Burkholderia pseudomallei Biofilm Promotes Adhesion, Internalization and Stimulates Proinflammatory Cytokines in Human Epithelial A549 Cells

d-Methionine-induced DNases disperse established Burkholderia pseudomallei biofilms and promotes ceftazidime susceptibility

Burkholderia pseudomallei biofilm is correlated with pathogenesis, antibiotic resistance, and relapsing cases of melioidosis, leading to challenges in clinical management. There is increasing interest in employing biofilm dispersal agents as adjunctive treatments for biofilm-associated infections. Methionine (Met) has shown promise as an anti-biofilm agent by inducing bacterial DNase production, resulting in the degradation of extracellular DNA (eDNA) and dispersion of bacterial biofilm. In this study, we investigated the impact of 0.05-50 μM D-Met and L-Met on the 24-h established biofilm of a clinical isolate, B. pseudomallei H777. Our findings revealed the ability of D-Met and L-Met to disperse the established biofilm in a non-dose-dependent manner accompanied by eDNA depletion. Real-time PCR analysis further identified an up-regulation of bacterial nuclease genes, including recJ, eddB, nth, xth, and recD, in the presence of 0.05 μM D-Met. Similarly, recJ and eddB in B. pseudomallei were up-regulated in response to the presence of 0.05 μM L-Met. Notably, D-Met enhanced the susceptibility of B. pseudomallei H777 biofilm cells to ceftazidime. Our findings indicate a correlation between methionine supplementation and the up-regulation of nuclease genes, leading to eDNA depletion and the dispersal of preformed B. pseudomallei H777 biofilm. This enhances the susceptibility of biofilm cells to ceftazidime, showing promise in combating biofilm-associated B. pseudomallei infections.

Influence of carbonyl cyanide m-chlorophenyl hydrazone on biofilm dynamics, protease, and siderophore production by Burkholderia pseudomallei

Efflux pump inhibitors are a potential therapeutic strategy for managing antimicrobial resistance and biofilm formation. This article evaluated the effect of carbonyl cyanide m-chlorophenyl hydrazone (CCCP) on the biofilm growth dynamics and the production of virulence factors by Burkholderia pseudomallei. The effects of CCCP on planktonic, growing, and mature biofilm, interaction with antibacterial drugs, and protease and siderophore production were assessed. CCCP MICs ranged between 128 and 256 µM. The CCCP (128 µM) had a synergic effect with all the antibiotics tested against biofilms. Additionally, CCCP reduced (p < .05) the biomass of biofilm growth and mature biofilms at 128 and 512 µM, respectively. CCCP also decreased (p < .05) protease production by growing (128 µM) and induced (p < .05) siderophore release by planktonic cells (128 µM) growing biofilms (12.8 and 128 µM) and mature biofilms (512 µM). CCCP demonstrates potential as a therapeutic adjuvant for disassembling B. pseudomallei biofilms and enhancing drug penetration.

Unraveling the role of toxin-antitoxin systems in Burkholderia pseudomallei: exploring bacterial pathogenesis and interactions within the HigBA families

Burkholderia pseudomallei (Bpm) is a Gram-negative intracellular pathogen that causes melioidosis in humans, a neglected, underreported, and lethal disease that can reach a fatal outcome in over 50% of the cases. It can produce both acute and chronic infections, the latter being particularly challenging to eliminate because of the intracellular life cycle of the bacteria and its ability to generate a "persister" dormant state. The molecular mechanism that allows the switch between growing and persister phenotypes is not well understood but it is hypothesized to be due at least in part to the participation of toxin-antitoxin (TA) systems. We have previously studied the link between one of those systems (defined as HigBA) with specific expression patterns associated with levofloxacin antibiotic exposure. Through in silico methods, we predicted the presence of another three pairs of genes encoding for additional putative HigBA systems. Therefore, our main goal was to establish which mechanisms are conserved as well as which pathways are specific among different Bpm TA systems from the same family. We hypothesize that the high prevalence, and sometimes even redundancy of these systems in the Bpm chromosomes indicates that they can interact with each other and not function as only individual systems, as it was traditionally thought, and might be playing an undefined role in Bpm lifecycle. Here, we show that both the toxin and the antitoxin of the different systems contribute to bacterial survival and that toxins from the same family can have a cumulative effect under environmental stressful conditions.

IMPORTANCE

Toxin-antitoxin (TA) systems play a significant role in bacterial persistence, a phenomenon where bacterial cells enter a dormant or slow-growing state to survive adverse conditions such as nutrient deprivation, antibiotic exposure, or host immune responses. By studying TA systems in Burkholderia pseudomallei, we can gain insights into how this pathogen survives and persists in the host environment, contributing to its virulence and ability to cause melioidosis chronic infections.

Role of type VI secretion system protein TssJ-3 in virulence and intracellular survival of Burkholderia pseudomallei

TssJ-3 is an outer-membrane lipoprotein and is one of the key components of the type VI secretion system in Burkholderia pseudomallei. TssJ translocates effector proteins to target cells to induce innate immune response in the host. However, the tssJ gene has not been identified in B. pseudomallei and its function in this bacterium has not yet been characterized. tssJ-3 knockout and tssJ-3-complemented B. pseudomallei strains were constructed to determine the effects of tssJ-3 on bacterial growth, biofilm formation, flagellum synthesis, motility, host cell infection, and gene expression in B. pseudomallei. We found that the ΔtssJ-3 mutant strain of B. pseudomallei showed significantly suppressed biofilm formation, flagellum synthesis, bacterial growth, motility, and bacterial invasion into host cells (A549 cells). Furthermore, the ΔtssJ-3 mutation downregulated multiple key genes, including biofilm and flagellum-related genes in B. pseudomallei and induced interleukin-8 gene expression in host cells. These results suggest that tssJ-3, an important gene controlling TssJ-3 protein expression, has regulatory effects on biofilm formation and flagellum synthesis in B. pseudomallei. In addition, B. pseudomallei-derived tssJ-3 contributes to cell infiltration and intracellular replication. This study provides a molecular basis of tssJ-3 for developing therapeutic strategies against B. pseudomallei infections.

