Microcolony formation: a novel biofilm model of Pseudomonas aeruginosa for the cystic fibrosis lung

Proteomics of bacterial pathogens: Pseudomonas aeruginosa infections in cystic fibrosis - a case study

Technology development in the high throughput sciences of genomics, transcriptomics and proteomics, has been driven by bacteriological research. These organisms are excellent models for testing new methodology due to their comparatively small genome size, the relative ease of culturing large amounts of material, and the inherent biomedical, environmental and biotechnological interest in their underlying biology. Techniques developed in prokaryotes have since become applicable to higher organisms and human disease, opening vast research opportunities for understanding complex molecular processes. Pseudomonas aeruginosa is an excellent example of a microbe with fascinating properties suitable for stretching the boundaries of technology, and with underlying biology that remains poorly understood. P. aeruginosa is an opportunistic pathogen in humans and contains one of the largest genetic capabilities for a single-celled organism (approximately 5500 genes), which allows it to encode a wide variety of surface-associated and secreted virulence factors, as well as adapt to harsh environments, forming resistance to an array of antibacterial agents. While it is a major threat as a nosocomial pathogen, and particularly in the immunocompromised, it is also the most significant cause of mortality in patients suffering from the genetic disorder, cystic fibrosis. This review examines the role of proteomics in gaining a better understanding of the molecular basis of P. aeruginosa infection and persistence in the lungs of cystic fibrosis patients.

Small Heat Shock Proteins Produced by Pseudomonas Aeruginosa Clonal Variants Isolated from Diverse Niches

Genomic islands interspersed in the chromosome of P. aeruginosa led to inter- and intraclonal diversity. Recently, a particular clone of P. aeruginosa called clone C was isolated from cystic fibrosis (CF) patients, clinical and non-clinical habitats throughout Europe and in Canada. P. aeruginosa clone C strains harbour up to several hundred acquired genes involved in the adaptation of bacteria to diverse niches. Two genes ( hp25 and hp18) from one of the hypervariable regions in the chromosome of clone C strains were highly expressed under standard culture conditions as well as under conditions that mimicked CF sputum environment. Protein sequence analysis revealed that Hp25 and Hp18 belonged to small heat shock protein (sHSP) family. Hp25 protein possessed α-crystallin domain, which is a conserved region among heat shock proteins involved in diverse functions. Sequence homology search revealed that in the Methylobacillus flagellatus genome both genes were situated close to each other and the hp25 gene is found among a few other members of Proteobacteria. Expression of hp25 and hp18 by inter- and intraclonal strains of P. aeruginosa suggested that both genes were present in the stable part of the hypervariable region at the toxR locus and might play a role in their adaptation to diverse niches including the CF lung environment.

Signals, regulatory networks, and materials that build and break bacterial biofilms

Biofilms are communities of microorganisms that live attached to surfaces. Biofilm formation has received much attention in the last decade, as it has become clear that virtually all types of bacteria can form biofilms and that this may be the preferred mode of bacterial existence in nature. Our current understanding of biofilm formation is based on numerous studies of myriad bacterial species. Here, we review a portion of this large body of work including the environmental signals and signaling pathways that regulate biofilm formation, the components of the biofilm matrix, and the mechanisms and regulation of biofilm dispersal.

Neutrophil enhancement of Pseudomonas aeruginosa biofilm development: human F-actin and DNA as targets for therapy

In the cystic fibrosis (CF) airway, chronic infection by Pseudomonas aeruginosa results from biofilm formation in a neutrophil-rich environment. We tested the capacity of human neutrophils to modify early biofilm formation of P. aeruginosa strain PAO1, and an isogenic CF strain isolated early and years later in infection. In a static reactor, P. aeruginosa biofilm density of all strains was enhanced at 24 h in the presence of neutrophils, with the greatest relative increase associated with the lowest inoculum of P. aeruginosa tested. Previously, neutrophil-induced biofilm enhancement was shown to largely result from the incorporation of F-actin and DNA polymers into the bacterial biofilm. This finding was advanced by the comparison of biofilm enhancement from intact unstimulated neutrophils and from lysed or apoptotic neutrophils. Apoptotic neutrophils, with an intact cell membrane, were unable to contribute to biofilm enhancement, while lysed neutrophils evoked a similar response to that of intact cells. Using F-actin and DNA as targets, the capacity of negatively charged poly(amino acids) to disrupt, or prevent, early biofilm formation was tested. Anionic poly(aspartic acid) effectively prevented or disrupted biofilm formation. Combination of poly(aspartic acid) with DNase resulted in a synergistic increase in biofilm disruption. These results demonstrate that the presence of dying neutrophils can facilitate the initial stages of biofilm development by low inocula of P. aeruginosa. Neutrophil F-actin represents a potential new therapeutic target for disruption of pathogenic biofilms.

