Burkholderia pseudomallei kynB plays a role in AQ production, biofilm formation, bacterial swarming and persistence

Role of bacterial multidrug efflux pumps during infection

Multidrug efflux pumps are protein complexes located in the cell envelope that enable bacteria to expel, not only antibiotics, but also a wide array of molecules relevant for infection. Hence, they are important players in microbial pathogenesis. On the one hand, efflux pumps can extrude exogenous compounds, including host-produced antimicrobial molecules. Through this extrusion, pathogens can resist antimicrobial agents and evade host defenses. On the other hand, efflux pumps also have a role in the extrusion of endogenous compounds, such as bacterial intercommunication signaling molecules, virulence factors or metabolites. Therefore, efflux pumps are involved in the modulation of bacterial behavior and virulence, as well as in the maintenance of the bacterial homeostasis under different stresses found within the host. This review delves into the multifaceted roles that efflux pumps have, shedding light on their impact on bacterial virulence and their contribution to bacterial infection. These observations suggest that strategies targeting bacterial efflux pumps could both reinvigorate the efficacy of existing antibiotics and modulate the bacterial pathogenicity to the host. Thus, a comprehensive understanding of bacterial efflux pumps can be pivotal for the development of new effective strategies for the management of infectious diseases.

Efflux pumps and microbial biofilm formation

Biofilm-related infections are resistant forms of pathogens that are regarded as a medical problem, particularly due to the spread of multiple drug resistance. One of the factors associated with biofilm drug resistance is the presence of various types of efflux pumps in bacteria. Efflux pumps also play a role in biofilm formation by influencing Physical-chemical interactions, mobility, gene regulation, quorum sensing (QS), extracellular polymeric substances (EPS), and toxic compound extrusion. According to the findings of studies based on efflux pump expression analysis, their role in the anatomical position within the biofilm will differ depending on the biofilm formation stage, encoding gene expression level, the type and concentration of substrate. In some cases, the function of the efflux pumps can overlap with each other, so it seems necessary to accurate identify the efflux pumps of biofilm-forming bacteria along with their function in this process. Such studies will help to choose treatment strategy, at least in combination with antibiotics. Furthermore, if the goal of treatment is an efflux pump manipulation, we should not limit it to inhibition.

In vitro effect of the iron chelator deferiprone on the antimicrobial susceptibility and biofilms of Burkholderia pseudomallei

This study evaluated the effect of the iron chelator deferiprone (DFP) on antimicrobial susceptibility and biofilm formation and maintenance by Burkholderia pseudomallei. Planktonic susceptibility to DFP alone and in combination with antibiotics was evaluated by broth microdilution and biofilm metabolic activity was determined with resazurin. DFP minimum inhibitory concentration (MIC) range was 4-64 µg/mL and in combination reduced the MIC for amoxicillin/clavulanate and meropenem. DFP reduced the biomass of biofilms by 21 and 12% at MIC and MIC/2, respectively. As for mature biofilms, DFP reduced the biomass by 47%, 59%, 52% and 30% at 512, 256, 128 and 64 µg/mL, respectively, but did not affect B. pseudomallei biofilm viability nor increased biofilm susceptibility to amoxicillin/clavulanate, meropenem and doxycycline. DFP inhibits planktonic growth and potentiates the effect of β-lactams against B. pseudomallei in the planktonic state and reduces biofilm formation and the biomass of B. pseudomallei biofilms.

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.

Bacterial Alkyl-4-quinolones: Discovery, Structural Diversity and Biological Properties

The alkyl-4-quinolones (AQs) are a class of metabolites produced primarily by members of the Pseudomonas and Burkholderia genera, consisting of a 4-quinolone core substituted by a range of pendant groups, most commonly at the C-2 position. The history of this class of compounds dates back to the 1940s, when a range of alkylquinolones with notable antibiotic properties were first isolated from Pseudomonas aeruginosa. More recently, it was discovered that an alkylquinolone derivative, the Pseudomonas Quinolone Signal (PQS) plays a key role in bacterial communication and quorum sensing in Pseudomonas aeruginosa. Many of the best-studied examples contain simple hydrocarbon side-chains, but more recent studies have revealed a wide range of structurally diverse examples from multiple bacterial genera, including those with aromatic, isoprenoid, or sulfur-containing side-chains. In addition to their well-known antimicrobial properties, alkylquinolones have been reported with antimalarial, antifungal, antialgal, and antioxidant properties. Here we review the structural diversity and biological activity of these intriguing metabolites.