Type VI Secretion System Accessory Protein TagAB-5 Promotes Burkholderia pseudomallei Pathogenicity in Human Microglia

Central nervous system (CNS) melioidosis caused by Burkholderia pseudomallei is being increasingly reported. Because of the high mortality associated with CNS melioidosis, understanding the underlying mechanism of B. pseudomallei pathogenesis in the CNS needs to be intensively investigated to develop better therapeutic strategies against this deadly disease. The type VI secretion system (T6SS) is a multiprotein machine that uses a spring-like mechanism to inject effectors into target cells to benefit the infection process. In this study, the role of the T6SS accessory protein TagAB-5 in B. pseudomallei pathogenicity was examined using the human microglial cell line HCM3, a unique resident immune cell of the CNS acting as a primary mediator of inflammation. We constructed B. pseudomallei tagAB-5 mutant and complementary strains by the markerless allele replacement method. The effects of tagAB-5 deletion on the pathogenicity of B. pseudomallei were studied by bacterial infection assays of HCM3 cells. Compared with the wild type, the tagAB-5 mutant exhibited defective pathogenic abilities in intracellular replication, multinucleated giant cell formation, and induction of cell damage. Additionally, infection by the tagAB-5 mutant elicited a decreased production of interleukin 8 (IL-8) in HCM3, suggesting that efficient pathogenicity of B. pseudomallei is required for IL-8 production in microglia. However, no significant differences in virulence in the Galleria mellonella model were observed between the tagAB-5 mutant and the wild type. Taken together, this study indicated that microglia might be an important intracellular niche for B. pseudomallei, particularly in CNS infection, and TagAB-5 confers B. pseudomallei pathogenicity in these cells.

Burkholderia pseudomallei biofilm resists Acanthamoeba sp. grazing and produces 8-O-4'-diferulic acid, a superoxide scavenging metabolite after passage through the amoeba

Burkholderia pseudomallei, an etiological agent of melioidosis is an environmental bacterium that can survive as an intracellular pathogen. The biofilm produced by B. pseudomallei is crucial for cellular pathogenesis of melioidosis. The purpose of this investigation is to explore the role of biofilm in survival of B. pseudomallei during encounters with Acanthamoeba sp. using B. pseudomallei H777 (a biofilm wild type), M10 (a biofilm defect mutant) and C17 (a biofilm-complemented strain). The results demonstrated similar adhesion to amoebae by both the biofilm wild type and biofilm mutant strains. There was higher initial internalisation, but the difference diminished after longer encounter with the amoeba. Interestingly, confocal laser scanning microscopy demonstrated that pre-formed biofilm of B. pseudomallei H777 and C17 were markedly more persistent in the face of Acanthamoeba sp. grazing than that of M10. Metabolomic analysis revealed a significant increased level of 8-O-4'-diferulic acid, a superoxide scavenger metabolite, in B. pseudomallei H777 serially passaged in Acanthamoeba sp. The interaction between B. pseudomallei with a free-living amoeba may indicate the evolutionary pathway that enables the bacterium to withstand superoxide radicals in intracellular environments. This study supports the hypothesis that B. pseudomallei biofilm persists under grazing by amoebae and suggests a strategy of metabolite production that turns this bacterium from saprophyte to intracellular pathogen.

Antibiofilm properties of cathelicidin LL-37: an in-depth review

Notwithstanding ceaseless endeavors toward developing effective antibiofilm chemotherapeutics, biofilm-associated infections continue to be one of the most perplexing challenges confronting medicine today. Endogenous host defense peptides, such as the human cathelicidin LL-37, are being propounded as promising options for treating such infectious diseases. Over the past decennium, LL-37 has duly received tremendous research attention by virtue of its broad-spectrum antimicrobial activity and immunomodulatory properties. No attempt has hitherto been made, as far as we are aware, to comprehensively review the antibiofilm effects of LL-37. Accordingly, the intent in this paper is to provide a fairly all-embracing review of the literature available on the subject. Accumulating evidence suggests that LL-37 is able to prevent biofilm establishment by different bacterial pathogens such as Acinetobacter baumannii, Aggregatibacter actinomycetemcomitans, Bacteroides fragilis, Burkholderia thailandensis, Cutibacterium acnes, Escherichia coli, Francisella tularensis, Helicobacter pylori, Klebsiella pneumoniae, Pseudomonas aeruginosa, Staphylococcus aureus, and Streptococcus pyogenes. Inhibition of bacterial adhesion, downregulation of biofilm-associated genes, suppression of quorum-sensing pathways, degradation of biofilm matrix, and eradication of biofilm-residing cells are the major mechanisms responsible for antibiofilm properties of LL-37. In terms of its efficacy and safety in vivo, there are still many questions to be answered. Undoubtedly, LL-37 can open up new windows of opportunity to prevent and treat obstinate biofilm-mediated infections.

Vitamin D (1α,25(OH)2D3) supplementation minimized multinucleated giant cells formation and inflammatory response during Burkholderia pseudomallei infection in human lung epithelial cells

