Modulation of quorum sensing in Pseudomonas aeruginosa through alteration of membrane properties

Inhibition of biofilm formation, quorum sensing and virulence factor production in Pseudomonas aeruginosa PAO1 by selected LasR inhibitors

The quorum sensing network of Pseudomonas aeruginosa mediates the regulation of genes controlling biofilm formation and virulence factors. The rise of drug resistance to Pseudomonas aeruginosa infections has made quorum sensing-regulated biofilm formation in clinical settings a major issue. In the present study, LasR inhibitors identified in our previous study were evaluated for their antibiofilm and antiquorum sensing activities against P. aeruginosa PAO1. The compounds selected were (3-[2-(3,4-dimethoxyphenyl)-2-(1H-indol-3-yl)ethyl]-1-(2-fluorophenyl)urea) (C1), (3-(4-fluorophenyl)-2-[(3-methylquinoxalin-2-yl)methylsulfanyl]quinazolin-4-one) (C2) and (2-({4-[4-(2-methoxyphenyl)piperazin-1-yl]pyrimidin-2-yl}sulfanyl)-N-(2,4,6-trimethylphenyl)acetamide) (C3). The minimum inhibitory concentrations of C1 and C2 were 1000 μM, whereas that of C3 was 500 μM. At sub-MICs, the compounds showed potent antibiofilm activity without affecting the growth of P. aeruginosa PAO1. Electron microscopy confirmed the disruption of biofilm by the selected compounds. The antiquorum sensing activity of the compounds was revealed by the inhibition of violacein in Chromobacterium violaceum and the inhibition of swimming and swarming motilities in P. aeruginosa PAO1. Furthermore, the compounds also attenuated the production of quorum sensing-mediated virulence factors. The qRT-PCR revealed the downregulation of quorum sensing regulatory genes, namely lasI, lasR, rhlI, rhlR, lasB, pqsA and pqsR. The selected compounds also exhibited lower cytotoxicity against peripheral blood lymphocytes. Thus, this study could pave a way to explore these compounds for the development of therapeutic agent against Pseudomonas aeruginosa biofilm-related infections.

Suppressor mutants demonstrate the metabolic plasticity of unsaturated fatty acid synthesis in Pseudomonas aeruginosa PAO1

Pseudomonas aeruginosa PAO1 has two aerobic pathways for synthesis of unsaturated fatty acids (UFAs), DesA and DesB plus the oxygen independent FabAB pathway. The DesA desaturase acts on saturated acyl chains of membrane phospholipid bilayers whereas the substrates of the DesB desaturase are thought to be long chain saturated acyl-CoA thioesters derived from exogeneous saturated fatty acids that are required to support DesB-dependent growth. Under suitable aerobic conditions either of these membrane-bound desaturates can support growth of P. aeruginosa ∆fabA strains lacking the oxygen independent FabAB pathway. We previously studied function of the desA desaturase of P. putida in a P. aeruginosa ∆fabA ∆desA strain that required supplementation with a UFA for growth and noted bypass suppression of the P. aeruginosa ∆fabA ∆desA strain that restored UFA synthesis. We report three genes encoding lipid metabolism proteins that give rise to suppressor strains that bypass loss of the DesA and oxygen independent FabAB pathways.

Different cellular fatty acid pattern and gene expression of planktonic and biofilm state Listeria monocytogenes under nutritional stress

Listeria monocytogenes is a Gram-positive bacterium frequently involved in food-borne disease outbreaks and is widely distributed in the food-processing environment. This work aims to depict the impact of nutrition deficiency on the survival strategy of L. monocytogenes both in planktonic and biofilm states. In the present study, cell characteristics (autoaggression, hydrophobicity and motility), membrane fatty acid composition of MRL300083 (Lm83) in the forms of planktonic and biofilm-associated cells cultured in TSB-YE and 10-fold dilutions of TSB-YE (DTSB-YE) were investigated. Additionally, the relative expression of related genes were also determined by RT-qPCR. It was observed that cell growth in different bacterial life modes under nutritional stress rendered the cells a distinct phenotype. The higher autoaggression (AAG) and motility of the planktonic cells in DTSB-YE is associated with better biofilm formation. An increased proportion of unsaturated fatty acid/saturated fatty acid (USFA/SFA) indicates more fluidic biophysical properties for cell membranes of L. monocytogenes in planktonic and biofilm cells in DTSB-YE. Biofilm cells produced a higher percentage of USFA and straight fatty acids than the corresponding planktonic cells. An appropriate degree of membrane fluidity is crucial for survival, and alteration of membrane lipids is an essential adaptive response. The adaptation of bacteria to stress is a multifactorial cellular process, the expression of flagella-related genes fliG, fliP, flgE and the two-component chemotactic system cheA/Y genes of planktonic cells in DTSB-YE significantly increased compared to that in TSB-YE (p < 0.05). This study provides new information on the role of the physiological adaptation and gene expression of L. monocytogenes for planktonic and biofilm growth under nutritional stress.

Extracellular Vesicles of Pseudomonas: Friends and Foes

Extracellular vesicles (Evs) are small spherical vesicles capable of transporting molecules (such as proteins, nucleic acids and lipids) from one cell to another. They have been implicated in processes such as cell-to-cell communication, pathogenicity, biofilm formation and metabolism. In parallel, Evs have been proposed as interesting biotechnological tools. In recent years, antibiotic resistance has become a major problem for human health worldwide. A pathogen singled out as among the most lethal antibiotic-resistant organisms is Pseudomonas aeruginosa, an important Gram-negative bacterium that has been extensively studied for the production and characterization of Evs. Here, we describe the advances made in the last decade regarding understanding of the role of Evs in the pathogenicity of Pseudomonas. We also examine the potential of Evs for the development of new treatment strategies.

Pf4 Phage Variant Infection Reduces Virulence-Associated Traits in Pseudomonas aeruginosa

Pf4 is a filamentous bacteriophage integrated as a prophage into the genome of Pseudomonas aeruginosa PAO1. Pf4 virions can be produced without killing P. aeruginosa. However, cell lysis can occur during superinfection when Pf virions successfully infect a host lysogenized by a Pf superinfective variant. We have previously shown that infection of P. aeruginosa PAO1 with a superinfective Pf4 variant abolished twitching motility and altered biofilm architecture. More precisely, most of the cells embedded into the biofilm were showing a filamentous morphology, suggesting the activation of the cell envelope stress response involving both AlgU and SigX extracytoplasmic function sigma factors. Here, we show that Pf4 variant infection results in a drastic dysregulation of 3,360 genes representing about 58% of P. aeruginosa genome; of these, 70% of the virulence factors encoding genes show a dysregulation. Accordingly, Pf4 variant infection (termed Pf4*) causes in vivo reduction of P. aeruginosa virulence and decreased production of N-acyl-homoserine lactones and 2-alkyl-4-quinolones quorum-sensing molecules and related virulence factors, such as pyocyanin, elastase, and pyoverdine. In addition, the expression of genes involved in metabolism, including energy generation and iron homeostasis, was affected, suggesting further relationships between virulence and central metabolism. Altogether, these data show that Pf4 phage variant infection results in complex network dysregulation, leading to reducing acute virulence in P. aeruginosa. This study contributes to the comprehension of the bacterial response to filamentous phage infection. IMPORTANCE Filamentous bacteriophages can become superinfective and infect P. aeruginosa, even though they are inserted in the genome as lysogens. Despite this productive infection, growth of the host is only mildly affected, allowing the study of the interaction between the phage and the host, which is not possible in the case of lytic phages killing rapidly their host. Here, we demonstrate by transcriptome and phenotypic analysis that the infection by a superinfective filamentous phage variant causes a massive disruption in gene expression, including those coding for virulence factors and metabolic pathways.

