Pseudomonas aeruginosa: new insights into pathogenesis and host defenses

Synergy of silver nanoparticles and aztreonam against Pseudomonas aeruginosa PAO1 biofilms

Pathogenic bacterial biofilms, such as those found in the lungs of patients with cystic fibrosis (CF), exhibit increased antimicrobial resistance, due in part to the inherent architecture of the biofilm community. The protection provided by the biofilm limits antimicrobial dispersion and penetration and reduces the efficacy of antibiotics that normally inhibit planktonic cell growth. Thus, alternative antimicrobial strategies are required to combat persistent infections. The antimicrobial properties of silver have been known for decades, but silver and silver-containing compounds have recently seen renewed interest as antimicrobial agents for treating bacterial infections. The goal of this study was to assess the efficacy of citrate-capped silver nanoparticles (AgNPs) of various sizes, alone and in combination with the monobactam antibiotic aztreonam, to inhibit Pseudomonas aeruginosa PAO1 biofilms. Among the different sizes of AgNPs examined, 10-nm nanoparticles were most effective in inhibiting the recovery of P. aeruginosa biofilm cultures and showed synergy of inhibition when combined with sub-MIC levels of aztreonam. Visualization of biofilms treated with combinations of 10-nm AgNPs and aztreonam indicated that the synergistic bactericidal effects are likely caused by better penetration of the small AgNPs into the biofilm matrix, which enhances the deleterious effects of aztreonam against the cell envelope of P. aeruginosa within the biofilms. These data suggest that small AgNPs synergistically enhance the antimicrobial effects of aztreonam against P. aeruginosa in vitro, and they reveal a potential role for combinations of small AgNPs and antibiotics in treating patients with chronic infections.

Burn Patients Infected With Metallo-Beta-Lactamase-Producing Pseudomonas aeruginosa: Multidrug-Resistant Strains

BACKGROUND

Metallo-beta-lactamase (MBL) producing Pseudomonas aeruginosa in the burn patients is a leading cause of morbidity and mortality and remains a serious health concern among the clinicians.

OBJECTIVES

The aim of this study was to detect MBL-producing P. aeruginosa in burn patients and determine multidrug-resistant (MDR) strains, and respective resistance patterns.

PATIENTS AND METHODS

In this cross-sectional study, 270 strains of P. aeruginosa were isolated from the burn patients referred to Ghotbeddin Burn Hospital, Shiraz, Iran. Among them, 55 MBL-producing P. aeruginosa strains were isolated from 55 patients hospitalized in burn unit. Minimum inhibitory concentrations (MICs) and MBLs were determined by the E-test method.

RESULTS

Of the 55 burn cases, 29 (53%) were females and 26 (47%) males. Injured burn patients' ages ranged from 16 to 87 years, with maximum number of cases in the age group of 16 to 36 years (n, 40; 72.7%). Overall, 32 cases were accidental (60%), and 22 were suicidal burns (40%). Of the 55 burn patients, 17 cases were expired (30%). All deaths were due to chemical exposures. In antibiotic susceptibility testing by E-test method, ceftazidime was the most effective one and 35 isolates (63.5%) were resistant to all the 11 tested antibiotics.

CONCLUSIONS

Routine microbiological surveillance and careful in vitro testing of antibiotics prior to prescription and strict adherence to hospital antibiotic policy may help to prevent, treat, and control MDR and pandrug-resistant (PDR) P. aeruginosa strains in burn units.

Mimicking the host and its microenvironment in vitro for studying mucosal infections by Pseudomonas aeruginosa

Why is a healthy person protected from Pseudomonas aeruginosa infections, while individuals with cystic fibrosis or damaged epithelium are particularly susceptible to this opportunistic pathogen? To address this question, it is essential to thoroughly understand the dynamic interplay between the host microenvironment and P. aeruginosa. Therefore, using model systems that represent key aspects of human mucosal tissues in health and disease allows recreating in vivo host-pathogen interactions in a physiologically relevant manner. In this review, we discuss how factors of mucosal tissues, such as apical-basolateral polarity, junctional complexes, extracellular matrix proteins, mucus, multicellular complexity (including indigenous microbiota), and other physicochemical factors affect P. aeruginosa pathogenesis and are thus important to mimic in vitro. We highlight in vitro cell and tissue culture model systems of increasing complexity that have been used over the past 35 years to study the infectious disease process of P. aeruginosa, mainly focusing on lung models, and their respective advantages and limitations. Continued improvements of in vitro models based on our expanding knowledge of host microenvironmental factors that participate in P. aeruginosa pathogenesis will help advance fundamental understanding of pathogenic mechanisms and increase the translational potential of research findings from bench to the patient's bedside.