Melioidosis is an infectious disease with high mortality rates in human, caused by the bacterium Burkholderia pseudomallei. As an intracellular pathogen, B. pseudomallei can escape from the phagosome and induce multinucleated giant cells (MNGCs) formation resulting in antibiotic resistance and immune evasion. A novel strategy to modulate host response against B. pseudomallei pathogenesis is required. In this study, an active metabolite of vitamin D3 (1α,25-dihydroxyvitamin D3 or 1α,25(OH)2D3) was selected to interrupt pathogenesis of B. pseudomallei in a human lung epithelium cell line, A549. The results demonstrated that pretreatment with 10-6 M 1α,25(OH)2D3 could reduce B. pseudomallei internalization to A549 cells at 4 h post infection (P < 0.05). Interestingly, the presence of 1α,25(OH)2D3 gradually reduced MNGC formation at 8, 10 and 12 h compared to that of the untreated cells (P < 0.05). Furthermore, pretreatment with 10-6 M 1α,25(OH)2D3 considerably increased hCAP-18/LL-37 mRNA expression (P < 0.001). Additionally, pro-inflammatory cytokines, including MIF, PAI-1, IL-18, CXCL1, CXCL12 and IL-8, were statistically decreased (P < 0.05) in 10-6 M 1α,25(OH)2D3-pretreated A549 cells by 12 h post-infection. Taken together, this study indicates that pretreatment with 10-6 M 1α,25(OH)2D3 has the potential to reduce the internalization of B. pseudomallei into host cells, decrease MNGC formation and modulate host response during B. pseudomallei infection by minimizing the excessive inflammatory response. Therefore, 1α,25(OH)2D3 supplement may provide an effective supportive treatment for melioidosis patients to combat B. pseudomallei infection and reduce inflammation in these patients.

Biofilm Signaling, Composition and Regulation in Burkholderia pseudomallei

The incidence of melioidosis cases caused by the gram-negative pathogen Burkholderia pseudomallei (BP) is seeing an increasing trend that has spread beyond its previously known endemic regions. Biofilms produced by BP have been associated with antimicrobial therapy limitation and relapse melioidosis, thus making it urgently necessary to understand the mechanisms of biofilm formation and their role in BP biology. Microbial cells aggregate and enclose within a self-produced matrix of extracellular polymeric substances (EPSs) to form biofilm. The transition mechanism of bacterial cells from planktonic state to initiate biofilm formation, which involves the formation of surface attachment microcolonies and the maturation of the biofilm matrix, is a dynamic and complex process. Despite the emerging findings on the biofilm formation process, systemic knowledge on the molecular mechanisms of biofilm formation in BP remains fractured. This review provides insights into the signaling systems, matrix composition, and the biosynthesis regulation of EPSs (exopolysaccharide, eDNA and proteins) that facilitate the formation of biofilms in order to present an overview of our current knowledge and the questions that remain regarding BP biofilms.

DNase I and chitosan enhance efficacy of ceftazidime to eradicate Burkholderia pseudomallei biofilm cells

Biofilm-associated Burkholderia pseudomallei infection contributes to antibiotic resistance and relapse of melioidosis. Burkholderia pseudomallei biofilm matrix contains extracellular DNA (eDNA) that is crucial for biofilm establishment. However, the contribution of eDNA to antibiotic resistance by B. pseudomallei remains unclear. In this study, we first demonstrated in vitro that DNase I with the administration of ceftazidime (CAZ) at 24 h considerably inhibited the 2-day biofilm formation and reduced the number of viable biofilm cells of clinical B. pseudomallei isolates compared to biofilm treated with CAZ alone. A 3-4 log reduction in numbers of viable cells embedded in the 2-day biofilm was observed when CAZ was combined with DNase I. Confocal laser-scanning microscope visualization emphasized the competence of DNase I followed by CAZ supplementation to significantly limit B. pseudomallei biofilm development and to eradicate viable embedded B. pseudomallei biofilm cells. Furthermore, DNase I supplemented with chitosan (CS) linked with CAZ (CS/CAZ) significantly eradicated shedding planktonic and biofilm cells. These findings indicated that DNase I effectively degraded eDNA leading to biofilm inhibition and dispersion, subsequently allowing CAZ and CS/CAZ to eradicate both shedding planktonic and embedded biofilm cells. These findings provide efficient strategies to interrupt biofilm formation and improve antibiotic susceptibility of biofilm-associated infections.

Extracellular Vesicle-Encapsulated microRNAs as Novel Biomarkers of Lung Health

Rationale: Early detection of respiratory diseases is critical to facilitate delivery of disease-modifying interventions. Extracellular vesicle-enriched microRNAs (EV-miRNAs) may represent reliable markers of early lung injury. Objectives: Evaluate associations of plasma EV-miRNAs with lung function. Methods: The prospective NAS (Normative Aging Study) collected plasma EV-miRNA measurements from 1996-2015 and spirometry every 3-5 years through 2019. Associations of EV-miRNAs with baseline lung function were modeled using linear regression. To complement the individual miRNA approach, unsupervised machine learning was used to identify clusters of participants with distinct EV-miRNA profiles. Associations of EV-miRNA profiles with multivariate latent longitudinal lung function trajectories were modeled using log binomial regression. Biological functions of significant EV-miRNAs were explored using pathway analyses. Results were replicated in an independent sample of NAS participants and in the HEALS (Health Effects of Arsenic Longitudinal Study). Measurements and Main Results: In the main cohort of 656 participants, 51 plasma EV-miRNAs were associated with baseline lung function (false discovery rate-adjusted P value < 0.05), 28 of which were replicated in the independent NAS sample and/or in the HEALS cohort. A subset of participants with distinct EV-miRNA expression patterns had increased risk of declining lung function over time, which was replicated in the independent NAS sample. Significant EV-miRNAs were shown in pathway analyses to target biological pathways that regulate respiratory cellular immunity, the lung inflammatory response, and airway structural integrity. Conclusions: Plasma EV-miRNAs may represent a robust biomarker of subclinical lung injury and may facilitate early identification and treatment of patients at risk of developing overt lung disease.