Efflux unbalance in Pseudomonas aeruginosa isolates from cystic fibrosis patients

Retrospective analysis of 189 nonredundant strains of Pseudomonas aeruginosa sequentially recovered from the sputum samples of 46 cystic fibrosis (CF) patients over a 10-year period (1998 to 2007) revealed that 53 out of 189 (28%) samples were hypersusceptible to the beta-lactam antibiotic ticarcillin (MIC < or = 4 microg/ml) (phenotype dubbed Tic(hs)). As evidenced by trans-complementation and gene inactivation experiments, the mutational upregulation of the efflux system MexXY was responsible for various degrees of resistance to aminoglycosides in a selection of 11 genotypically distinct strains (gentamicin MICs from 2 to 64 microg/ml). By demonstrating for the first time that the MexXY pump may evolve in CF strains, we found that a mutation leading to an F1018L change in the resistance-nodulation-cell division (RND) transporter MexY was able to increase pump-promoted resistance to aminoglycosides, cefepime, and fluoroquinolones twofold. The inactivation of the mexB gene (which codes for the RND transporter MexB) in the 11 selected strains showed that the Tic(hs) phenotype was due to a mutational or functional loss of function of MexAB-OprM, the multidrug efflux system known to contribute to the natural resistance of P. aeruginosa to beta-lactams (e.g., ticarcillin and aztreonam), fluoroquinolones, tetracycline, and novobiocin. Two of the selected strains synthesized abnormally low amounts of the MexB protein, and 3 of 11 strains expressed truncated MexB (n = 2) or MexA (n = 1) polypeptide as a result of mutations in the corresponding genes, while 7 of 11 strains produced wild-type though nonfunctional MexAB-OprM pumps at levels similar to or even higher than that of reference strain PAO1. Overall, our data indicate that while MexXY is necessary for P. aeruginosa to adapt to the hostile environment of the CF lung, the MexAB-OprM pump is dispensable and tends to be lost or inactivated in subpopulations of P. aeruginosa.

Viable ultramicrocells in drinking water

AIMS

To examine the diversity of cultivable 0.2 micron filtrate biofilm forming bacteria from drinking water systems.

METHODS AND RESULTS

Potable chlorinated drinking water hosts phylogenetically diverse ultramicrocells (UMC) (0.2 and 0.1 microm filterable). UMC (starved or dwarf bacteria) were isolated by cultivation on minimal medium from a flow system wall model with polyvinyl chloride (PVC) pipes. All cultivated cells (25 different isolates) did not maintain their ultra-size after passages on rich media. Cultured UMC were identified by their 16S ribosomal DNA sequences. The results showed that they were closely related to uncultured and cultured members of the Proteobacteria, Actinobacteria and Firmicutes. The isolates of phylum Actinobacteria included representatives of a diverse set of Actinobacterial families: Micrococcaceae, Microbacteriaceae, Dermabacteraceae, Nocardiaceae and Nocardioidaceae.

CONCLUSIONS

This study is the first to show an abundance of cultivable UMC of various phyla in drinking water system, including a high frequency of bacteria known to be involved in opportunistic infections, such as Stenotrophomonas maltophilia, Microbacterium sp., Pandoraea sp. and Afipia strains.

SIGNIFICANCE AND IMPACT OF THE STUDY

Chlorinated tap water filtrate (0.2 and 0.1 microm) still harbours opportunistic micro-organisms that can pose some health threat.