NTBC treatment of the pyomelanogenic Pseudomonas aeruginosa clinical isolate PA1111 inhibits pigment production and increases sensitivity to oxidative stress

Pyomelanin is a brown/black extracellular pigment with antioxidant and iron acquisition properties that is produced by a number of different bacteria. Production of pyomelanin in Pseudomonas aeruginosa contributes to increased resistance to oxidative stress and persistence in chronic infections. We demonstrate that pyomelanin production can be inhibited by 2-[2-nitro-4-(trifluoromethyl) benzoyl]-1,3-cyclohexanedione (NTBC). This treatment increases sensitivity of pyomelanogenic P. aeruginosa strains to oxidative stress, without altering the growth rate or resistance to aminoglycosides. As such, NTBC has potential to function as an anti-virulence factor in treating pyomelanogenic bacterial infections.

The C. elegans healthspan and stress-resistance assay toolkit

A wealth of knowledge on the genetic mechanisms that govern aging has emerged from the study of mutants that exhibit enhanced longevity and exceptional resilience to adverse environmental conditions. In these studies, lifespan has been an excellent proxy for establishing the rate of aging, but it is not always correlated with qualitative measures of healthy aging or 'healthspan'. Although the attributes of healthspan have been challenging to define, they share some universal features that are increasingly being incorporated into aging studies. Here we describe methods used to determine Caenorhabditis elegans healthspan. These include assessments of tissue integrity and functionality and resistance to a variety of biotic and abiotic stressors. We have chosen to include simple, rapid assays in this collection that can be easily undertaken in any C. elegans laboratory, and can be relied on to provide a preliminary but thorough insight into the healthspan of a population.

Importance of positioning for microbial evolution

Microbes commonly live in dense surface-attached communities where cells layer on top of one another such that only those at the edges have unimpeded access to limiting nutrients and space. Theory predicts that this simple spatial effect, akin to plants competing for light in a forest, generates strong natural selection on microbial phenotypes. However, we require direct empirical tests of the importance of this spatial structuring. Here we show that spontaneous mutants repeatedly arise, push their way to the surface, and dominate colonies of the bacterium Pseudomonas fluorescens Pf0-1. Microscopy and modeling suggests that these mutants use secretions to expand and push themselves up to the growth surface to gain the best access to oxygen. Physically mixing the cells in the colony, or introducing space limitations, largely removes the mutant's advantage, showing a key link between fitness and the ability of the cells to position themselves in the colony. We next follow over 500 independent adaptation events and show that all occur through mutation of a single repressor of secretions, RsmE, but that the mutants differ in competitiveness. This process allows us to map the genetic basis of their adaptation at high molecular resolution and we show how evolutionary competitiveness is explained by the specific effects of each mutation. By combining population level and molecular analyses, we demonstrate how living in dense microbial communities can generate strong natural selection to reach the growing edge.

CRISPR-Cas systems: new players in gene regulation and bacterial physiology

CRISPR-Cas systems are bacterial defenses against foreign nucleic acids derived from bacteriophages, plasmids or other sources. These systems are targeted in an RNA-dependent, sequence-specific manner, and are also adaptive, providing protection against previously encountered foreign elements. In addition to their canonical function in defense against foreign nucleic acid, their roles in various aspects of bacterial physiology are now being uncovered. We recently revealed a role for a Cas9-based Type II CRISPR-Cas system in the control of endogenous gene expression, a novel form of prokaryotic gene regulation. Cas9 functions in association with two small RNAs to target and alter the stability of an endogenous transcript encoding a bacterial lipoprotein (BLP). Since BLPs are recognized by the host innate immune protein Toll-like Receptor 2 (TLR2), CRISPR-Cas-mediated repression of BLP expression facilitates evasion of TLR2 by the intracellular bacterial pathogen Francisella novicida, and is essential for its virulence. Here we describe the Cas9 regulatory system in detail, as well as data on its role in controlling virulence traits of Neisseria meningitidis and Campylobacter jejuni. We also discuss potential roles of CRISPR-Cas systems in the response to envelope stress and other aspects of bacterial physiology. Since ~45% of bacteria and ~83% of Archaea encode these machineries, the newly appreciated regulatory functions of CRISPR-Cas systems are likely to play broad roles in controlling the pathogenesis and physiology of diverse prokaryotes.

Draft Genome Sequence of an Invasive Multidrug-Resistant Strain, Pseudomonas aeruginosa BK1, Isolated from a Keratitis Patient

Pseudomonas aeruginosa infections are difficult to treat due to the presence of a multitude of virulence factors and antibiotic resistance. Here, we report the draft genome sequence of P. aeruginosa BK1, an invasive and multidrug-resistant strain, isolated from a bacterial keratitis patient in southern India.