Burkholderia pseudomallei biofilm phenotypes confined but surviving in neutrophil extracellular traps of varying appearance

Melioidosis is a fatal infectious disease caused by Burkholderia pseudomallei. Complications following treatment are usually due to antibiotic resistance and relapse is mainly caused by B. pseudomallei biofilm. Although the release of neutrophil extracellular traps (NETs) is crucial to capture and eliminate bacterial pathogens, to date response of NETs to B. pseudomallei biofilm is poorly understood. Here we compare the NETs produced by neutrophils in response to B. pseudomallei H777 (a biofilm-producing strain containing the bpsl0618 gene), a biofilm-defect strain lacking this gene (B. pseudomallei M10) and a bpsl0618 biofilm-complemented strain, B. pseudomallei C17, in which function of bpsl0618 was restored. Co-cultivation of these strains with healthy human neutrophils at MOI 10 with or without cytochalasin D demonstrated that H777 significantly resisted neutrophil-mediated killing and non-phagocytotic mechanisms compared to M10 (p < 0.0001). Three distinct morphotypes of NETs were seen: “aggregated”, “spiky” and “cloudy”. These were induced in different proportions by the different bacterial strains. All types of NETs were shown to confine all B. pseudomallei strains. Strains H777 and C17 could stimulate production of twice as much extracellular DNA (234.62 ng/mL and 205.43 ng/mL, respectively) as did M10 (111.87 ng/mL). Cells of H777 and C17 were better able to survive in the presence of neutrophil killing mechanisms relative to M10 (p < 0.0001) and NET formation (p < 0.0001 and 0.05). These findings suggest that NET stimulation was insufficient to eradicate B. pseudomallei H777 and C17 despite their possession of bpsl0618, a sugar-transferase gene associated with biofilm formation ability. Our findings demonstrate that B. pseudomallei biofilm phenotype may be a key factor in assisting pathogens to escape killing by neutrophils. This work provides a better understanding of how B. pseudomallei biofilm-associated infections induce and survive NET formation, resulting in bacterial persistence and increased severity of disease.

Evaluating the Contribution of the Predicted Toxin-Antitoxin System HigBA to Persistence, Biofilm Formation, and Virulence in Burkholderia pseudomallei

Melioidosis is an underreported human disease caused by the Gram-negative intracellular pathogen Burkholderia pseudomallei (Bpm). Both the treatment and the clearance of the pathogen are challenging, with high relapse rates leading to latent infections. This has been linked to the bacterial persistence phenomenon, a growth arrest strategy that allows bacteria to survive under stressful conditions, as in the case of antibiotic treatment, within a susceptible clonal population. At a molecular level, this phenomenon has been associated with the presence of toxin-antitoxin (TA) systems. We annotated the Bpm K96243 genome and selected 11 pairs of genes encoding for these TA systems, and their expression was evaluated under different conditions (supralethal antibiotic conditions; intracellular survival bacteria). The predicted HigB toxin (BPSL3343) and its predicted antitoxin HigA (BPS_RS18025) were further studied using mutant construction. The phenotypes of two mutants (ΔhigB and ΔhigB ΔhigA) were evaluated under different conditions compared to the wild-type (WT) strain. The ΔhigB toxin mutant showed a defect in intracellular survival on macrophages, a phenotype that was eliminated after levofloxacin treatment. We found that the absence of the toxin provides an advantage over the WT strain, in both in vitro and in vivo models, during persister conditions induced by levofloxacin. The lack of the antitoxin also resulted in differential responses to the conditions evaluated, and under some conditions, it restored the WT phenotype, overall suggesting that both toxin and antitoxin components play a role in the persister-induced phenotype in Bpm.

A type IVB pilin influences twitching motility and in vitro adhesion to epithelial cells in Burkholderia pseudomallei

Type IV pili are involved in adhesion, twitching motility, aggregation, biofilm formation and virulence in a variety of Gram-negative bacteria. Burkholderia pseudomallei, the causative agent of melioidosis and a Tier 1 biological select agent, is a Gram-negative bacterium with eight type IV pili-associated loci (TFP1 to TFP8). Most have not been fully characterized. In this study, we investigated BPSS2185, an uncharacterized TFP8 gene that encodes a type IVB pilus protein subunit. Using genetic deletion and complementation analysis in B. pseudomallei JW270, we demonstrate that BPSS2185 plays an important role in twitching motility and adhesion to A549 human alveolar epithelial cells. Compared to JW270, the JW270 ΔBPSS2185 mutant failed to display twitching motility and did not adhere to the epithelial cells. These phenotypes were partially reversed by the complementation of BPSS2185 in the mutant strain. The study also shows that BPSS2185 is expressed only during the onset of mature biofilm formation and at the dispersal of a biofilm, suggesting that the motility characteristic is required to form a biofilm. Our study is the first to suggest that the BPSS2185 gene in TFP8 contributes to twitching motility, adhesion and biofilm formation, indicating that the gene may contribute to B. pseudomallei virulence.

Chitosan biological molecule improves bactericidal competence of ceftazidime against Burkholderia pseudomallei biofilms

Biofilm-associated Burkholderia pseudomallei infections (melioidosis) are problematic because of reduced sensitivity to antibiotics and high frequency of relapse. Biofilm dispersal agents are essential to liberate the biofilm-encased cells, which then become planktonic and are more susceptible to antibiotics. This study aimed to evaluate the ability of deacetylated chitosan (dCS), an antimicrobial and antibiofilm biological macromolecule, to disrupt established biofilms, thus enabling ceftazidime (CAZ) to kill biofilm-embedded B. pseudomallei. We combined dCS with CAZ using a mechanical stirring method to generate dCS/CAZ. In combination, 1.25-2.5 mg ml^-1 dCS/1-2 μg ml^-1 CAZ acted synergistically to kill cells more effectively than did either dCS or CAZ alone. Notably, a combination of 5-10 mg ml^-1 dCS with 256-512 μg ml^-1 CAZ, prepared either by mechanical stirring (dCS/CAZ) or mixing (dCS + CAZ), drastically improved bactericidal activities against biofilm cells leading to a 3-6 log CFU reduction. Confocal laser-scanning microscope (CLSM) images revealed that 10 mg ml^-1 dCS/512 μg ml^-1 CAZ is by far the best formulation to diminish B. pseudomallei biofilm biomass and produces the lowest live/dead cell ratios of B. pseudomallei in biofilm matrix. Collectively, these findings emphasize the potential of novel therapeutic antibacterial and antibiofilm agents to fight against antibiotic-tolerant B. pseudomallei biofilm-associated infections.