Adaptive expression of foreign genes in the clonal variants of bacteria: from proteomics to clinical application

Clonal variants of bacteria are able to colonize environmental niches and patients. The factors, that determine the interplay between the colonization of diverse habitats and adaptation, are acquired through horizontal gene transfer. Elucidation of mechanisms, which lead to the prevalence of dominant bacterial clones in patients and the environment, requires the knowledge of complex phenotypes. It was found in the genomes of most bacteria, that upon a conserved chromosomal backbone there were regions of plasticity achieved by insertions, deletions and rearrangements of genomic islands and islets as well as large chromosomal inversions. However, it had been shown that environmental and clinical isolates are indistinguishable in certain pathogenic and biodegradative properties. For example, clonal variants of Pseudomonas aeruginosa exhibit convergent phenotypes despite the presence of numerous DNA insertions in the genome. Apart from this feature, expression of a few genes from the acquired genetic material is important for niche-based adaptation of this organism. Protein expression patterns at the cellular and sub-cellular levels showed common virulence factors and novel drug targets among clonal variants of bacteria. This review will give a short overview on proteomics of different clonal variants of bacteria with respect to clinical applications.

Fitness of isogenic colony morphology variants of Pseudomonas aeruginosa in murine airway infection

Chronic lung infections with Pseudomonas aeruginosa are associated with the diversification of the persisting clone into niche specialists and morphotypes, a phenomenon called 'dissociative behaviour'. To explore the potential of P. aeruginosa to change its morphotype by single step loss-of-function mutagenesis, a signature-tagged mini-Tn5 plasposon library of the cystic fibrosis airway isolate TBCF10839 was screened for colony morphology variants under nine different conditions in vitro. Transposon insertion into 1% of the genome changed colony morphology into eight discernable morphotypes. Half of the 55 targets encode features of primary or secondary metabolism whereby quinolone production was frequently affected. In the other half the transposon had inserted into genes of the functional categories transport, regulation or motility/chemotaxis. To mimic dissociative behaviour of isogenic strains in lungs, pools of 25 colony morphology variants were tested for competitive fitness in an acute murine airway infection model. Six of the 55 mutants either grew better or worse in vivo than in vitro, respectively. Metabolic proficiency of the colony morphology variant was a key determinant for survival in murine airways. The most common morphotype of self-destructive autolysis did unexpectedly not impair fitness. Transposon insertions into homologous genes of strain PAO1 did not reproduce the TBCF10839 mutant morphotypes for 16 of 19 examined loci pointing to an important role of the genetic background on colony morphology. Depending on the chosen P. aeruginosa strain, functional genome scans will explore other areas of the evolutionary landscape. Based on our discordant findings of mutant phenotypes in P. aeruginosa strains PAO1, PA14 and TBCF10839, we conclude that the current focus on few reference strains may miss modes of niche adaptation and dissociative behaviour that are relevant for the microevolution of complex traits in the wild.

Pseudomonas aeruginosa biofilm formation in the cystic fibrosis airway

The cystic fibrosis (CF) lung is chronically inflamed and infected by Pseudomonas aeruginosa, which is a major cause of morbidity and mortality in this genetic disease. Although aerosolization of Tobramycin into the airway of CF patients improves outcomes, the lungs of CF patients, even those receiving antibiotic therapy, are persistently colonized by P. aeruginosa. Recent studies suggest that the antibiotic resistance of P. aeruginosa in the CF lung is due to the formation of drug resistant biofilms, which are defined as communities of microbes associated with surfaces or interfaces, and whose growth is facilitated by thick and dehydrated mucus in the CF lung. In this review, we discuss some of the current models used to study biofilm formation in the context of biotic surfaces, such as airway cells, as well as the contribution of host-derived factors, including DNA, actin and mucus, to the formation of these microbial communities. We suggest that better in vitro models are required, both to understand the interaction of P. aeruginosa with the host airway, and as models to validate new therapeutics, whether targeted at bacteria or host.