Contribution of Membrane Vesicle to Reprogramming of Bacterial Membrane Fluidity in Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen capable of resisting environmental insults by applying various strategies, including regulating membrane fluidity and producing membrane vesicles (MVs). This study examined the difference in membrane fluidity between planktonic and biofilm modes of growth in P. aeruginosa and whether the ability to alter membrane rigidity in P. aeruginosa could be transferred via MVs. To this end, planktonic and biofilm P. aeruginosa were compared with respect to the lipid composition of their membranes and their MVs and the expression of genes contributing to alteration of membrane fluidity. Additionally, viscosity maps of the bacterial membrane in planktonic and biofilm lifestyles and under the effect of incubation with bacterial MVs were obtained. Further, the growth rate and biofilm formation capability of P. aeruginosa in the presence of MVs were compared. Results showed that the membrane of the biofilm bacteria is significantly less fluid than the membrane of the planktonic bacteria and is enriched with saturated fatty acids. Moreover, the enzymes involved in altering the structure of existing lipids and favoring membrane rigidification are overexpressed in the biofilm bacteria. MVs of biofilm P. aeruginosa elicit membrane rigidification and delay the bacterial growth in the planktonic lifestyle; conversely, they enhance biofilm development in P. aeruginosa. Overall, the study describes the interplay between the planktonic and biofilm bacteria by shedding light on the role of MVs in altering membrane fluidity.

IMPORTANCE Membrane rigidification is a survival strategy in Pseudomonas aeruginosa exposed to stress. Despite various studies dedicated to the mechanism behind this phenomenon, not much attention has been paid to the contribution of the bacterial membrane vesicles (MVs) in this regard. This study revealed that P. aeruginosa rigidifies its membrane in the biofilm mode of growth. Additionally, the capability of decreasing membrane fluidity is transferable to the bacterial population via the bacterial MVs, resulting in reprogramming of bacterial membrane fluidity. Given the importance of membrane rigidification for decreasing the pathogen’s susceptibility to antimicrobials, elucidation of the conditions leading to such biophysicochemical modulation of the P. aeruginosa membrane should be considered for the purpose of developing therapeutic approaches against this resistant pathogen.

Homeophasic Adaptation in Response to UVA Radiation in Pseudomonas aeruginosa: Changes of Membrane Fatty Acid Composition and Induction of desA and desB Expression

In bacteria, exposure to changes in environmental conditions can alter membrane fluidity, thereby affecting its essential functions in cell physiology. To adapt to these changes, bacteria maintain appropriate fluidity by varying the composition of the fatty acids of membrane phospholipids, a phenomenon known as homeophasic adaptation. In Pseudomonas aeruginosa, this response is achieved mainly by two mechanisms of fatty acid desaturation: the FabA-FabB and DesA-DesB systems. This study analyzed the effect of ultraviolet-A (UVA) radiation-the major fraction of solar UV radiation reaching the Earth's surface-on the homeophasic process. The prototypical strain PAO1 was grown under sublethal UVA doses or in the dark, and the profiles of membrane fatty acids were compared at early logarithmic, logarithmic and stationary growth phases. In the logarithmic growth phase, it was observed that growth under sublethal UVA doses induced the expression of the desaturase-encoding genes desA and desB and increased the proportion of unsaturated fatty acids; in addition, membrane fluidity could also increase, as suggested by the indices used as indicators of this parameter. The opposite effect was observed in the stationary growth phase. These results demonstrate the relevant role of UVA on the homeophasic response at transcriptional level.

In situ approaches show the limitation of the spoilage potential of Juniperus phoenicea L. essential oil against cold-tolerant Pseudomonas fluorescens KM24

Abstract

The present study aimed to elucidate the effect of subinhibitory concentrations (sub-MICs) of juniper essential oil (EO), α-pinene, and sabinene on the quorum-sensing (QS)–mediated proteolytic and lipolytic properties of Pseudomonas fluorescens KM24. These activities were verified under in situ conditions, in which sub-MICs of the agents altered the morphology of KM24 cells. RNA-Seq studies revealed key coding sequences (CDSs)/genes related to QS and the proteolytic/lipolytic activities of pseudomonads. In this work, all the examined agents decreased autoinducer synthesis and influenced the mRNA expression of the encoding acyltransferase genes lptA, lptD, and plsB. The highest reduction on the 3^rd and 5^th days of cultivation was observed for the genes lptD (−5.5 and −5.61, respectively) and lptA (−3.5 and −4.0, respectively) following treatment with EO. Inhibition of the lptA, lptD, and plsB genes by singular constituents of EO was on average, from −0.4 to −0.7. At 5 days of cultivation the profile of AHLs of the reference P. fluorescens KM24 strain consisted of 3-oxo-C14-HSL, 3-oxo-C6-HSL, C4-HSL, and N-[(RS)-3-hydroxybutyryl]-HSL, the concentrations of which were 0.570, 0.018, 3.744, and 0.554 μg ml⁻¹, respectively. Independent of the incubation time, EO, α-pinene, and sabinene also suppressed the protease genes prlC (−1.5, −0.5, and −0.5, respectively) and ctpB (−1.5, −0.7, and −0.4, respectively). Lipolysis and transcription of the lipA/lipB genes were downregulated by the agents on average from −0.3 to −0.6. α-Pinene- and sabinene-rich juniper EO acts as an anti-quorum-sensing agent and can repress the spoilage phenotype of pseudomonads.

Key points:

Juniper EO, α-pinene, sabinene exhibited anti-QS potential toward KM24.

RNA-Seq revealed key CDSs/genes related to QS/proteolytic/lipolytic activities of KM24.

Agents at sub-MIC levels influenced the mRNA expression of QS/lipase/protease genes.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-021-11338-3.

Investigating the action of the microalgal pigment marennine on Vibrio splendidus by in vivo2H and 31P solid-state NMR

This work investigates the potential probiotic effect of marennine - a natural pigment produced by the diatom Haslea ostrearia - on Vibrio splendidus. These marine bacteria are often considered a threat for aquaculture; therefore, chemical antibiotics can be required to reduce bacterial outbreaks. In vivo²H solid-state NMR was used to probe the effects of marennine on the bacterial membrane in the exponential and stationary phases. Comparisons were made with polymyxin B (PxB) - an antibiotic used in aquaculture and known to interact with Gram(-) bacteria membranes. We also investigated the effect of marennine using ³¹P solid-state NMR on model membranes. Our results show that marennine has little effect on phospholipid headgroups dynamics, but reduces the acyl chain fluidity. Our data suggest that the two antimicrobial agents perturb V. splendidus membranes through different mechanisms. While PxB would alter the bacterial outer and inner membranes, marennine would act through a membrane stiffening mechanism, without affecting the bilayer integrity. Our study proposes this microalgal pigment, which is harmless for humans, as a potential treatment against vibriosis.

PcsR2 Is a LuxR-Type Regulator That Is Upregulated on Wheat Roots and Is Unique to Pseudomonas chlororaphis

LuxR solos are common in plant-associated bacteria and increasingly recognized for playing important roles in plant-microbe interkingdom signaling. Unlike the LuxR-type transcriptional regulators of prototype LuxR/LuxI quorum sensing systems, luxR solos do not have a LuxI-type autoinducer synthase gene associated with them. LuxR solos in plant-pathogenic bacteria are important for virulence and in plant endosymbionts contribute to symbiosis. In the present study, we characterized an atypical LuxR solo, PcsR2, in the biological control species Pseudomonas chlororaphis 30-84 that is highly conserved among sequenced P. chlororaphis strains. Unlike most LuxR solos in the plant-associated bacteria characterized to date, pcsR2 is not associated with a proline iminopeptidase gene and the protein has an atypical N-terminal binding domain. We created a pcsR2 deletion mutant and used quantitative RT-PCR to show that the expression of pcsR2 and genes in the operon immediately downstream was upregulated ∼10-fold when the wild type strain was grown on wheat roots compared to planktonic culture. PcsR2 was involved in upregulation. Using a GFP transcriptional reporter, we found that expression of pcsR2 responded specifically to root-derived substrates as compared to leaf-derived substrates but not to endogenous AHLs. Compared to the wild type, the mutant was impaired in the ability to utilize root carbon and nitrogen sources in wheat root macerate and to colonize wheat roots. Phenazine production and most biofilm traits previously shown to be correlated with phenazine production also were diminished in the mutant. Gene expression of several of the proteins in the phenazine regulatory network including PhzR, Pip (phenazine inducing protein) and RpeA/RpeB were reduced in the mutant, and overexpression of these genes in trans restored phenazine production in the mutant to wild-type levels, indicating PcsR2 affects the activity of the these regulatory genes upstream of RpeA/RpeB via an undetermined mechanism. Our results indicate PcsR2 upregulates the expression of the adjacent operon in response to unknown wheat root-derived signals and belongs to a novel subfamily of LuxR-type transcriptional regulators found in sequenced P. chlororaphis strains.