Outer Membrane Protein F Is Involved in Biofilm Formation, Virulence and Antibiotic Resistance in Cronobacter sakazakii

In some Gram-negative bacteria, ompF encodes outer membrane protein F (OmpF), which is a cation-selective porin and is responsible for the passive transport of small molecules across the outer membrane. However, there are few reports about the functions of this gene in Cronobacter sakazakii. To investigate the role of ompF in detail, an ompF disruption strain (ΔompF) and a complementation strain (cpompF) were successfully obtained. We find that OmpF can affect the ability of biofilm formation in C. sakazakii. In addition, the variations in biofilm composition of C. sakazakii were examined using Raman spectroscopy analyses caused by knocking out ompF, and the result indicated that the levels of certain biofilm components, including lipopolysaccharide (LPS), were significantly decreased in the mutant (ΔompF). Then, SDS-PAGE was used to further analyze the LPS content, and the result showed that the LPS levels were significantly reduced in the absence of ompF. Therefore, we conclude that OmpF affects biofilm formation in C. sakazakii by reducing the amount of LPS. Furthermore, the ΔompF mutant showed decreased (2.7-fold) adhesion to and invasion of HCT-8 cells. In an antibiotic susceptibility analysis, the ΔompF mutant showed significantly smaller inhibition zones than the WT, indicating that OmpF had a positive effect on the influx of antibiotics into the cells. In summary, ompF plays a positive regulatory role in the biofilm formation and adhesion/invasion, which is achieved by regulating the amount of LPS, but is a negative regulator of antibiotic resistance in C. sakazakii.

Hijacking of the Host's Immune Surveillance Radars by Burkholderia pseudomallei

Burkholderia pseudomallei (B. pseudomallei) causes melioidosis, a potentially fatal disease for which no licensed vaccine is available thus far. The host-pathogen interactions in B. pseudomallei infection largely remain the tip of the iceberg. The pathological manifestations are protean ranging from acute to chronic involving one or more visceral organs leading to septic shock, especially in individuals with underlying conditions similar to COVID-19. Pathogenesis is attributed to the intracellular ability of the bacterium to ‘step into’ the host cell’s cytoplasm from the endocytotic vacuole, where it appears to polymerize actin filaments to spread across cells in the closer vicinity. B. pseudomallei effectively evades the host’s surveillance armory to remain latent for prolonged duration also causing relapses despite antimicrobial therapy. Therefore, eradication of intracellular B. pseudomallei is highly dependent on robust cellular immune responses. However, it remains ambiguous why certain individuals in endemic areas experience asymptomatic seroconversion, whereas others succumb to sepsis-associated sequelae. Here, we propose key insights on how the host’s surveillance radars get commandeered by B. pseudomallei.

Biofilm Targeting Strategy in the Eradication of Burkholderia Infections: A Mini-Review

Burkholderia are intracellular pathogenic bacteria which can produce biofilm. This biofilm protects the intracellular pathogenic bacteria from antibiotic treatment and the immunological system of the host. Therefore, this review aims to describe the capacity of Burkholderia to form a biofilm, the regulation of its biofilm formation, the efficacy of antibiotics to eradicate biofilm, and the novel therapy which targets its biofilm. Burkholderia's biofilm is characterized by its lipopolysaccharides, exopolysaccharides (EPSs), biofilm-associated proteins, and eDNA. Its regulation is made by quorum sensing, c-di-AMP, sRNA, and two component systems. Many antibiotics have been used as sole or mixture agents; however, they are not always effective in eradicating the biofilm-forming Burkholderia. Inhibitors of quorum sensing and other non-conventional antibiotic approaches are promising to discover effective treatment of Burkholderia infections.

Tannic Acid-Stabilized Silver Nanoparticles Used in Biomedical Application as an Effective Antimelioidosis and Prolonged Efflux Pump Inhibitor against Melioidosis Causative Pathogen

Burkholderia pseudomallei is the causative pathogen of melioidosis and this bacterium is resistant to several antibiotics. Silver nanoparticles (AgNPs) are an interesting agent to develop to solve this bacterial resistance. Here, we characterize and assess the antimelioidosis activity of AgNPs against these pathogenic bacteria. AgNPs were characterized and displayed a maximum absorption band at 420 nm with a spherical shape, being well-monodispersed and having high stability in solution. The average size of AgNPs is 7.99 ± 1.46 nm. The antibacterial efficacy of AgNPs was evaluated by broth microdilution. The bactericidal effect of AgNPs was further assessed by time-kill kinetics assay. Moreover, the effect of AgNPs on the inhibition of the established biofilm was investigated by the crystal violet method. In parallel, a study of the resistance induction development of B. pseudomallei towards AgNPs with efflux pump inhibiting effect was performed. We first found that AgNPs had strong antibacterial activity against both susceptible and ceftazidime-resistant (CAZ-resistant) strains, as well as being efficiently active against B. pseudomallei CAZ-resistant strains with a fast-killing mode via a bactericidal effect within 30 min. These AgNPs did not only kill planktonic bacteria in broth conditions, but also in established biofilm. Our findings first documented that the resistance development was not induced in B. pseudomallei toward AgNPs in the 30th passage. We found that AgNPs still showed an effective efflux pump inhibiting effect against these bacteria after prolonged exposure to AgNPs at sublethal concentrations. Thus, AgNPs have valuable properties for being a potent antimicrobial agent to solve the antibiotic resistance problem in pathogens.