Effects of iron on DNA release and biofilm development by Pseudomonas aeruginosa

Extracellular DNA is one of the major matrix components in Pseudomonas aeruginosa biofilms. It functions as an intercellular connector and plays a role in stabilization of the biofilms. Evidence that DNA release in P. aeruginosa PAO1 biofilms is controlled by the las-rhl and pqs quorum-sensing systems has been previously presented. This paper provides evidence that DNA release in P. aeruginosa PAO1 biofilms is also under iron regulation. Experiments involving cultivation of P. aeruginosa in microtitre trays suggested that pqs expression, DNA release and biofilm formation were favoured in media with low iron concentrations (5 microM FeCl(3)), and decreased with increasing iron concentrations. Experiments involving cultivation of P. aeruginosa in a flow-chamber system suggested that a high level of iron (100 microM FeCl(3)) in the medium suppressed DNA release, structural biofilm development, and the development of subpopulations with increased tolerance toward antimicrobial compounds. Experiments with P. aeruginosa strains harbouring fluorescent reporters suggested that expression of the pqs operon was induced in particular subpopulations of the biofilm cells under low-iron conditions (1 microM FeCl(3)), but repressed in the biofilm cells under high-iron conditions (100 microM FeCl(3)).

Nutritional cues control Pseudomonas aeruginosa multicellular behavior in cystic fibrosis sputum

The sputum (mucus) layer of the cystic fibrosis (CF) lung is a complex substrate that provides Pseudomonas aeruginosa with carbon and energy to support high-density growth during chronic colonization. Unfortunately, the CF lung sputum layer has been difficult to mimic in animal models of CF disease, and mechanistic studies of P. aeruginosa physiology during growth in CF sputum are hampered by its complexity. In this study, we performed chromatographic and enzymatic analyses of CF sputum to develop a defined, synthetic CF sputum medium (SCFM) that mimics the nutritional composition of CF sputum. Importantly, P. aeruginosa displays similar phenotypes during growth in CF sputum and in SCFM, including similar growth rates, gene expression profiles, carbon substrate preferences, and cell-cell signaling profiles. Using SCFM, we provide evidence that aromatic amino acids serve as nutritional cues that influence cell-cell signaling and antimicrobial activity of P. aeruginosa during growth in CF sputum.

Transcriptomic analysis of the sulfate starvation response of Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen that causes a number of infections in humans, but is best known for its association with cystic fibrosis. It is able to use a wide range of sulfur compounds as sources of sulfur for growth. Gene expression in response to changes in sulfur supply was studied in P. aeruginosa E601, a cystic fibrosis isolate that displays mucin sulfatase activity, and in P. aeruginosa PAO1. A large family of genes was found to be upregulated by sulfate limitation in both isolates, encoding sulfatases and sulfonatases, transport systems, oxidative stress proteins, and a sulfate-regulated TonB/ExbBD complex. These genes were localized in five distinct islands on the genome and encoded proteins with a significantly reduced content of cysteine and methionine. Growth of P. aeruginosa E601 with mucin as the sulfur source led not only to a sulfate starvation response but also to induction of genes involved with type III secretion systems.

Cystic fibrosis: a disease of vulnerability to airway surface dehydration

Cystic fibrosis (CF) lung disease involves chronic bacterial infection of retained airway secretions (mucus). Recent data suggest that CF lung disease pathogenesis reflects the vulnerability of airway surfaces to dehydration and collapse of mucus clearance. This predisposition is caused by mutations in the CF transmembrane conductance regulator (CFTR) gene, resulting in (i) the absence of CFTR-mediated Cl- secretion and regulation of epithelial Na+ channel (ENaC) function; and (ii) the sole dependence on extracellular ATP to rebalance these ion transport processes through P2 purinoceptor signaling. Recent clinical studies indicate that inhalation of hypertonic saline osmotically draws sufficient water onto CF airway surfaces to provide clinical benefit.

Diffusible signal factor-dependent cell-cell signaling and virulence in the nosocomial pathogen Stenotrophomonas maltophilia

The genome of Stenotrophomonas maltophilia encodes a cell-cell signaling system that is highly related to the diffusible signal factor (DSF)-dependent system of the phytopathogen Xanthomonas campestris. Here we show that in S. maltophilia, DSF signaling controls factors contributing to the virulence and antibiotic resistance of this important nosocomial pathogen.