Predicting toxins found in toxin-antitoxin systems with a role in host-induced Burkholderia pseudomallei persistence

Burkholderia pseudomallei (Bpm) is a bacterial pathogen that causes Melioidosis, a disease with up to 40% mortality and an infection relapse of 15-23% despite antibiotic treatment. Ineffective clearance of Bpm by antibiotics is believed to be due to persistence, a hibernation-like survival mechanism modulated, in part, by toxin-antitoxin systems (TAS). Several organisms possess a repertoire of TASs but defining environmental cues eliciting their activity is hindered by laborious in vitro experiments, especially when there are many toxins with redundant function. Here, we identified which of 103 proteins in Bpm that share features found in toxins of the TAS and repurposed transcriptional data to identify which ones play a role in surviving intracellular host defenses. Putative toxins with the strongest transcriptional response were found to have low conservation between Bpm strains, while toxins that were constitutively expressed were highly conserved. Further examination of highly conserved toxins BPSS0899, BPSS1321, and BPSL1494 showed that they were functional, and their mutation led to reduce survival within macrophages and reduced in vivo persistence-associated pathology (abscesses) during treatment, but did not affect macrophages persistence. These findings highlight the utility of a data-driven approach to select putative toxins and suggests a selective role for some TAS in host survival.

A Quantitative Survey of Bacterial Persistence in the Presence of Antibiotics: Towards Antipersister Antimicrobial Discovery

Background: Bacterial persistence to antibiotics relates to the phenotypic ability to survive lethal concentrations of otherwise bactericidal antibiotics. The quantitative nature of the time-kill assay, which is the sector's standard for the study of antibiotic bacterial persistence, is an invaluable asset for global, unbiased, and cross-species analyses. Methods: We compiled the results of antibiotic persistence from antibiotic-sensitive bacteria during planktonic growth. The data were extracted from a sample of 187 publications over the last 50 years. The antibiotics used in this compilation were also compared in terms of structural similarity to fluorescent molecules known to accumulate in Escherichia coli. Results: We reviewed in detail data from 54 antibiotics and 36 bacterial species. Persistence varies widely as a function of the type of antibiotic (membrane-active antibiotics admit the fewest), the nature of the growth phase and medium (persistence is less common in exponential phase and rich media), and the Gram staining of the target organism (persistence is more common in Gram positives). Some antibiotics bear strong structural similarity to fluorophores known to be taken up by E. coli, potentially allowing competitive assays. Some antibiotics also, paradoxically, seem to allow more persisters at higher antibiotic concentrations. Conclusions: We consolidated an actionable knowledge base to support a rational development of antipersister antimicrobials. Persistence is seen as a step on the pathway to antimicrobial resistance, and we found no organisms that failed to exhibit it. Novel antibiotics need to have antipersister activity. Discovery strategies should include persister-specific approaches that could find antibiotics that preferably target the membrane structure and permeability of slow-growing cells.

Evaluating the role of Burkholderia pseudomallei K96243 toxins BPSS0390, BPSS0395, and BPSS1584 in persistent infection

Burkholderia pseudomallei is the causative agent of melioidosis, a disease with a mortality rate of up to 40% even with treatment. Despite the ability of certain antibiotics to control initial infection, relapse occurs in treated patients. The inability of antibiotics to clear this bacterial infection is in part due to persistence, an evasion mechanism against antibiotics and the effect of host defenses. Evaluation of antibiotic efficacy against B. pseudomallei revealed that up to 48% of in vitro grown populations can survive in a persister state. Toxin-antitoxin (TA) systems have been previously implicated in modulating bacterial persistence. We generated three isogenic TA mutants and found that loss of each toxin gene did not alter antibiotic persistence or macrophage survival. In response to macrophage-induced persistence, all three toxin mutants demonstrated increased intracellular susceptibility to levofloxacin which in part was due to the inability of the mutants to induce persistence after nitric oxide or nutrient starvation. In an inhalational model of murine melioidosis, both ΔBPSS0395 and ΔBPSS1584 strains were attenuated, and treatment with levofloxacin led to significant reduction in lung colonisation and reduced splenic colonisation by ΔBPSS0395. Based on our findings, these toxins deserve additional evaluation as putative therapeutic targets.