Interaction between bovine mammary epithelial cells and planktonic or biofilm Staphylococcus aureus: The bacterial lifestyle determines its internalization ability and the pathogen recognition

The main cause of mastitis, one of the most costly diseases in the dairy industry, is bacterial intramammary infection. Many of these bacteria are biofilm formers. Biofilms have been associated with resistance to antibiotics and to the host immune system. Here, we evaluated different experimental models representing bacterial biofilm lifestyle with the aim to study bacterial invasion into bovine mammary epithelial cells and the interaction of these cells with planktonic or biofilm Staphylococcus aureus. Staphylococcus aureus V329, its nonbiofilm-forming mutant and bovine mammary alveolar cells (MAC-T) were used. Bacterial invasion was studied using the gentamicin exclusion test, cell viability by trypan blue exclusion technique, TLR2 expression by flow cytometry, IL1β/IL6 production by ELISA and IL8/TNFα gene expression by real-time polymerase chain reaction. Biofilm and planktonic S. aureus showed differences in their invasion ability, with the biofilm mode showing a lower ability. Planktonic S. aureus reduced MAC-T viability after 6 h of co-culture, while biofilms did so at 24 h. MAC-T infected with planktonic bacteria showed increased TLR2 expression. Both lifestyles increased IL8 expression and IL1β/IL6 production but did not modify TNFα expression. Our results demonstrate that the bacterial lifestyle affects the invasion behavior, suggesting that biofilms reduce the bacteria-epithelial cell interaction. Planktonic cultures seem to induce higher cellular activation than biofilms. Further knowledge about the complex host-biofilm interaction is necessary to design more efficient therapies against bovine mastitis.

Tetraspanins are involved in Burkholderia pseudomallei-induced cell-to-cell fusion of phagocytic and non-phagocytic cells

Tetraspanins are four-span transmembrane proteins of host cells that facilitate infections by many pathogens. Burkholderia pseudomallei is an intracellular bacterium and the causative agent of melioidosis, a severe disease in tropical regions. This study investigated the role of tetraspanins in B. pseudomallei infection. We used flow cytometry to determine tetraspanins CD9, CD63, and CD81 expression on A549 and J774A.1 cells. Their roles in B. pseudomallei infection were investigated in vitro using monoclonal antibodies (MAbs) and recombinant large extracellular loop (EC2) proteins to pretreat cells before infection. Knockout of CD9 and CD81 in cells was performed using CRISPR Cas9 to confirm the role of tetraspanins. Pretreatment of A549 cells with MAb against CD9 and CD9-EC2 significantly enhanced B. pseudomallei internalization, but MAb against CD81 and CD81-EC2 inhibited MNGC formation. Reduction of MNGC formation was consistently observed in J774.A1 cells pretreated with MAbs specific to CD9 and CD81 and with CD9-EC2 and CD81-EC2. Data from knockout experiments confirmed that CD9 enhanced bacterial internalization and that CD81 inhibited MNGC formation. Our data indicate that tetraspanins are host cellular factors that mediated internalization and membrane fusion during B. pseudomallei infection. Tetraspanins may be the potential therapeutic targets for melioidosis.

Host and Pathogen Communication in the Respiratory Tract: Mechanisms and Models of a Complex Signaling Microenvironment

Chronic lung diseases are a leading cause of morbidity and mortality across the globe, encompassing a diverse range of conditions from infections with pathogenic microorganisms to underlying genetic disorders. The respiratory tract represents an active interface with the external environment having the primary immune function of resisting pathogen intrusion and maintaining homeostasis in response to the myriad of stimuli encountered within its microenvironment. To perform these vital functions and prevent lung disorders, a chemical and biological cross-talk occurs in the complex milieu of the lung that mediates and regulates the numerous cellular processes contributing to lung health. In this review, we will focus on the role of cross-talk in chronic lung infections, and discuss how different cell types and signaling pathways contribute to the chronicity of infection(s) and prevent effective immune clearance of pathogens. In the lung microenvironment, pathogens have developed the capacity to evade mucosal immunity using different mechanisms or virulence factors, leading to colonization and infection of the host; such mechanisms include the release of soluble and volatile factors, as well as contact dependent (juxtracrine) interactions. We explore the diverse modes of communication between the host and pathogen in the lung tissue milieu in the context of chronic lung infections. Lastly, we review current methods and approaches used to model and study these host-pathogen interactions in vitro, and the role of these technological platforms in advancing our knowledge about chronic lung diseases.

Genomic loss in environmental and isogenic morphotype isolates of Burkholderia pseudomallei is associated with intracellular survival and plaque-forming efficiency

BACKGROUND

Burkholderia pseudomallei is an environmental bacterium that causes melioidosis. A facultative intracellular pathogen, B. pseudomallei can induce multinucleated giant cells (MNGCs) leading to plaque formation in vitro. B. pseudomallei can switch colony morphotypes under stress conditions. In addition, different isolates have been reported to have varying virulence in vivo, but genomic evolution and the relationship with plaque formation is poorly understood.

METHODOLOGY/PRINCIPLE FINDINGS

To gain insights into genetic underpinnings of virulence of B. pseudomallei, we screened plaque formation of 52 clinical isolates and 11 environmental isolates as well as 4 isogenic morphotype isolates of B. pseudomallei strains K96243 (types II and III) and 153 (types II and III) from Thailand in A549 and HeLa cells. All isolates except one environmental strain (A4) and K96243 morphotype II were able to induce plaque formation in both cell lines. Intracellular growth assay and confocal microscopy analyses demonstrated that the two plaque-forming-defective isolates were also impaired in intracellular replication, actin polymerization and MNGC formation in infected cells. Whole genome sequencing analysis and PCR revealed that both isolates had a large genomic loss on the same region in chromosome 2, which included Bim cluster, T3SS-3 and T6SS-5 genes.

CONCLUSIONS/SIGNIFICANCE

Our plaque screening and genomic studies revealed evidence of impairment in plaque formation in environmental isolates of B. pseudomallei that is associated with large genomic loss of genes important for intracellular multiplication and MNGC formation. These findings suggest that the genomic and phenotypic differences of environmental isolates may be associated with clinical infection.