Airway surface dehydration in cystic fibrosis: pathogenesis and therapy

Cystic fibrosis (CF) lung disease reflects the failure of airways defense against chronic bacterial infection. Studies of CF cultures, transgenic mice, and CF patients suggest that the initiating event in CF airways disease pathogenesis is reduced airway surface liquid (ASL) volume, i.e., dehydration. CF ASL volume regulation depends on a single extracellular signaling system, ATP, which renders CF airways more vulnerable to disease-causing insults (e.g., viruses) than are normal airways, which regulate ASL volume by dual ATP and adenosine signaling pathways. Clinical studies have explored the hypothesis that treating the dehydration of CF airways will be therapeutically beneficial. Inhaled hypertonic saline osmotically draws water onto airway surfaces, improves mucus clearance and pulmonary function, and reduces acute exacerbations in CF patients. Thus, rehydration therapies may slow the progression of CF lung disease in patients with established bacterial infection and may prevent the onset of CF lung disease if initiated early in life.

Biofilms formed by nontypeable Haemophilus influenzae in vivo contain both double-stranded DNA and type IV pilin protein

Nontypeable Haemophilus influenzae (NTHI) strains are members of the normal human nasopharyngeal flora, as well as frequent opportunistic pathogens of both the upper and lower respiratory tracts. Recently, it has been shown that NTHI can form biofilms both in vitro and in vivo. NTHI strains within in vitro-formed biofilms differentially express both epitopes of lipooligosaccharide (LOS) and the outer membrane proteins P2, P5, and P6, whereas those generated either in a 96-well plate assay in vitro or in a mammalian host have been shown to incorporate a specific glycoform of sialylated LOS within the biofilm matrix. While DNA has been identified as a key component of the biofilm matrix formed in vitro by several bacterial pathogens, here we demonstrate for the first time that in addition to sialylated LOS, the biofilm formed by NTHI in vivo contains both type IV pilin protein and a significant amount of double-stranded DNA. The DNA appeared to be arranged in a dense interlaced meshwork of fine strands as well as in individual thicker "ropes" that span water channels, suggesting that DNA could be imparting structural stability to the biofilm produced by NTHI in vivo. The presence of type IV pilin protein both appearing as small aggregates within the biofilm matrix and tracking along DNA strands supports our observations which showed that type IV pili are expressed by NTHI during experimental otitis media when these bacteria form a biofilm in the middle ear space.

Anaerobic culture conditions favor biofilm-like phenotypes inPseudomonas aeruginosaisolates from patients with cystic fibrosis

Pseudomonas aeruginosa causes chronic infections in the lungs of cystic fibrosis (CF) individuals and remains the leading cause of morbidity and mortality associated with the disease. Biofilm growth and phenotypic diversification are factors thought to contribute to this organism's persistence. Most studies have focused on laboratory isolates such as strain PAO1, and there are relatively few reports characterizing the properties of CF strains, especially under decreased oxygen conditions such as occur in the CF lung. This study compared the phenotypic and functional properties of P. aeruginosa from chronically infected CF adults with those of strain PAO1 and other clinical non-CF isolates under aerobic and anaerobic culture conditions. The CF isolates overall displayed a reduced ability to form biofilms in standard in vitro short-term models. They also grew more slowly in culture, and exhibited decreased adherence to glass and decreased motilities (swimming, swarming and twitching). All of these characteristics were markedly accentuated by anaerobic growth conditions. Moreover, the CF strain phenotypes were not readily reversed by culture manipulations designed to encourage planktonic growth. The CF strains were thus inherently different from strain PAO1 and most of the other non-CF clinical P. aeruginosa isolates tested. In vitro models used to research CF isolate biofilm growth need to take the above properties of these strains into account.

A physical linkage between cystic fibrosis airway surface dehydration and Pseudomonas aeruginosa biofilms