Activation of the Cell Wall Stress Response in Pseudomonas aeruginosa Infected by a Pf4 Phage Variant

Pseudomonas aeruginosa PAO1 has an integrated Pf4 prophage in its genome, encoding a relatively well-characterized filamentous phage, which contributes to the bacterial biofilm organization and maturation. Pf4 variants are considered as superinfectives when they can re-infect and kill the prophage-carrying host. Herein, the response of P. aeruginosa H103 to Pf4 variant infection was investigated. This phage variant caused partial lysis of the bacterial population and modulated H103 physiology. We show by confocal laser scanning microscopy that a Pf4 variant-infection altered P. aeruginosa H103 biofilm architecture either in static or dynamic conditions. Interestingly, in the latter condition, numerous cells displayed a filamentous morphology, suggesting a link between this phenotype and flow-related forces. In addition, Pf4 variant-infection resulted in cell envelope stress response, mostly mediated by the AlgU and SigX extracytoplasmic function sigma factors (ECFσ). AlgU and SigX involvement may account, at least partly, for the enhanced expression level of genes involved in the biosynthesis pathways of two matrix exopolysaccharides (Pel and alginates) and bis-(3′-5′)-cyclic dimeric guanosine monophosphate (c-di-GMP) metabolism.

The Temperature-Regulation of Pseudomonas aeruginosa cmaX-cfrX-cmpX Operon Reveals an Intriguing Molecular Network Involving the Sigma Factors AlgU and SigX

Pseudomonas aeruginosa is a highly adaptable Gram-negative opportunistic pathogen, notably due to its large number of transcription regulators. The extracytoplasmic sigma factor (ECFσ) AlgU, responsible for alginate biosynthesis, is also involved in responses to cell wall stress and heat shock via the RpoH alternative σ factor. The SigX ECFσ emerged as a major regulator involved in the envelope stress response via membrane remodeling, virulence and biofilm formation. However, their functional interactions to coordinate the envelope homeostasis in response to environmental variations remain to be determined. The regulation of the putative cmaX-cfrX-cmpX operon located directly upstream sigX was investigated by applying sudden temperature shifts from 37°C. We identified a SigX- and an AlgU- dependent promoter region upstream of cfrX and cmaX, respectively. We show that cmaX expression is increased upon heat shock through an AlgU-dependent but RpoH independent mechanism. In addition, the ECFσ SigX is activated in response to valinomycin, an agent altering the membrane structure, and up-regulates cfrX-cmpX transcription in response to cold shock. Altogether, these data provide new insights into the regulation exerted by SigX and networks that are involved in maintaining envelope homeostasis.

Role of quorum sensing in UVA-induced biofilm formation in Pseudomonas aeruginosa

Pseudomonas aeruginosa, a versatile bacterium present in terrestrial and aquatic environments and a relevant opportunistic human pathogen, is largely known for the production of robust biofilms. The unique properties of these structures complicate biofilm eradication, because they make the biofilms very resistant to diverse antibacterial agents. Biofilm development and establishment is a complex process regulated by multiple regulatory genetic systems, among them is quorum sensing (QS), a mechanism employed by bacteria to regulate gene transcription in response to population density. In addition, environmental factors such as UVA radiation (400-315 nm) have been linked to biofilm formation. In this work, we further investigate the mechanism underlying the induction of biofilm formation by UVA, analysing the role of QS in this phenomenon. We demonstrate that UVA induces key genes of the Las and Rhl QS systems at the transcriptional level. We also report that pelA and pslA genes, which are essential for biofilm formation and whose transcription depends in part on QS, are significantly induced under UVA exposure. Finally, the results demonstrate that in a relA strain (impaired for ppGpp production), the UVA treatment does not induce biofilm formation or QS genes, suggesting that the increase of biofilm formation due to exposure to UVA in P. aeruginosa could rely on a ppGpp-dependent QS induction.

A Novel Lysophosphatidic Acid Acyltransferase of Escherichia coli Produces Membrane Phospholipids with a cis-vaccenoyl Group and Is Related to Flagellar Formation

Lysophosphatidic acid acyltransferase (LPAAT) introduces fatty acyl groups into the sn-2 position of membrane phospholipids (PLs). Various bacteria produce multiple LPAATs, whereas it is believed that Escherichia coli produces only one essential LPAAT homolog, PlsC-the deletion of which is lethal. However, we found that E. coli possesses another LPAAT homolog named YihG. Here, we show that overexpression of YihG in E. coli carrying a temperature-sensitive mutation in plsC allowed its growth at non-permissive temperatures. Analysis of the fatty acyl composition of PLs from the yihG-deletion mutant (∆yihG) revealed that endogenous YihG introduces the cis-vaccenoyl group into the sn-2 position of PLs. Loss of YihG did not affect cell growth or morphology, but ∆yihG cells swam well in liquid medium in contrast to wild-type cells. Immunoblot analysis showed that FliC was highly expressed in ∆yihG cells, and this phenotype was suppressed by expression of recombinant YihG in ∆yihG cells. Transmission electron microscopy confirmed that the flagellar structure was observed only in ∆yihG cells. These results suggest that YihG has specific functions related to flagellar formation through modulation of the fatty acyl composition of membrane PLs.

Diversity of physical properties of bacterial extracellular membrane vesicles revealed through atomic force microscopy phase imaging

Bacteria release nanometer-scale extracellular membrane vesicles (MVs) to mediate a variety of biological processes. We analyzed individual MVs under physiological conditions by phase imaging of high-speed atomic force microscopy to assess the physiological heterogeneity of MVs isolated from bacterial cultures. Phase imaging makes it possible to map the physical properties of an individual, fragile MV in an isolated MV population containing a broad variety of vesicle diameters, from 20 to 150 nm. We also developed a method for quantitatively comparing the physical properties of MVs among samples. This allowed for the comparison of the physical properties of MVs isolated from different bacterial species. We compared bacterial MVs isolated from four bacterial species and artificially synthesized liposomes. We demonstrate that each bacterial species generates physically heterogeneous types of MVs, unlike the physical homogeneity displayed by liposomes. These results indicate that the physical heterogeneity of bacterial MVs is mainly caused by compositional differences mediated through biological phenomena and could be unique to each species. We provide a new methodology using phase imaging that would pave the way for single-vesicle analysis of extracellular vesicles of a broad size range.