Rhamnolipid enhances Burkholderia pseudomallei biofilm susceptibility, disassembly and production of virulence factors

Aim: This study evaluated the effect of the biosurfactant rhamnolipid on the antimicrobial susceptibility, biofilm growth dynamics and production of virulence factors by Burkholderia pseudomallei. Materials & methods: The effects of rhamnolipid on planktonic and biofilm growth and its interaction with antibacterial drugs were evaluated. Then, its effects on growing and mature biofilms and on protease and siderophore production were assessed. Results: Rhamnolipid did not inhibit B. pseudomallei growth, but significantly enhanced the activity of meropenem and amoxicillin-clavulanate against mature biofilms. Rhamnolipid significantly reduced the biomass of mature biofilms, significantly increased protease production by growing and mature biofilms and siderophore release by growing biofilms. Conclusion: Rhamnolipid enhances the antimicrobial activity against B. pseudomallei, assists biofilm disassembly and alters protease and siderophore production by bacterial biofilms.

Burkholderia pseudomallei pathogenesis and survival in different niches

Burkholderia pseudomallei (Bp) is the causative agent of melioidosis, a disease of the tropics with high clinical mortality rates. To date, no vaccines are approved for melioidosis and current treatment relies on antibiotics. Conversely, common misdiagnosis and high pathogenicity of Bp hamper efforts to fight melioidosis. This bacterium can be isolated from a wide range of niches such as waterlogged fields, stagnant water bodies, salt water bodies and from human and animal clinical specimens. Although extensive studies have been undertaken to elucidate pathogenesis mechanisms of Bp, little is known about how a harmless soil bacterium adapts to different environmental conditions, in particular, the shift to a human host to become a highly virulent pathogen. The bacterium has a large genome encoding an armory of factors that assist the pathogen in surviving under stressful conditions and assuming its role as a deadly intracellular pathogen. This review presents an overview of what is currently known about how the pathogen adapts to different environments. With in-depth understanding of Bp adaptation and survival, more effective therapies for melioidosis can be developed by targeting related genes or proteins that play a major role in the bacteria's survival.

Peptidyl-Prolyl Isomerase ppiB Is Essential for Proteome Homeostasis and Virulence in Burkholderia pseudomallei

Burkholderia pseudomallei is the causative agent of melioidosis, a disease endemic to Southeast Asia and northern Australia. Mortality rates in these areas are high even with antimicrobial treatment, and there are few options for effective therapy. Therefore, there is a need to identify antibacterial targets for the development of novel treatments. Cyclophilins are a family of highly conserved enzymes important in multiple cellular processes. Cyclophilins catalyze the cis-trans isomerization of xaa-proline bonds, a rate-limiting step in protein folding which has been shown to be important for bacterial virulence. B. pseudomallei carries a putative cyclophilin B gene, ppiB, the role of which was investigated. A B. pseudomallei ΔppiB (BpsΔppiB) mutant strain demonstrates impaired biofilm formation and reduced motility. Macrophage invasion and survival assays showed that although the BpsΔppiB strain retained the ability to infect macrophages, it had reduced survival and lacked the ability to spread cell to cell, indicating ppiB is essential for B. pseudomallei virulence. This is reflected in the BALB/c mouse infection model, demonstrating the requirement of ppiB for in vivo disease dissemination and progression. Proteomic analysis demonstrates that the loss of PpiB leads to pleiotropic effects, supporting the role of PpiB in maintaining proteome homeostasis. The loss of PpiB leads to decreased abundance of multiple virulence determinants, including flagellar machinery and alterations in type VI secretion system proteins. In addition, the loss of ppiB leads to increased sensitivity toward multiple antibiotics, including meropenem and doxycycline, highlighting ppiB inhibition as a promising antivirulence target to both treat B. pseudomallei infections and increase antibiotic efficacy.

Extracellular DNA facilitates bacterial adhesion during Burkholderia pseudomallei biofilm formation

The biofilm-forming ability of Burkholderia pseudomallei is crucial for its survival in unsuitable environments and is correlated with antibiotic resistance and relapsing cases of melioidosis. Extracellular DNA (eDNA) is an essential component for biofilm development and maturation in many bacteria. The aim of this study was to investigate the eDNA released by B. pseudomallei during biofilm formation using DNase treatment. The extent of biofilm formation and quantity of eDNA were assessed by crystal-violet staining and fluorescent dye-based quantification, respectively, and visualized by confocal laser scanning microscopy (CLSM). Variation in B. pseudomallei biofilm formation and eDNA quantity was demonstrated among isolates. CLSM images of biofilms stained with FITC-ConA (biofilm) and TOTO-3 (eDNA) revealed the localization of eDNA in the biofilm matrix. A positive correlation of biofilm biomass with quantity of eDNA during the 2-day biofilm-formation observation period was found. The increasing eDNA quantity over time, despite constant living/dead ratios of bacterial cells during the experiment suggests that eDNA is delivered from living bacterial cells. CLSM images demonstrated that depletion of eDNA by DNase I significantly lessened bacterial attachment (if DNase added at 0 h) and biofilm developing stages (if added at 24 h) but had no effect on mature biofilm (if added at 45 h). Collectively, our results reveal that eDNA is released from living B. pseudomallei and is correlated with biofilm formation. It was also apparent that eDNA is essential during bacterial cell attachment and biofilm-forming steps. The depletion of eDNA by DNase may provide an option for the prevention or dispersal of B. pseudomallei biofilm.

Robust biofilm assay for quantification and high throughput screening applications

Bacterial biofilms are populations of bacteria within a self-produced adherent extracellular matrix that are notoriously resistant to treatment. Existing methods for biofilm quantification are often limited in their dynamic range of detection (signal-to-background), throughput, and require modifications to the protocol depending on the bacterial species. To address these limitations, a broad utility, high-throughput (HTP) method was required. Using a fluorescent dye, FM1-43, we stained the biofilm, followed by solvent extraction and quantitation of biofilm employing a fluorescent plate reader. Utilizing eight different bacterial pathogens, we demonstrate that this method is widely applicable for biofilm quantification. Depending on the species, this biofilm assay offered a large dynamic range of 8-146 fold change compared to 2-22 fold for crystal violet staining under similar conditions. In addition to routine biofilm quantification using this new assay, as a proof-of-concept, 1200 compounds were screened against two different bacterial species to identify biofilm inhibitors. In our HTP screens we successfully identified compounds rifabutin and ethavarine as potential biofilm inhibitors of Burkholderia pseudomallei Bp82 and Acinetobacter baumannii biofilm production respectively. This newly validated biofilm assay is robust and can be readily adapted for antibiofilm screening campaigns and can supplant other less sensitive and low throughput methods.