Influence of O polysaccharides on biofilm development and outer membrane vesicle biogenesis in Pseudomonas aeruginosa PAO1

Pseudomonas aeruginosa is a common opportunistic human pathogen known for its ability to adapt to changes in its environment during the course of infection. These adaptations include changes in the expression of cell surface lipopolysaccharide (LPS), biofilm development, and the production of a protective extracellular exopolysaccharide matrix. Outer membrane vesicles (OMVs) have been identified as an important component of the extracellular matrix of P. aeruginosa biofilms and are thought to contribute to the development and fitness of these bacterial communities. The goal of this study was to examine the relationships between changes in the cell surface expression of LPS O polysaccharides, biofilm development, and OMV biogenesis in P. aeruginosa. We compared wild-type P. aeruginosa PAO1 with three chromosomal knockouts. These knockouts have deletions in the rmd, wbpM, and wbpL genes that produce changes in the expression of common polysaccharide antigen (CPA), O-specific antigen (OSA), or both. Our results demonstrate that changes in O polysaccharide expression do not significantly influence OMV production but do affect the size and protein content of OMVs derived from both CPA(-) and OSA(-) cells; these mutant cells also exhibited different physical properties from wild-type cells. We further examined biofilm growth of the mutants and determined that CPA(-) cells could not develop into robust biofilms and exhibit changes in cell morphology and biofilm matrix production. Together these results demonstrate the importance of O polysaccharide expression on P. aeruginosa OMV composition and highlight the significance of CPA expression in biofilm development.

2,3-dihydroxybenzoic acid-containing nanofiber wound dressings inhibit biofilm formation by Pseudomonas aeruginosa

Pseudomonas aeruginosa forms biofilms in wounds, which often leads to chronic infections that are difficult to treat with antibiotics. Free iron enhances biofilm formation, delays wound healing, and may even be responsible for persistent inflammation, increased connective tissue destruction, and lipid peroxidation. Exposure of P. aeruginosa Xen 5 to the iron chelator 2,3-dihydroxybenzoic acid (DHBA), electrospun into a nanofiber blend of poly(d,l-lactide) (PDLLA) and poly(ethylene oxide) (PEO), referred to as DF, for 8 h decreased biofilm formation by approximately 75%. This was shown by a drastic decline in cell numbers, from 7.1 log10 CFU/ml to 4.8 log10 CFU/ml when biofilms were exposed to DF in the presence of 2.0 mM FeCl3 6H2O. A similar decline in cell numbers was recorded in the presence of 3.0 mM FeCl3 6H2O and DF. The cells were more mobile in the presence of DHBA, supporting the observation of less biofilm formation at lower iron concentrations. DHBA at MIC levels (1.5 mg/ml) inhibited the growth of strain Xen 5 for at least 24 h. Our findings indicate that DHBA electrospun into nanofibers inhibits cell growth for at least 4 h, which is equivalent to the time required for all DHBA to diffuse from DF. This is the first indication that DF can be developed into a wound dressing to treat topical infections caused by P. aeruginosa.

Preventing bacterial colonization using colloidal crystals

A monolayer of polystyrene particles inhibits colony formation of Pseudomonas aeruginosa, and causes adsorption in particular locations.

Draft genomes of 12 host-adapted and environmental isolates of Pseudomonas aeruginosa and their positions in the core genome phylogeny

Pseudomonas aeruginosa is a Gram-negative opportunistic pathogen particularly associated with the inherited disease cystic fibrosis (CF). Pseudomonas aeruginosa is well known to have a large and adaptable genome that enables it to colonise a wide range of ecological niches. Here, we have used a comparative genomics approach to identify changes that occur during infection of the CF lung. We used the mucoid phenotype as an obvious marker of host adaptation and compared these genomes to analyse SNPs, indels and islands within near-isogenic pairs. To commence the correction of the natural bias towards clinical isolates in genomics studies and to widen our understanding of the genomic diversity of P. aeruginosa, we included four environmental isolates in our analysis. Our data suggest that genome plasticity plays an important role in chronic infection and that the strains sequenced in this study are representative of the two major phylogenetic groups as determined by core genome SNP analysis.

Human host defense peptide LL-37 stimulates virulence factor production and adaptive resistance in Pseudomonas aeruginosa