A Screen for Antibiotic Resistance Determinants Reveals a Fitness Cost of the Flagellum in Pseudomonas aeruginosa

The intrinsic resistance of Pseudomonas aeruginosa to many antibiotics limits treatment options for pseudomonal infections. P. aeruginosa's outer membrane is highly impermeable and decreases antibiotic entry into the cell. We used an unbiased high-throughput approach to examine mechanisms underlying outer membrane-mediated antibiotic exclusion. Insertion sequencing (INSeq) identified genes that altered fitness in the presence of linezolid, rifampin, and vancomycin, antibiotics to which P. aeruginosa is intrinsically resistant. We reasoned that resistance to at least one of these antibiotics would depend on outer membrane barrier function, as previously demonstrated in Escherichia coli and Vibrio cholerae This approach demonstrated a critical role of the outer membrane barrier in vancomycin fitness, while efflux pumps were primary contributors to fitness in the presence of linezolid and rifampin. Disruption of flagellar assembly or function was sufficient to confer a fitness advantage to bacteria exposed to vancomycin. These findings clearly show that loss of flagellar function alone can confer a fitness advantage in the presence of an antibiotic.IMPORTANCE The cell envelopes of Gram-negative bacteria render them intrinsically resistant to many classes of antibiotics. We used insertion sequencing to identify genes whose disruption altered the fitness of a highly antibiotic-resistant pathogen, Pseudomonas aeruginosa, in the presence of antibiotics usually excluded by the cell envelope. This screen identified gene products involved in outer membrane biogenesis and homeostasis, respiration, and efflux as important contributors to fitness. An unanticipated fitness cost of flagellar assembly and function in the presence of the glycopeptide antibiotic vancomycin was further characterized. These findings have clinical relevance for individuals with cystic fibrosis who are infected with P. aeruginosa and undergo treatment with vancomycin for a concurrent Staphylococcus aureus infection.

Metabolomic analysis of quorum sensing inhibitor hordenine on Pseudomonas aeruginosa

Proton magnetic resonance-based metabolomics analysis was performed to determine the global metabolite changes in pathogenic bacterium Pseudomonas aeruginosa PAO1 following exposure to quorum sensing (QS) inhibitor hordenine. Pyocyanin inhibition assay confirmed that hordenine exhibited potent QS inhibitory activity. A total of 40 metabolites were assigned by PMR spectra. Hordenine treatment resulted in the destruction of QS system in P. aeruginosa PAO1 by downregulating the expressions of genes involved in QS. The synthesis of antioxidant enzymes was repressed and the oxidative stress was enhanced due to the dysfunctional QS system of P. aeruginosa PAO1. The enhanced oxidative stress induced by the dysfunctional QS system of P. aeruginosa PAO1 altered the membrane components, enhanced membrane permeability, and disturbed energy metabolism, amino acid metabolism, and nucleotide metabolism, and would ultimately attenuate the pathogenicity of P. aeruginosa PAO1. Hordenine may have promising potential for controlling nosocomial pathogens.

Ethanol Stimulates Trehalose Production through a SpoT-DksA-AlgU-Dependent Pathway in Pseudomonas aeruginosa

Pseudomonas aeruginosa frequently resides among ethanol-producing microbes, making its response to the microbially produced concentrations of ethanol relevant to understanding its biology. Our transcriptome analysis found that genes involved in trehalose metabolism were induced by low concentrations of ethanol, and biochemical assays showed that levels of intracellular trehalose increased significantly upon growth with ethanol. The increase in trehalose was dependent on the TreYZ pathway but not other trehalose-metabolic enzymes (TreS or TreA). The sigma factor AlgU (AlgT), a homolog of RpoE in other species, was required for increased expression of the treZ gene and trehalose levels, but induction was not controlled by the well-characterized proteolysis of its anti-sigma factor, MucA. Growth with ethanol led to increased SpoT-dependent (p)ppGpp accumulation, which stimulates AlgU-dependent transcription of treZ and other AlgU-regulated genes through DksA, a (p)ppGpp and RNA polymerase binding protein. Ethanol stimulation of trehalose also required acylhomoserine lactone (AHL)-mediated quorum sensing (QS), as induction was not observed in a ΔlasR ΔrhlR strain. A network analysis using a model, eADAGE, built from publicly available P. aeruginosa transcriptome data sets (J. Tan, G. Doing, K. A. Lewis, C. E. Price, et al., Cell Syst 5:63-71, 2017, https://doi.org/10.1016/j.cels.2017.06.003) provided strong support for our model in which treZ and coregulated genes are controlled by both AlgU- and AHL-mediated QS. Consistent with (p)ppGpp- and AHL-mediated quorum-sensing regulation, ethanol, even when added at the time of culture inoculation, stimulated treZ transcript levels and trehalose production in cells from post-exponential-phase cultures but not in cells from exponential-phase cultures. These data highlight the integration of growth and cell density cues in the P. aeruginosa transcriptional response to ethanol.IMPORTANCE Pseudomonas aeruginosa is often found with bacteria and fungi that produce fermentation products, including ethanol. At concentrations similar to those produced by environmental microbes, we found that ethanol stimulated expression of trehalose-biosynthetic genes and cellular levels of trehalose, a disaccharide that protects against environmental stresses. The induction of trehalose by ethanol required the alternative sigma factor AlgU through DksA- and SpoT-dependent (p)ppGpp. Trehalose accumulation also required AHL quorum sensing and occurred only in post-exponential-phase cultures. This work highlights how cells integrate cell density and growth cues in their responses to products made by other microbes and reveals a new role for (p)ppGpp in the regulation of AlgU activity.

Exploring the Links between Nucleotide Signaling and Quorum Sensing Pathways in Regulating Bacterial Virulence

The survival of all organisms depends on implementation of appropriate phenotypic responses upon perception of relevant environmental stimuli. Sensory inputs are propagated via interconnected biochemical and/or electrical cascades mediated by diverse signaling molecules, including gases, metal cations, lipids, peptides, and nucleotides. These networks often comprise second messenger signaling systems in which a ligand (the primary messenger) binds to an extracellular receptor, thereby altering the intracellular concentration of a second messenger molecule which ultimately modulates gene expression through interaction with various effectors. The identification of intersections of these signaling pathways, such as nucleotide second messengers and quorum sensing, provides new insights into the mechanisms by which bacteria use multiple inputs to regulate cellular metabolism and phenotypes. Further investigations of the overlap between bacterial signaling pathways may yield new targets and methods to control bacterial behavior, such as biofilm formation and virulence.

Roles of Cholesteryl-α-Glucoside Transferase and Cholesteryl Glucosides in Maintenance of Helicobacter pylori Morphology, Cell Wall Integrity, and Resistance to Antibiotics

Infection of the human stomach caused by Helicobacter pylori is very common, as the pathogen colonizes more than half of the world's population. It is associated with varied outcomes of infection, such as peptic ulcer disease, gastric ulcers, and mucosa-associated lymphoid tissue lymphoma, and is generally considered a risk factor for the development of gastric adenocarcinoma. Cholesteryl glucosides (CGs) constitute a vital component of the cell wall of H. pylori and contribute to its pathogenicity and virulence. The hp0421 gene, which encodes cholesteryl-α-glucoside transferase (CGT), appears critical for the enzymatic function of integrating unique CGs into the cell wall of H. pylori, and deletion of this gene leads to depletion of CGs and their variants. Herein, we report that the deletion of hp0421 and consequent deficiency of cholesterol alter the morphology, shape, and cell wall composition of H. pylori cells, as demonstrated by high-resolution confocal microscopy and flow cytometry analyses of two different type strains of H. pylori, their isogenic knockouts as well as a reconstituted strain. Moreover, measurement of ethidium bromide (EtBr) influx by flow cytometry showed that lack of CGs increased cell wall permeability. Antimicrobial susceptibility testing revealed that the hp0421 isogenic knockout strains (Hp26695Δ421 and Hp76Δ421) were sensitive to antibiotics, such as fosfomycin, polymyxin B, colistin, tetracycline, and ciprofloxacin, in contrast to the wild-type strains that were resistant to the above antibiotics and tended to form denser biofilms. Lipid profile analysis of both Hp76 and Hp76Δ421 strains showed an aberrant profile of lipopolysaccharides (LPS) in the Hp76Δ421 strain. Taken together, we herein provide a set of mechanistic evidences to demonstrate that CGs play critical roles in the maintenance of the typical spiral morphology of H. pylori and its cell wall integrity, and any alteration in CG content affects the characteristic morphological features and renders the H. pylori susceptible to various antibiotics.IMPORTANCEHelicobacter pylori is an important cause of chronic gastritis leading to peptic ulcer and is a major risk factor for gastric malignancies. Failure in the eradication of H. pylori infection and increasing antibiotic resistance are two major problems in preventing H. pylori colonization. Hence, a deeper understanding of the bacterial survival strategies is needed to tackle the increasing burden of H. pylori infection by an appropriate intervention. Our study demonstrated that the lack of cholesteryl glucosides (CGs) remarkably altered the morphology of H. pylori and increased permeability of the bacterial cell wall. Further, this study highlighted the substantial role of CGs in maintaining the typical H. pylori morphology that is essential for retaining its pathogenic potential. We also demonstrated that the loss of CGs in H. pylori renders the bacterium susceptible to different antibiotics.