A vexing problem in cystic fibrosis (CF) pathogenesis has been to explain the high prevalence of Pseudomonas aeruginosa biofilms in CF airways. We speculated that airway surface liquid (ASL) hyperabsorption generates a concentrated airway mucus that interacts with P. aeruginosa to promote biofilms. To model CF vs. normal airway infections, normal (2.5% solids) and CF-like concentrated (8% solids) mucus were prepared, placed in flat chambers, and infected with an approximately 5 x 10(3) strain PAO1 P. aeruginosa. Although bacteria grew to 10(10) cfu/ml in both mucus concentrations, macrocolony formation was detected only in the CF-like (8% solids) mucus. Biophysical and functional measurements revealed that concentrated mucus exhibited properties that restrict bacterial motility and small molecule diffusion, resulting in high local bacterial densities with high autoinducer concentrations. These properties also rendered secondary forms of antimicrobial defense, e.g., lactoferrin, ineffective in preventing biofilm formation in a CF-like mucus environment. These data link airway surface liquid hyperabsorption to the high incidence of P. aeruginosa biofilms in CF via changes in the hydration-dependent physical-chemical properties of mucus and suggest that the thickened mucus gel model will be useful to develop therapies of P. aeruginosa biofilms in CF airways.

Delivery of photosensitisers and light through mucus: investigations into the potential use of photodynamic therapy for treatment of Pseudomonas aeruginosa cystic fibrosis pulmonary infection

Respiratory disease is the main cause of morbidity and mortality in patients with cystic fibrosis (CF). In such patients chronic Pseudomonas aeruginosa infection is virtually impossible to eradicate using antibiotic therapy. Photodynamic antimicrobial chemotherapy (PACT) could be one potential alternative antimicrobial method. As photosensitisers could be delivered to the lungs of CF patients via inhalation, the current in vitro study investigated the potential use of PACT in the treatment of P. aeruginosa CF pulmonary infection. Delivery of red light (635 nm) and two photosensitisers (toluidine blue O (TBO) and meso-tetra (N-methyl-4-pyridyl) porphine tetra tosylate (TMP)) across artificial CF mucus was successfully achieved. Artificial CF mucus reduced the measured fluence of incident light in an almost exponential manner with increasing depth. The presence of dissolved photosensitisers also reduced light fluence. TMP diffused more efficiently across artificial CF mucus than TBO. However, receiver compartment concentrations of both drugs after 6 h were of the same order as those required to achieve high rates of kill (>99%) of P. aeruginosa isolates growing both planktonically and in biofilms. TMP required significantly higher concentrations (2.5 mg ml(-1)) than TBO to achieve high rates of kill (>99%) of P. aeruginosa isolates growing planktonically. Higher concentrations (5.0 mg ml(-1)) of both photosensitisers were required to achieve high rates of kill (>99%) of P. aeruginosa isolates growing in biofilms. When photosensitisers were prepared in artificial mucus, higher concentrations were required to achieve reasonably high kill rates (>80%) of P. aeruginosa (PAO1) growing both planktonically and in biofilm.

Proteomic analysis of Escherichia coli biofilms reveals the overexpression of the outer membrane protein OmpA

Bacterial colonisation and biofilm formation on the surface of urinary catheters is a common cause of nosocomial infection, and as such is a major impediment to their long-term use. Understanding the mechanisms of biofilm formation on urinary catheters is critical to their control and will aid the future development of materials used in their manufacture. In this report we have used proteomic analysis coupled with immunoassays to show that the major outer membrane protein (OmpA) of Escherichia coli is overexpressed during biofilm formation. A series of synthetic hydrogels being developed for potential use as catheter coatings were used as the substrata and OmpA expression was increased in biofilms on all these surfaces, as well as being a feature of both a laboratory and a clinical strain of E. coli. Up-regulation of OmpA may, therefore, be a common feature of E. coli biofilms. These findings present OmpA as a potential target for biofilm inhibition and may contribute to the rational design of biofilm inhibiting hydrogel coatings for urinary catheters.

Consequences of combining cystic fibrosis- and non-cystic fibrosis-derived Pseudomonas aeruginosa antibiotic susceptibility results in hospital antibiograms

BACKGROUND

In preparing hospital antibiograms for individual organisms and antibiotics, laboratories often combine susceptibility data for isolates from a variety of sources and patient types. If results from patients with known resistance patterns that vary from normal are included, the overall susceptibility value for the institution could be misleadingly skewed.

OBJECTIVE

To assess the degree of bias introduced into a hospital antibiogram by combining cystic fibrosis (CF) and non-CF isolates of Pseudomonas aeruginosa to produce one hospital-wide percent susceptible figure for each tested antibiotic.