Potential of the Burkholderia cepacia Complex to Produce 4-Hydroxy-3-Methyl-2-Alkyquinolines

A few Burkholderia species, especially Burkholderia pseudomallei, Burkholderia thailandensis, Burkholderia ambifaria, and Burkholderia cepacia, are known to produce and release various 4-hydroxy-3-methyl-2-alkylquinolines (HMAQs), a family of molecules analogous to the 4-hydroxy-2-alkylquinolines [aka 2-n-alkyl-4(1H)-quinolones] of Pseudomonas aeruginosa, which include the Pseudomonas quinolone signal (PQS). However, while these exoproducts play several roles in P. aeruginosa virulence and survival, the available literature is very limited on their distribution and function in Burkholderia. In this perspective article, we studied the distribution of the hmqABCDEFG operon, which encodes the enzymes involved in the biosynthesis of HMAQs, in the Burkholderia cepacia complex (Bcc) group. Based on the available sequence data, about one third of Bcc species carry a homolog of the hmqABCDEFG, and not all sequenced strains in a given species possess this operon. Looking at the synteny of genes surrounding the hmqABCDEFG operon, we found that for some species, the operon seems to have been deleted or replaced by other genes. Finally, we review the literature on the possible function of HMAQs. Understanding the Hmq system may provide clues concerning their functions in Bcc.

SmpB down-regulates proton-motive force for the persister tolerance to aminoglycosides in Aeromonas veronii

Bacterial persisters comprise a small fraction of phenotypically heterogeneous variants with transient capability for survival when exposed to high concentrations of antibiotic. In aquatic pathogenic bacteria Aeromonas veronii, Small Protein B (SmpB), the core factor of trans-translation system, was identified as a new persistence-related gene. The SmpB deletion exhibited a higher susceptibility and lower persister cell formation under aminoglycosides antibiotics pressure compared with wild type. The transcriptional and translational activities of smpB gene were significantly enhanced by the gentamicin challenge in exponential phase, but not changed in stationary phase. The transcriptomic analysis revealed that the smpB deletion stimulated the production of proton-motive force (PMF). The cell survival induced by carbonyl cyanide m-chlorophenyl hydrazone (CCCP) further verified that SmpB variation affected the quantities of PMF. Taken together, these results uncovered a novel mechanism of persister formation mediated by SmpB under aminoglycosides treatments.

Redox-Sensitive MarR Homologue BifR from Burkholderia thailandensis Regulates Biofilm Formation

Biofilm formation by pathogenic Burkholderia species is a serious complication as it renders the bacteria resistant to antibiotics and host defenses. Using B. thailandensis, we report here a novel redox-sensitive member of the multiple antibiotic resistance regulator (MarR) protein family, BifR, which represses biofilm formation. BifR is encoded as part of the emrB-bifR operon; emrB-bifR is divergent to ecsC, which encodes a putative LasA protease. In Pseudomonas aeruginosa, LasA has been implicated in virulence by contributing to cleavage of elastase. BifR repressed the expression of ecsC and emrB-bifR, and expression was further repressed under oxidizing conditions. BifR bound two sites in the intergenic region between ecsC and emrB-bifR with nanomolar affinity under both reducing and oxidizing conditions; however, oxidized BifR formed a disulfide-linked dimer-of-dimers, a covalent linkage that was absent in BifR-C104A in which the redox-active cysteine was replaced with alanine. BifR also repressed an operon encoding enzymes required for synthesis of phenazine antibiotics, which function as alternate respiratory electron receptors, and inactivation of bifR resulted in enhanced biofilm formation. Taken together, our data suggest that BifR functions to control LasA production and expression of genes involved in biofilm formation, in part by regulating synthesis of alternate electron acceptors that promote survival in the oxygen-limiting environment of a biofilm. The correlation between increased repression of emrB-bifR under oxidative conditions and the formation of a covalently linked BifR dimer-of-dimers suggests that BifR may modulate gene activity in response to cellular redox state.

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

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

The art of persistence-the secrets to Burkholderia chronic infections

The Gram-negative proteobacteria genus Burkholderia encompasses multiple bacterial species that are pathogenic to humans and other vertebrates. Two pathogenic species of interest within this genus are Burkholderia pseudomallei (Bpm) and the B. cepacia complex (Bcc); the former is the causative agent of melioidosis in humans and other mammals, and the latter is associated with pneumonia in immunocompromised patients. One understudied and shared characteristic of these two pathogenic groups is their ability to persist and establish chronic infection within the host. In this review, we will explore the depth of knowledge about chronic infections caused by persistent Bpm and Bcc. We examine the host risk factors and immune responses associated with more severe chronic infections. We also discuss host adaptation and phenotypes associated with persistent Burkholderia species. Lastly, we survey how other intracellular bacteria associated with chronic infections are combatted and explore possible future applications to target Burkholderia Our goal is to highlight understudied areas that should be addressed for a more thorough understanding of chronic Burkholderia infections and how to combat them.