The absence of SigX results in impaired carbon metabolism and membrane fluidity in Pseudomonas aeruginosa

In Pseudomonas aeruginosa, SigX is an extra-cytoplasmic function σ factor that belongs to the cell wall stress response network. In previous studies, we made the puzzling observation that sigX mutant growth was severely affected in rich lysogeny broth (LB) but not in minimal medium. Here, through comparative transcriptomic and proteomic analysis, we show that the absence of SigX results in dysregulation of genes, whose products are mainly involved in transport, carbon and energy metabolisms. Production of most of these genes is controlled by carbon catabolite repression (CCR), a key regulatory system than ensures preferential carbon source uptake and utilization, substrate prioritization and metabolism. The strong CCR response elicited in LB was lowered in a sigX mutant, suggesting altered nutrient uptake. Since the absence of SigX affects membrane composition and fluidity, we suspected membrane changes to cause such phenotype. The detergent polysorbate 80 (PS80) can moderately destabilize the envelope resulting in non-specific increased nutrient intake. Remarkably, growth, membrane fluidity and expression of dysregulated genes in the sigX mutant strain were restored in LB supplemented with PS80. Altogether, these data suggest that SigX is indirectly involved in CCR regulation, possibly via its effects on membrane integrity and fluidity.

Inhaled nanomaterials and the respiratory microbiome: clinical, immunological and toxicological perspectives

Our development and usage of engineered nanomaterials has grown exponentially despite concerns about their unfavourable cardiorespiratory consequence, one that parallels ambient ultrafine particle exposure from vehicle emissions. Most research in the field has so far focused on airway inflammation in response to nanoparticle inhalation, however, little is known about nanoparticle-microbiome interaction in the human airway and the environment. Emerging evidence illustrates that the airway, even in its healthy state, is not sterile. The resident human airway microbiome is further altered in chronic inflammatory respiratory disease however little is known about the impact of nanoparticle inhalation on this airway microbiome. The composition of the airway microbiome, which is involved in the development and progression of respiratory disease is dynamic, adding further complexity to understanding microbiota-host interaction in the lung, particularly in the context of nanoparticle exposure. This article reviews the size-dependent properties of nanomaterials, their body deposition after inhalation and factors that influence their fate. We evaluate what is currently known about nanoparticle-microbiome interactions in the human airway and summarise the known clinical, immunological and toxicological consequences of this relationship. While associations between inhaled ambient ultrafine particles and host immune-inflammatory response are known, the airway and environmental microbiomes likely act as intermediaries and facilitate individual susceptibility to inhaled nanoparticles and toxicants. Characterising the precise interaction between the environment and airway microbiomes, inhaled nanoparticles and the host immune system is therefore critical and will provide insight into mechanisms promoting nanoparticle induced airway damage.

Mechanistic understanding of Phenyllactic acid mediated inhibition of quorum sensing and biofilm development in Pseudomonas aeruginosa

Pseudomonas aeruginosa depends on its quorum sensing (QS) system for its virulence factors' production and biofilm formation. Biofilms of P. aeruginosa on the surface of indwelling catheters are often resistant to antibiotic therapy. Alternative approaches that employ QS inhibitors alone or in combination with antibiotics are being developed to tackle P. aeruginosa infections. Here, we have studied the mechanism of action of 3-Phenyllactic acid (PLA), a QS inhibitory compound produced by Lactobacillus species, against P. aeruginosa PAO1. Our study revealed that PLA inhibited the expression of virulence factors such as pyocyanin, protease, and rhamnolipids that are involved in the biofilm formation of P. aeruginosa PAO1. Swarming motility, another important criterion for biofilm formation of P. aeruginosa PAO1, was also inhibited by PLA. Gene expression, mass spectrometric, functional complementation assays, and in silico data indicated that the quorum quenching and biofilm inhibitory activities of PLA are attributed to its ability to interact with P. aeruginosa QS receptors. PLA antagonistically binds to QS receptors RhlR and PqsR with a higher affinity than its cognate ligands N-butyryl-L-homoserine lactone (C₄-HSL) and 2-heptyl-3,4-dihydroxyquinoline (PQS; Pseudomonas quinolone signal). Using an in vivo intraperitoneal catheter-associated medaka fish infection model, we proved that PLA inhibited the initial attachment of P. aeruginosa PAO1 on implanted catheter tubes. Our in vitro and in vivo results revealed the potential of PLA as anti-biofilm compound against P. aeruginosa.

Targeting Bacterial Cardiolipin Enriched Microdomains: An Antimicrobial Strategy Used by Amphiphilic Aminoglycoside Antibiotics

Some bacterial proteins involved in cell division and oxidative phosphorylation are tightly bound to cardiolipin. Cardiolipin is a non-bilayer anionic phospholipid found in bacterial inner membrane. It forms lipid microdomains located at the cell poles and division plane. Mechanisms by which microdomains are affected by membrane-acting antibiotics and the impact of these alterations on membrane properties and protein functions remain unclear. In this study, we demonstrated cardiolipin relocation and clustering as a result of exposure to a cardiolipin-acting amphiphilic aminoglycoside antibiotic, the 3′,6-dinonyl neamine. Changes in the biophysical properties of the bacterial membrane of P. aeruginosa, including decreased fluidity and increased permeability, were observed. Cardiolipin-interacting proteins and functions regulated by cardiolipin were impacted by the amphiphilic aminoglycoside as we demonstrated an inhibition of respiratory chain and changes in bacterial shape. The latter effect was characterized by the loss of bacterial rod shape through a decrease in length and increase in curvature. It resulted from the effect on MreB, a cardiolipin dependent cytoskeleton protein as well as a direct effect of 3′,6-dinonyl neamine on cardiolipin. These results shed light on how targeting cardiolipin microdomains may be of great interest for developing new antibacterial therapies.

The Type VI Secretion System: A Dynamic System for Bacterial Communication?

Numerous studies in Gram-negative bacteria have focused on the Type VISecretion Systems (T6SSs), Quorum Sensing (QS), and social behavior, such as in biofilms. These interconnected mechanisms are important for bacterial survival; T6SSs allow bacteria to battle other cells, QS is devoted to the perception of bacterial cell density, and biofilm formation is essentially controlled by QS. Here, we review data concerning T6SS dynamics and T6SS–QS cross-talk that suggest the existence of inter-bacterial communication via T6SSs.

Studies on the Behavior of Eucalyptol and Terpinen-4-ol-Natural Food Additives and Ecological Pesticides-in Model Lipid Membranes

Effective application of the essential oils requires detailed exploration of their mechanism of action and the origin of diverse activity of their components. In this work, the influence of eucalyptol and terpinen-4-ol on artificial membranes was studied to verify whether the differences in the activity of these compounds are related to their effect on membranes. The properties of monolayers formed from structurally different lipids in the presence of terpenes were examined based on the results of the surface pressure-area measurements, penetration studies, and Brewster angle microscopy experiments. Both compounds were able to incorporate into the membrane and alter lipid/lipid interactions, making the monolayer less stable and more fluid. These effects were determined by monolayer composition (but not by its condensation per se) and the resulting rheological properties and were stronger in the presence of terpinen-4-ol. These findings confirm the hypothesis that differences in the antimicrobial potency of these terpenes are membrane-related, and membrane composition may determine their selectivity.