A multitude of different virulence factors as well as the ability to rapidly adapt to adverse environmental conditions are important features for the high pathogenicity of Pseudomonas aeruginosa. Both virulence and adaptive resistance are tightly controlled by a complex regulatory network and respond to external stimuli, such as host signals or antibiotic stress, in a highly specific manner. Here, we demonstrate that physiological concentrations of the human host defense peptide LL-37 promote virulence factor production as well as an adaptive resistance against fluoroquinolone and aminoglycoside antibiotics in P. aeruginosa PAO1. Microarray analyses of P. aeruginosa cells exposed to LL-37 revealed an upregulation of gene clusters involved in the production of quorum sensing molecules and secreted virulence factors (PQS, phenazine, hydrogen cyanide (HCN), elastase and rhamnolipids) and in lipopolysaccharide (LPS) modification as well as an induction of genes encoding multidrug efflux pumps MexCD-OprJ and MexGHI-OpmD. Accordingly, we detected significantly elevated levels of toxic metabolites and proteases in bacterial supernatants after LL-37 treatment. Pre-incubation of bacteria with LL-37 for 2 h led to a decreased susceptibility towards gentamicin and ciprofloxacin. Quantitative Realtime PCR results using a PAO1-pqsE mutant strain present evidence that the quinolone response protein and virulence regulator PqsE may be implicated in the regulation of the observed phenotype in response to LL-37. Further experiments with synthetic cationic antimicrobial peptides IDR-1018, 1037 and HHC-36 showed no induction of pqsE expression, suggesting a new role of PqsE as highly specific host stress sensor.

Spectroscopic identification and anti-biofilm properties of polar metabolites from the medicinal plant Helichrysum italicum against Pseudomonas aeruginosa

Two new acylated styrylpyrones, one 5-methoxy-1(3H)-isobenzofuranone glucoside and a hydroxymethyl-orcinol derivative, along with sixteen known aromatic metabolites, including lignans, quinic acid derivatives low-molecular weight phenol glucosides, have been isolated from the methanol extract of Helichrysum italicum, a medicinal plant typical of the Mediterranean vegetation. The structures of these compounds have been elucidated on the basis of extensive 2D-NMR spectroscopic analyses, including COSY, TOCSY, HSQC, CIGAR-HMBC, H2BC and HSQC-TOCSY, along with Q-TOF HRMS(2) analysis. Selected compounds were evaluated for their anti-biofilm properties against Pseudomonas aeruginosa.

Small molecule screen yields inhibitors of Pseudomonas homoserine lactone-induced host responses

Pseudomonas aeruginosa infections are commonly associated with cystic fibrosis, pneumonias, neutropenia and burns. The P. aeruginosa quorum sensing molecule N-(3-oxo-dodecanoyl) homoserine lactone (C12) cause multiple deleterious host responses, including repression of NF-κB transcriptional activity and apoptosis. Inhibition of C12-mediated host responses is predicted to reduce P. aeruginosa virulence. We report here a novel, host-targeted approach for potential adjunctive anti-Pseudomonal therapy based on inhibition of C12-mediated host responses. A high-throughput screen was developed to identify C12 inhibitors that restore NF-κB activity in C12-treated, lipopolysaccharide (LPS)-stimulated cells. Triazolo[4,3-a]quinolines with nanomolar potency were identified as C12-inhibitors that restore NF-κB-dependent luciferase expression in LPS- and TNF-stimulated cell lines. In primary macrophages and fibroblasts, triazolo[4,3-a]quinolines inhibited C12 action to restore cytokine secretion in LPS-stimulated cells. Serendipitously, in the absence of an inflammatory stimulus, triazolo[4,3-a]quinolines prevented C12-mediated responses, including cytotoxicity, elevation of cytoplasmic calcium, and p38 MAPK phosphorylation. In vivo efficacy was demonstrated in a murine model of dermal inflammation involving intradermalC12 administration. The discovery of triazolo[4,3-a]quinolines provides a pharmacological tool to investigate C12-mediated host responses, and a potential host-targeted anti-Pseudomonal therapy.

Promise for plant pest control: root-associated pseudomonads with insecticidal activities

Insects are an important and probably the most challenging pest to control in agriculture, in particular when they feed on belowground parts of plants. The application of synthetic pesticides is problematic owing to side effects on the environment, concerns for public health and the rapid development of resistance. Entomopathogenic bacteria, notably Bacillus thuringiensis and Photorhabdus/Xenorhabdus species, are promising alternatives to chemical insecticides, for they are able to efficiently kill insects and are considered to be environmentally sound and harmless to mammals. However, they have the handicap of showing limited environmental persistence or of depending on a nematode vector for insect infection. Intriguingly, certain strains of plant root-colonizing Pseudomonas bacteria display insect pathogenicity and thus could be formulated to extend the present range of bioinsecticides for protection of plants against root-feeding insects. These entomopathogenic pseudomonads belong to a group of plant-beneficial rhizobacteria that have the remarkable ability to suppress soil-borne plant pathogens, promote plant growth, and induce systemic plant defenses. Here we review for the first time the current knowledge about the occurrence and the molecular basis of insecticidal activity in pseudomonads with an emphasis on plant-beneficial and prominent pathogenic species. We discuss how this fascinating Pseudomonas trait may be exploited for novel root-based approaches to insect control in an integrated pest management framework.