Multiple recurrent abscesses in a patient with undiagnosed IL-12 deficiency and infection by Burkholderia gladioli

We report the occurrence of two severe illnesses experienced by one patient over a 19 year period of time. Both illnesses were characterized by severe inflammation and tissue destruction. Signs and symptoms of the first illness were characteristic of lymphogranuloma venereum (LGV). The second illness mimicked scrofula. During the second illness the patient was discovered to have a rare immunodeficiency due to auto-antibodies to Interleukin (IL)-12 and infection by Burkholderia gladioli, a plant pathogen usually harmless in humans. We were able to retrieve biopsies from the first illness to establish that B. gladioli was already present during the original presentation. That first illness lasted 5 year s, but she survived without the correct pathogen ever being identified, and without a diagnosis of immunodeficiency. After a remission of 10 year s, she experienced her second illness. The responses to treatment before and after the correct diagnoses were established provide us with an excellent opportunity to consider and discuss how disease expression reflects complex relationships between host defenses and microbial characteristics.

Burkholderia pseudomallei Adaptation for Survival in Stressful Conditions

Burkholderia pseudomallei is a Gram-negative bacterium that causes melioidosis, which can be fatal in humans. Melioidosis is prevalent in the tropical regions of Southeast Asia and Northern Australia. Ecological data have shown that this bacterium can survive as a free-living organism in environmental niches, such as soil and water, as well as a parasite living in host organisms, such as ameba, plants, fungi, and animals. This review provides an overview of the survival and adaptation of B. pseudomallei to stressful conditions induced by hostile environmental factors, such as salinity, oxidation, and iron levels. The adaptation of B. pseudomallei in host cells is also reviewed. The adaptive survival mechanisms of this pathogen mainly involve modulation of gene and protein expression, which could cause alterations in the bacteria's cell membrane, metabolism, and virulence. Understanding the adaptations of this organism to environmental factors provides important insights into the survival and pathogenesis of B. pseudomallei, which may lead to the development of novel strategies for the control, prevention, and treatment of melioidosis in the future.

A Negative Regulator of Cellulose Biosynthesis, bcsR, Affects Biofilm Formation, and Adhesion/Invasion Ability of Cronobacter sakazakii

Cronobacter sakazakii is an important foodborne pathogen that causes neonatal meningitis and sepsis, with high mortality in neonates. However, very little information is available regarding the pathogenesis of C. sakazakii at the genetic level. In our previous study, a cellulose biosynthesis-related gene (bcsR) was shown to be involved in C. sakazakii adhesion/invasion into epithelial cells. In this study, the detailed functions of this gene were investigated using a gene knockout technique. A bcsR knockout mutant (ΔbcsR) of C. sakazakii ATCC BAA-894 showed decreased adhesion/invasion (3.9-fold) in human epithelial cell line HCT-8. Biofilm formation by the mutant was reduced to 50% of that exhibited by the wild-type (WT) strain. Raman spectrometry was used to detect variations in biofilm components caused by bcsR knockout, and certain components, including carotenoids, fatty acids, and amides, were significantly reduced. However, another biofilm component, cellulose, was increased in ΔbcsR, suggesting that bcsR negatively affects cellulose biosynthesis. This result was also verified via RT-PCR, which demonstrated up-regulation of five crucial cellulose synthesis genes (bcsA, B, C, E, Q) in ΔbcsR. Furthermore, the expression of other virulence or biofilm-related genes, including flagellar assembly genes (fliA, C, D) and toxicity-related genes (ompA, ompX, hfq), was studied. The expression of fliC and ompA in the ΔbcsR mutant was found to be remarkably reduced compared with that in the wild-type and the others were also affected excepted ompX. In summary, bcsR is a negative regulator of cellulose biosynthesis but positively regulates biofilm formation and the adhesion/invasion ability of C. sakazakii.

PKC-η-MARCKS Signaling Promotes Intracellular Survival of Unopsonized Burkholderia thailandensis

Pathogenic Burkholderia rely on host factors for efficient intracellular replication and are highly refractory to antibiotic treatment. To identify host genes that are required by Burkholderia spp. during infection, we performed a RNA interference (RNAi) screen of the human kinome and identified 35 host kinases that facilitated Burkholderia thailandensis intracellular survival in human monocytic THP-1 cells. We validated a selection of host kinases using imaging flow cytometry to assess efficiency of B. thailandensis survival in the host upon siRNA-mediated knockdown. We focused on the role of the novel protein kinase C isoform, PKC-η, in Burkholderia infection and characterized PKC-η/MARCKS signaling as a key event that promotes the survival of unopsonized B. thailandensis CDC2721121 within host cells. While infection of lung epithelial cells with unopsonized Gram-negative bacteria stimulated phosphorylation of Ser175/160 in the MARCKS effector domain, siRNA-mediated knockdown of PKC-η expression reduced the levels of phosphorylated MARCKS by >3-fold in response to infection with Bt CDC2721121. We compared the effect of the conventional PKC-α and novel PKC-η isoforms on the growth of B. thailandensis CDC2721121 within monocytic THP-1 cells and found that ≥75% knock-down of PRKCH transcript levels reduced intracellular bacterial load 100% more efficiently when compared to growth in cells siRNA-depleted of the classical PKC-α, suggesting that the PKC-η isoform can specifically mediate Burkholderia intracellular survival. Based on imaging studies of intracellular B. thailandensis, we found that PKC-η function stimulates phagocytic pathways that promote B. thailandensis escape into the cytoplasm leading to activation of autophagosome flux. Identification of host kinases that are targeted by Burkholderia during infection provides valuable molecular insights in understanding Burkholderia pathogenesis, and ultimately, in designing effective host-targeted therapies against infectious disease caused by intracellular pathogens.