Molecular mechanism for sphingosine-induced Pseudomonas ceramidase expression through the transcriptional regulator SphR

Pseudomonas aeruginosa, an opportunistic, but serious multidrug-resistant pathogen, secretes a ceramidase capable of cleaving the N-acyl linkage of ceramide to generate fatty acids and sphingosine. We previously reported that the secretion of P. aeruginosa ceramidase was induced by host-derived sphingolipids, through which phospholipase C-induced hemolysis was significantly enhanced. We herein investigated the gene(s) regulating sphingolipid-induced ceramidase expression and identified SphR, which encodes a putative AraC family transcriptional regulator. Disruption of the sphR gene in P. aeruginosa markedly decreased the sphingomyelin-induced secretion of ceramidase, reduced hemolytic activity, and resulted in the loss of sphingomyelin-induced ceramidase expression. A microarray analysis confirmed that sphingomyelin significantly induced ceramidase expression in P. aeruginosa. Furthermore, an electrophoretic mobility shift assay revealed that SphR specifically bound free sphingoid bases such as sphingosine, dihydrosphingosine, and phytosphingosine, but not sphingomyelin or ceramide. A β-galactosidase-assisted promoter assay showed that sphingosine activated ceramidase expression through SphR at a concentration of 100 nM. Collectively, these results demonstrated that sphingosine induces the secretion of ceramidase by promoting the mRNA expression of ceramidase through SphR, thereby enhancing hemolytic phospholipase C-induced cytotoxicity. These results facilitate understanding of the physiological role of bacterial ceramidase in host cells.

Pseudomonas aeruginosa: targeting cell-wall metabolism for new antibacterial discovery and development

Pseudomonas aeruginosa is a leading cause of hospital-acquired infections and is resistant to most antibiotics. With therapeutic options against P. aeruginosa dwindling, and the lack of new antibiotics in advanced developmental stages, strategies for preserving the effectiveness of current antibiotics are urgently required. β-Lactam antibiotics are important agents for treating P. aeruginosa infections, thus, adjuvants that potentiate the activity of these compounds are desirable for extending their lifespan while new antibiotics - or antibiotic classes - are discovered and developed. In this review, we discuss recent research that has identified exploitable targets of cell-wall metabolism for the design and development of compounds that hinder resistance and potentiate the activity of antipseudomonal β-lactams.

EliA is required for inducing the stearyl alcohol-mediated expression of secretory proteins and production of polyester in Ralstonia sp. NT80

Addition of stearyl alcohol to the culture medium of Ralstonia sp. NT80 induced expression of a significant amount of secretory lipase. Comparative proteomic analysis of extracellular proteins from NT80 cells grown in the presence or absence of stearyl alcohol revealed that stearyl alcohol induced expression of several secretory proteins including lipase, haemolysin-coregulated protein and nucleoside diphosphate kinase. Expression of these secreted proteins was upregulated at the transcriptional level. Stearyl alcohol also induced the synthesis of polyhydroxyalkanoate. Secretory protein EliA was required for all these responses of NT80 cells to stearyl alcohol. Accordingly, the effects of stearyl alcohol were significantly reduced in the eliA deletion mutant cells of NT80 (ΔeliA). The remaining concentration of stearyl alcohol in the culture supernatant of the wild-type cells, but not that in the culture supernatant of the ΔeliA cells, clearly decreased during the course of growth. These observed phenotypes of the ΔeliA mutant were rescued by gene complementation. The results suggested that EliA is essential for these cells to respond to stearyl alcohol, and that it plays an important role in the recognition and assimilation of stearyl alcohol by NT80 cells.

The absence of the Pseudomonas aeruginosa OprF protein leads to increased biofilm formation through variation in c-di-GMP level

OprF is the major outer membrane porin in bacteria belonging to the Pseudomonas genus. In previous studies, we have shown that OprF is required for full virulence expression of the opportunistic pathogen Pseudomonas aeruginosa. Here, we describe molecular insights on the nature of this relationship and report that the absence of OprF leads to increased biofilm formation and production of the Pel exopolysaccharide. Accordingly, the level of c-di-GMP, a key second messenger in biofilm control, is elevated in an oprF mutant. By decreasing c-di-GMP levels in this mutant, both biofilm formation and pel gene expression phenotypes were restored to wild-type levels. We further investigated the impact on two small RNAs, which are associated with the biofilm lifestyle, and found that expression of rsmZ but not of rsmY was increased in the oprF mutant and this occurs in a c-di-GMP-dependent manner. Finally, the extracytoplasmic function (ECF) sigma factors AlgU and SigX displayed higher activity levels in the oprF mutant. Two genes of the SigX regulon involved in c-di-GMP metabolism, PA1181 and adcA (PA4843), were up-regulated in the oprF mutant, partly explaining the increased c-di-GMP level. We hypothesized that the absence of OprF leads to a cell envelope stress that activates SigX and results in a c-di-GMP elevated level due to higher expression of adcA and PA1181. The c-di-GMP level can in turn stimulate Pel synthesis via increased rsmZ sRNA levels and pel mRNA, thus affecting Pel-dependent phenotypes such as cell aggregation and biofilm formation. This work highlights the connection between OprF and c-di-GMP regulatory networks, likely via SigX (ECF), on the regulation of biofilm phenotypes.

Deciphering the role of coumarin as a novel quorum sensing inhibitor suppressing virulence phenotypes in bacterial pathogens

The rapid unchecked rise in antibiotic resistance over the last few decades has led to an increased focus on the need for alternative therapeutic strategies for the treatment and clinical management of microbial infections. In particular, small molecules that can suppress microbial virulence systems independent of any impact on growth are receiving increased attention. Quorum sensing (QS) is a cell-to-cell signalling communication system that controls the virulence behaviour of a broad spectrum of bacterial pathogens. QS systems have been proposed as an effective target, particularly as they control biofilm formation in pathogens, a key driver of antibiotic ineffectiveness. In this study, we identified coumarin, a natural plant phenolic compound, as a novel QS inhibitor, with potent anti-virulence activity in a broad spectrum of pathogens. Using a range of biosensor systems, coumarin was active against short, medium and long chain N-acyl-homoserine lactones, independent of any effect on growth. To determine if this suppression was linked to anti-virulence activity, key virulence systems were studied in the nosocomial pathogen Pseudomonas aeruginosa. Consistent with suppression of QS, coumarin inhibited biofilm, the production of phenazines and swarming motility in this organism potentially linked to reduced expression of the rhlI and pqsA quorum sensing genes. Furthermore, coumarin significantly inhibited biofilm formation and protease activity in other bacterial pathogens and inhibited bioluminescence in Aliivibrio fischeri. In light of these findings, coumarin would appear to have potential as a novel quorum sensing inhibitor with a broad spectrum of action.

The hierarchy quorum sensing network in Pseudomonas aeruginosa

Pseudomonas aeruginosa causes severe and persistent infections in immune compromised individuals and cystic fibrosis sufferers. The infection is hard to eradicate as P. aeruginosa has developed strong resistance to most conventional antibiotics. The problem is further compounded by the ability of the pathogen to form biofilm matrix, which provides bacterial cells a protected environment withstanding various stresses including antibiotics. Quorum sensing (QS), a cell density-based intercellular communication system, which plays a key role in regulation of the bacterial virulence and biofilm formation, could be a promising target for developing new strategies against P. aeruginosa infection. The QS network of P. aeruginosa is organized in a multi-layered hierarchy consisting of at least four interconnected signaling mechanisms. Evidence is accumulating that the QS regulatory network not only responds to bacterial population changes but also could react to environmental stress cues. This plasticity should be taken into consideration during exploration and development of anti-QS therapeutics.