METHODS

A retrospective analysis was conducted of an academic, tertiary care medical center's microbiology database. We examined quarterly and annual susceptibility data from 2004, comparing non-CF data with combined susceptibility data for 10 antibiotics within each quarter, as well as those reported in the annual antibiogram. Differences were assessed for statistical significance using chi(2) testing with Bonferroni correction.

RESULTS

Large differences were observed between non-CF and combined percent susceptible data in the 4 quarters (aminoglycosides 3% vs 20%, fluoroquinolones 2% vs 18%, respectively) and when comparing annual non-CF (n = 191) with annual combined (n = 266) data. With the annual figures, these differences were frequently statistically significant (70% vs 58%, 91% vs 83%, 85% vs 70%, and 72% vs 60% for gentamicin, tobramycin, amikacin, and gatifloxacin/levofloxacin, respectively; all p< or =0.01).

CONCLUSIONS

Combining CF and non-CF P. aeruginosa susceptibility into one percent susceptibility value for all isolates may produce figures that underestimate the activity of some antibiotic classes against non-CF isolates. Clinicians may make less than optimal empiric antibiotic selection choices based on such data.

Succinate-mediated catabolite repression control on the production of glycine betaine catabolic enzymes in Pseudomonas aeruginosa PAO1 under low and elevated salinities

Glycine betaine (GB) and its immediate precursors choline and carnitine, dimethylsulfonioacetate, dimethylsulfoniopropionate, ectoine and proline were effective osmoprotectants for Pseudomonas aeruginosa, but pipecolate, trehalose and sucrose had no osmoprotective effect. GB was accumulated stably or transiently when succinate or glucose, respectively, was used as a carbon and energy source. The catabolite repression mediated by succinate occurred at both low and high salinities, and it did not involve the global regulators Vfr and Crc. A proteomic analysis showed that at least 21 proteins were induced when GB was used as a carbon and energy source, and provided evidence that succinate repressed the synthesis of all these proteins. Many of the proteins induced by GB (sarcosine oxidase, serine hydroxymethyltransferase and serine dehydratase) are involved in GB catabolism. In addition, GB uptake was stimulated at high medium osmolalities but it was insensitive to catabolite repression by succinate. Despite its ability to inhibit betaine catabolism, succinate did not allow any better growth of P. aeruginosa cells under hyperosmotic constraint. Conversely, as observed for cells supplied with glucose, a transient accumulation of GB was sufficient to provide a significant cell osmoprotection.

Mechanism of enhanced activity of liposome-entrapped aminoglycosides against resistant strains of Pseudomonas aeruginosa

Pseudomonas aeruginosa is inherently resistant to most conventional antibiotics. The mechanism of resistance of this bacterium is mainly associated with the low permeability of its outer membrane to these agents. We sought to assess the bactericidal efficacy of liposome-entrapped aminoglycosides against resistant clinical strains of P. aeruginosa and to define the mechanism of liposome-bacterium interactions. Aminoglycosides were incorporated into liposomes, and the bactericidal efficacies of both free and liposomal drugs were evaluated. To define the mechanism of liposome-bacterium interactions, transmission electron microscopy (TEM), flow cytometry, lipid mixing assay, and immunocytochemistry were employed. Encapsulation of aminoglycosides into liposomes significantly increased their antibacterial activity against the resistant strains used in this study (MICs of > or =32 versus < or =8 microg/ml). TEM observations showed that liposomes interact intimately with the outer membrane of P. aeruginosa, leading to the membrane deformation. The flow cytometry and lipid mixing assays confirmed liposome-bacterial membrane fusion, which increased as a function of incubation time. The maximum fusion rate was 54.3% +/- 1.5% for an antibiotic-sensitive strain of P. aeruginosa and 57.8% +/- 1.9% for a drug-resistant strain. The fusion between liposomes and P. aeruginosa significantly enhanced the antibiotics' penetration into the bacterial cells (3.2 +/- 2.3 versus 24.2 +/- 6.2 gold particles/bacterium, P < or = 0.001). Our data suggest that liposome-entrapped antibiotics could successfully resolve infections caused by antibiotic-resistant P. aeruginosa through an enhanced mechanism of drug entry into the bacterial cells.