in susceptible patients, mainly people with cystic fibrosis. There is convincing evidence that B. cepacia complex bacteria can form biofilms, not only on abiotic surfaces (e.g., glass and plastics), but also on biotic surfaces such as epithelial cells, leading to the suggestion that biofilm formation plays a key role in persistent infection of cystic fibrosis lungs. This article presents an overview of the molecular mechanisms involved in B. cepacia complex biofilm formation, the increased resistance of sessile B. cepacia complex cells and the role of quorum sensing in B. cepacia complex biofilm formation.

Growing Burkholderia pseudomallei in biofilm stimulating conditions significantly induces antimicrobial resistance

Background

Burkholderia pseudomallei, a Gram-negative bacterium that causes melioidosis, was reported to produce biofilm. As the disease causes high relapse rate when compared to other bacterial infections, it therefore might be due to the reactivation of the biofilm forming bacteria which also provided resistance to antimicrobial agents. However, the mechanism on how biofilm can provide tolerance to antimicrobials is still unclear.

Methodology/Principal Findings

The change in resistance of B. pseudomallei to doxycycline, ceftazidime, imipenem, and trimethoprim/sulfamethoxazole during biofilm formation were measured as minimum biofilm elimination concentration (MBEC) in 50 soil and clinical isolates and also in capsule, flagellin, LPS and biofilm mutants. Almost all planktonic isolates were susceptible to all agents studied. In contrast, when they were grown in the condition that induced biofilm formation, they were markedly resistant to all antimicrobial agents even though the amount of biofilm production was not the same. The capsule and O-side chains of LPS mutants had no effect on biofilm formation whereas the flagellin-defective mutant markedly reduced in biofilm production. No alteration of LPS profiles was observed when susceptible form was changed to resistance. The higher amount of N-acyl homoserine lactones (AHLs) was detected in the high biofilm-producing isolates. Interestingly, the biofilm mutant which produced a very low amount of biofilm and was sensitive to antimicrobial agents significantly resisted those agents when grown in biofilm inducing condition.

Conclusions/Significance

The possible drug resistance mechanism of biofilm mutants and other isolates is not by having biofilm but rather from some factors that up-regulated when biofilm formation genes were stimulated. The understanding of genes related to this situation may lead us to prevent B. pseudomallei biofilms leading to the relapse of melioidosis.

Reciprocal regulation by the CepIR and CciIR quorum sensing systems in Burkholderia cenocepacia

Background

Burkholderia cenocepacia belongs to a group of closely related organisms called the B. cepacia complex (Bcc) which are important opportunistic human pathogens. B. cenocepacia utilizes a mechanism of cell-cell communication called quorum sensing to control gene expression including genes involved in virulence. The B. cenocepacia quorum sensing network includes the CepIR and CciIR regulatory systems.

Results

Global gene expression profiles during growth in stationary phase were generated using microarrays of B. cenocepacia cepR, cciR and cepRcciIR mutants. This is the first time CciR was shown to be a global regulator of quorum sensing gene expression. CepR was primarily responsible for positive regulation of gene expression while CciR generally exerted negative gene regulation. Many of the genes that were regulated by both quorum sensing systems were reciprocally regulated by CepR and CciR. Microarray analysis of the cepRcciIR mutant suggested that CepR is positioned upstream of CciR in the quorum sensing hierarchy in B. cenocepacia. A comparison of CepIR-regulated genes identified in previous studies and in the current study showed a substantial amount of overlap validating the microarray approach. Several novel quorum sensing-controlled genes were confirmed using qRT-PCR or promoter::lux fusions. CepR and CciR inversely regulated flagellar-associated genes, the nematocidal protein AidA and a large gene cluster on Chromosome 3. CepR and CciR also regulated genes required for iron transport, synthesis of extracellular enzymes and surface appendages, resistance to oxidative stress, and phage-related genes.

Conclusion

For the first time, the influence of CciIR on global gene regulation in B. cenocepacia has been elucidated. Novel genes under the control of the CepIR and CciIR quorum sensing systems in B. cenocepacia have been identified. The two quorum sensing systems exert reciprocal regulation of many genes likely enabling fine-tuned control of quorum sensing gene expression in B. cenocepacia strains carrying the cenocepacia island.

A Burkholderia cenocepacia orphan LuxR homolog is involved in quorum-sensing regulation

Burkholderia cenocepacia utilizes quorum sensing to control gene expression, including the expression of genes involved in virulence. In addition to CepR and CciR, a third LuxR homolog, CepR2, was found to regulate gene expression and virulence factor production. All B. cenocepacia strains examined contained this orphan LuxR homolog, which was not associated with an adjacent N-acyl-homoserine lactone synthase gene. Expression of cepR2 was negatively autoregulated and was negatively regulated by CciR in strain K56-2. Microarray analysis and quantitative reverse transcription-PCR determined that CepR2 did not influence expression of cepIR or cciIR. However, in strain K56-2, CepR2 negatively regulated expression of several known quorum-sensing-controlled genes, including genes encoding zinc metalloproteases. CepR2 exerted positive and negative regulation on genes on three chromosomes, including strong negative regulation of a gene cluster located adjacent to cepR2. In strain H111, which lacks the CciIR quorum-sensing system, CepR2 positively regulated pyochelin production by controlling transcription of one of the operons required for the biosynthesis of the siderophore in an N-acyl-homoserine lactone-independent manner. CepR2 activation of a luxI promoter was demonstrated in a heterologous Escherichia coli host, providing further evidence that CepR2 can function in the absence of signaling molecules. This study demonstrates that the orphan LuxR homolog CepR2 contributes to the quorum-sensing regulatory network in two distinct strains of B. cenocepacia.