Global regulator Anr represses PlcH phospholipase activity in Pseudomonas aeruginosa when oxygen is limiting

Haemolytic phospholipase C (PlcH) is a potent virulence and colonization factor that is expressed at high levels by Pseudomonas aeruginosa within the mammalian host. The phosphorylcholine liberated from phosphatidylcholine and sphingomyelin by PlcH is further catabolized into molecules that both support growth and further induce plcH expression. We have shown previously that the catabolism of PlcH-released choline leads to increased activity of Anr, a global transcriptional regulator that promotes biofilm formation and virulence. Here, we demonstrated the presence of a negative feedback loop in which Anr repressed plcH transcription and we proposed that this regulation allowed for PlcH levels to be maintained in a way that promotes productive host-pathogen interactions. Evidence for Anr-mediated regulation of PlcH came from data showing that growth at low oxygen (1%) repressed PlcH abundance and plcH transcription in the WT, and that plcH transcription was enhanced in an Δanr mutant. The plcH promoter featured an Anr consensus sequence that was conserved across all P. aeruginosa genomes and mutation of conserved nucleotides within the Anr consensus sequence increased plcH expression under hypoxic conditions. The Anr-regulated transcription factor Dnr was not required for this effect. The loss of Anr was not sufficient to completely derepress plcH transcription as GbdR, a positive regulator of plcH, was required for expression. Overexpression of Anr was sufficient to repress plcH transcription even at 21 % oxygen. Anr repressed plcH expression and phospholipase C activity in a cell culture model for P. aeruginosa-epithelial cell interactions.

Gluconacetobacter diazotrophicus PAL5 possesses an active quorum sensing regulatory system

The endophytic bacterium Gluconacetobacter diazotrophicus colonizes a broad range of host plants. Its plant growth-promoting capability is related to the capacity to perform biological nitrogen fixation, the biosynthesis of siderophores, antimicrobial substances and the solubilization of mineral nutrients. Colonization of and survival in these endophytic niche requires a complex regulatory network. Among these, quorum sensing systems (QS) are signaling mechanisms involved in the control of several genes related to microbial interactions, host colonization and stress survival. G. diazotrophicus PAL5 possesses a QS composed of a luxR and a luxI homolog, and produces eight molecules from the AHL family as QS signals. In this report data are provided showing that glucose concentration modifies the relative levels of these signal molecules. The activity of G. diazotrophicus PAL5 QS is also altered in presence of other carbon sources and under saline stress conditions. Inactivation of the QS system of G. diazotrophicus PAL5 by means of a quorum quenching strategy allowed the identification of extracellular and intracellular proteins under the control of this regulatory mechanism.

A novel role for an ECF sigma factor in fatty acid biosynthesis and membrane fluidity in Pseudomonas aeruginosa

Extracytoplasmic function (ECF) sigma factors are members of cell-surface signaling systems, abundant in the opportunistic pathogen Pseudomonas aeruginosa. Twenty genes coding for ECF sigma factors are present in P. aeruginosa sequenced genomes, most of them being part of TonB systems related to iron uptake. In this work, poorly characterized sigma factors were overexpressed in strain PA14, in an attempt to understand their role in the bacterium's physiology. Cultures overexpressing SigX displayed a biphasic growth curve, reaching stationary phase earlier than the control strain, followed by subsequent growth resumption. During the first stationary phase, most cells swell and die, but the remaining cells return to the wild type morphology and proceed to a second exponential growth. This is not due to compensatory mutations, since cells recovered from late time points and diluted into fresh medium repeated this behavior. Swollen cells have a more fluid membrane and contain higher amounts of shorter chain fatty acids. A proteomic analysis was performed to identify differentially expressed proteins due to overexpression of sigX, revealing the induction of several fatty acid synthesis (FAS) enzymes. Using qRT-PCR, we showed that at least one isoform from each of the FAS pathway enzymes were upregulated at the mRNA level in the SigX overexpressing strain thus pointing to a role for this ECF sigma factor in the FAS regulation in P. aeruginosa.

Pleiotropic effects of acyltransferases on various virulence-related phenotypes of Pseudomonas aeruginosa

Pseudomonas aeruginosa, an opportunistic pathogen causing various infections, expresses various virulence factors under the control of quorum sensing (QS), a cell density-sensing mechanism. Because the major signal molecules of QS are acyl homoserine lactones (acyl-HSLs), acyltransferases, the enzymes that act upon acyl group transfer could affect the QS signaling and QS-related virulence phenotypes. In this study, we overexpressed acyltransferases of P. aeruginosa and screened them for the activity influencing the QS and QS-related virulence phenotypes. Among seven acyltransferases tested in this study, two acyltransferases, PA3984 (apolipoprotein N-acyltransferase) and PA2537 (putative acyltransferase), significantly affected both growth of P. aeruginosa and the activity of LasR, a major QS regulator, when overexpressed. These acyltransferases also reduced virulence and swarming motility of P. aeruginosa. The other acyltransferase, PA3646 (UDP-3-O-[3-hydroxylauroyl] glucosamine N-acyltransferase), reduced the LasR activity, swarming motility, protease production and virulence without any influence on growth. These effects by PA3646 over-expression were caused by less production of QS signal. PA3644 (UDP-N-acetylglucosamine acyltransferase) enhanced biofilm formation and swarming motility with no effect on the growth and QS activity. These results suggest that acyltransferases may be an important factor regulating the cellular activity about virulence-related phenotypes.

From biocontrol to cancer, probiotics and beyond

This invited commentary covers the period 1997-2012 and has seen changes in terminology that progressed from "basic" and "applied" to "translational" research. In the context of Bioengineered, these changes map readily onto the processes of identifying microbial characteristics appropriate for specific applications, isolation of suitable cultures, strain or genome manipulation and exploitation of these or their metabolomes across a range of settings.   To a great degree, this commentary and my career reflect an engagement with molecular microbiology and the trialling of bacteria and derived constructs in applications ranging from intensive-scale crop protection to amelioration of gastrointestinal disease. This engagement began with laboratory and field evaluations of biocontrol, specifically use of pseudomonads effective against nematode and fungal plant pathogens, characterization of mechanisms mediating beneficial effects of probiotic lactobacilli and bifidobacteria and assessment of functional foods in multinational clinical trials relating to inflammatory bowel disease. Subsequent work focused on (1) intellectual property (IP)-based medical devices for localized delivery of systemically toxic and gene cancer therapies; (2) growth of the science base supporting expansion of a multinational business including company acquisitions; (3) complementing existing inter-institutional research capabilities through development of a national industry-led collaboration; and, most recently, (4) strategic research programs at Ireland's newest medical school. My activities as outlined above parallel two distinct aspects of translational research: (1) involvement in knowledge-driven (commercial and research) organizations that brought together necessary resources and infrastructure and (2) availability of scale research funding from European Framework and Irish national programs.

Respiratory pathogens adopt a chronic lifestyle in response to bile

Chronic respiratory infections are a major cause of morbidity and mortality, most particularly in Cystic Fibrosis (CF) patients. The recent finding that gastro-esophageal reflux (GER) frequently occurs in CF patients led us to investigate the impact of bile on the behaviour of Pseudomonas aeruginosa and other CF-associated respiratory pathogens. Bile increased biofilm formation, Type Six Secretion, and quorum sensing in P. aeruginosa, all of which are associated with the switch from acute to persistent infection. Furthermore, bile negatively influenced Type Three Secretion and swarming motility in P. aeruginosa, phenotypes associated with acute infection. Bile also modulated biofilm formation in a range of other CF-associated respiratory pathogens, including Burkholderia cepacia and Staphylococcus aureus. Therefore, our results suggest that GER-derived bile may be a host determinant contributing to chronic respiratory infection.

Determinants of intrinsic aminoglycoside resistance in Pseudomonas aeruginosa

Screening of a transposon insertion mutant library of Pseudomonas aeruginosa for increased susceptibility to paromomycin identified a number of genes whose disruption enhanced susceptibility of this organism to multiple aminoglycosides, including tobramycin, amikacin, and gentamicin. These included genes associated with lipid biosynthesis or metabolism (lptA, faoA), phosphate uptake (pstB), and two-component regulators (amgRS, PA2797-PA2798) and a gene of unknown function (PA0392). Deletion mutants lacking these showed enhanced panaminoglycoside susceptibility that was reversed by the cloned genes, confirming their contribution to intrinsic panaminoglycoside resistance. None of these mutants showed increased aminoglycoside permeation of the cell envelope, indicating that increased susceptibility was not related to enhanced aminoglycoside uptake owing to a reduced envelope barrier function. Several mutants (pstB, faoA, PA0392, amgR) did, however, show increased cytoplasmic membrane depolarization relative to wild type following gentamicin exposure, consistent with the membranes of these mutants being more prone to perturbation, likely by gentamicin-generated mistranslated polypeptides. Mutants lacking any two of these resistance genes in various combinations invariably showed increased aminoglycoside susceptibility relative to single-deletion mutants, confirming their independent contribution to resistance and highlighting the complexity of the intrinsic aminoglycoside resistome in P. aeruginosa. Deletion of these genes also compromised the high-level panaminoglycoside resistance of clinical isolates, emphasizing their important contribution to acquired resistance.

Heterologous expression of a plant RelA-SpoT homologue results in increased stress tolerance in Saccharomyces cerevisiae by accumulation of the bacterial alarmone ppGpp

The bacterial alarmone ppGpp is present only in bacteria and the chloroplasts of plants, but not in mammalian cells or eukaryotic micro-organisms such as yeasts and fungi. The importance of the ppGpp signalling system in eukaryotes has therefore been largely overlooked. Here, we demonstrated that heterologous expression of a relA-spoT homologue (Sj-RSH) isolated from the halophilic plant Suaeda japonica in the yeast Saccharomyces cerevisiae results in accumulation of ppGpp, accompanied by enhancement of tolerance against various stress stimuli, such as osmotic stress, ethanol, hydrogen peroxide, high temperature and freezing. Unlike bacterial ppGpp accumulation, ppGpp was accumulated in the early growth phase but not in the late growth phase. Moreover, nutritional downshift resulted in a decrease in ppGpp level, suggesting that the observed Sj-RSH activity to synthesize ppGpp is not starvation-dependent, contrary to our expectations based on bacteria. Accumulated ppGpp was found to be present solely in the cytosolic fraction and not in the mitochondrial fraction, perhaps reflecting the ribosome-independent ppGpp synthesis in S. cerevisiae cells. Unlike bacterial inosine monophosphate (IMP) dehydrogenases, the IMP dehydrogenase of S. cerevisiae was insensitive to ppGpp. Microarray analysis showed that ppGpp accumulation gave rise to marked changes in gene expression, with both upregulation and downregulation, including changes in mitochondrial gene expression. The most prominent upregulation (38-fold) was detected in the hypothetical gene YBR072C-A of unknown function, followed by many other known stress-responsive genes. S. cerevisiae may provide new opportunities to uncover and analyse the ppGpp signalling system in eukaryotic cells.

Absence of membrane phosphatidylcholine does not affect virulence and stress tolerance phenotypes in the opportunistic pathogen Pseudomonas aeruginosa

During growth in presence of choline, both laboratory and clinical Pseudomonas aeruginosa strains synthesize phosphatidylcholine (PC), and PC makes up ∼4% of the total membrane phospholipid content. In all the strains tested, PC synthesis occurred only when choline is provided exogenously. Mutants defective in synthesis of PC were generated in the strain backgrounds PAO1 and PA14. Minimum inhibitory concentration studies testing sensitivity of PC-deficient strains towards various antibiotics and cationic antimicrobial peptides revealed no differences as compared to wild-type strains. Mutants incapable of synthesizing PC were also found to be unaffected in motility and biofilm formation on abiotic surfaces, colonization of biotic surfaces and virulence in a mouse infection model. A global phenotypic microarray was further used to identify conditions wherein membrane PC may play a role of in P. aeruginosa. No culture conditions were identified wherein wild-type and PC-deficient mutants showed phenotypic differences. Membrane PC may serve a highly specific role during P. aeruginosa interactions with its eukaryotic hosts based on all the clinical strains tested retaining the ability to synthesize it during availability of choline.

Multifunctional membrane vesicles in Pseudomonas aeruginosa

Gram-negative bacteria secrete small particles called membrane vesicles (MVs) into the extracellular milieu. While MVs have important roles in delivering toxins from pathogenic bacteria to eukaryotic cells, these vesicles also play ecological roles necessary for survival in various environmental conditions. Pseudomonas aeruginosa, which lives in soil, ocean, plant, animal and human environments, has become a model organism for studying these small extracellular particles. Such studies have increased our understanding of the function and biogenesis of bacterial MVs. Pseudomonas aeruginosa MVs possess versatile components and chemical substances with unique structures. These characteristics allow MVs to play their multifunctional biological roles, including microbial interaction, maintenance of biofilm structure and host infection. This review summarizes the comprehensive biochemical and physiochemical properties of MVs derived from P. aeruginosa. These studies will help us understand their biological roles of MVs not only in pathogenicity but also in microbial ecology. Also, the mechanisms of MV production, as currently understood, are discussed.

Quorum sensing: regulating the regulators

Many bacteria use 'quorum sensing' (QS) as a mechanism to regulate gene induction in a population-dependent manner. In its simplest sense this involves the accumulation of a signaling metabolite during growth; the binding of this metabolite to a regulator or multiple regulators activates induction or repression of gene expression. However QS regulation is seldom this simple, because other inputs are usually involved. In this review we have focussed on how those other inputs influence QS regulation and as implied by the title, this often occurs by environmental or physiological effects regulating the expression or activity of the QS regulators. The rationale of this review is to briefly introduce the main QS signals used in Gram-negative bacteria and then introduce one of the earliest understood mechanisms of regulation of the regulator, namely the plant-mediated control of expression of the TraR QS regulator in Agrobacterium tumefaciens. We then describe how in several species, multiple QS regulatory systems can act as integrated hierarchical regulatory networks and usually this involves the regulation of QS regulators. Such networks can be influenced by many different physiological and environmental inputs and we describe diverse examples of these. In the final section, we describe different examples of how eukaryotes can influence QS regulation in Gram-negative bacteria.

A novel two-component system BqsS-BqsR modulates quorum sensing-dependent biofilm decay in Pseudomonas aeruginosa

Pseudomonas aeruginosa can grow either as planktonic- or biofilm-form in response to environmental changes. Recent studies show that switching from biofilm to planktonic lifestyle requires rhamnolipids. Here we report the identification of a novel twocomponent system BqsS-BqsR that regulates biofilm decay in P. aeruginosa. BqsS is a multidomain sensor kinase and BqsR is an OmpR-like response regulator. Deletion of either bqsS or bqsR in P. aeruginosa mPAO1 resulted in a significant increase in biofilm formation. Time course analysis showed that the bqsS-bqsR mutants were defective in biofilm dispersal and in rhamnolipid production. Mutation of the BqsS-BqsR two-component system did not affect the biosynthesis of long chain quorum sensing (QS) signal N-3-oxo-dodecanoyl-homoserine lactone (3OC12HSL) but resulted in reduced production of the short chain QS signal N-butyryl-L-homoserine lactone (C4HSL) and the Pseudomonas quinolone signal (PQS). Exogenous addition of C4HSL, PQS or rhamnolipids to the bqsS mutant reduced the biofilm formation to the wild-type level. Evidence suggests that the BqsS-BqsR two-component system might promote conversion of anthranilate to PQS. Taken together, these results establish BqsS-BqsR as a novel two-component system that regulates biofilm decay in P. aeruginosa by modulating biosynthesis of QS signals and rhamnolipids.