Selection for Staphylococcus aureus small-colony variants due to growth in the presence of Pseudomonas aeruginosa

Implications of interspecies signaling for virulence of bacterial and fungal pathogens

Despite the broad armory of vaccines, antibiotics and other weapons at our disposal, pathogenic bacteria and fungi continue to present a serious threat to human health. These pathogens have proved very versatile and many are associated with infections of vulnerable individuals, often in hospital settings. Evidence is accumulating that certain infections, for example, of medical devices, the cystic fibrosis lung, the oral cavity, the GI tract and wounds, are in fact polymicrobial, with more than one microbe involved. To understand diseases and formulate intervention strategies, it is necessary to know the extent of contact and communication between microbes in these mixed infections. It is now emerging that the signals that microbes use to coordinate expression of viruence factors within a species may also be perceived by other microbes in the community. This article addresses such interspecies signaling and examines the consequences of such signaling between bacterial and fungal pathogens for expression of virulence traits.

Microbial competition between Bacillus subtilis and Staphylococcus aureus monitored by imaging mass spectrometry

Microbial competition exists in the general environment, such as soil or aquatic habitats, upon or within unicellular or multicellular eukaryotic life forms. The molecular actions that govern microbial competition, leading to niche establishment and microbial monopolization, remain undetermined. The emerging technology of imaging mass spectrometry (IMS) enabled the observation that there is directionality in the metabolic output of the organism Bacillus subtilis when co-cultured with Staphylococcus aureus. The directionally released antibiotic alters S. aureus virulence factor production and colonization. Therefore, IMS provides insight into the largely hidden nature of competitive microbial encounters and niche establishment, and provides a paradigm for future antibiotic discovery.

Review: Staphylococcus aureus and MRSA in cystic fibrosis

BACKGROUND

Staphylococcus aureus (S. aureus) is one of the earliest bacteria detected in infants and children with cystic fibrosis (CF). The rise of methicillin resistant S. aureus (MRSA) in the last 10 years has caused a lot of attention to this organism.

RESULTS

The aim of this review is to provide a general overview of methicillin sensitive S. aureus (MSSA) and MRSA, discuss special aspects of S. aureus in cystic fibrosis, and to review treatment concepts. Microbiology of the organism will be reviewed along with data regarding the epidemiology of both MSSA and MRSA. Antibiotic treatments both in regards to acute management and eradication of MSSA and MRSA will be reviewed. Prophylaxis of MSSA in CF remains controversial. Treatment with anti-staphylococcal agents reduces the infection rate with MSSA but may lead to a higher rate of infection with P. aeruginosa. In regards to MRSA, there is a paucity of clinical data regarding approaches to eradication.

CONCLUSIONS

To advance the care of CF patients, controlled clinical trials are urgently needed to find the optimal approach to treating CF patients who are infected with either MSSA or MRSA.

The nature and consequences of coinfection in humans

Objective

Many fundamental patterns of coinfection (multi-species infections) are undescribed, including the relative frequency of coinfection by various pathogens, differences between single-species infections and coinfection, and the burden of coinfection on human health. We aimed to address the paucity of general knowledge on coinfection by systematically collating and analysing data from recent publications to understand the types of coinfection and their effects.

Methods

From an electronic search to find all publications from 2009 on coinfection and its synonyms in humans we recorded data on i) coinfecting pathogens and their effect on ii) host health and iii) intensity of infection.

Results

The most commonly reported coinfections differ from infections causing highest global mortality, with a notable lack of serious childhood infections in reported coinfections. We found that coinfection is generally reported to worsen human health (76% publications) and exacerbate infections (57% publications). Reported coinfections included all kinds of pathogens, but were most likely to contain bacteria.

Conclusions

These results suggest differences between coinfected patients and those with single infections, with coinfection having serious health effects. There is a pressing need to quantify the tendency towards negative effects and to evaluate any sampling biases in the coverage of coinfection research.

Microbial ecology and adaptation in cystic fibrosis airways

Chronic infections in the respiratory tracts of cystic fibrosis (CF) patients are important to investigate, both from medical and from fundamental ecological points of view. Cystic fibrosis respiratory tracts can be described as natural environments harbouring persisting microbial communities with Pseudomonas aeruginosa as a dominant pathogen. Various factors contribute to the complexity of this ecosystem, including community composition, dynamics and interactions, as well as heterogeneous distribution and fluctuation of components of the immune system, antibiotics and nutrients. All these elements constitute the selective forces that drive the evolution of the microbes after they migrate from the outer environment to human airways. Pseudomonas aeruginosa adapts to the new environment through genetic changes and exhibits a special lifestyle in chronic CF airways. Understanding the persistent colonization of microbial pathogens in CF patients in the context of ecology and evolution will expand our knowledge of the pathogenesis of chronic infections and improve therapeutic strategies.

Production of Pseudomonas aeruginosa Intercellular Small Signaling Molecules in Human Burn Wounds

Pseudomonas aeruginosa has developed a complex cell-to-cell communication system that relies on low-molecular weight excreted molecules to control the production of its virulence factors. We previously characterized the transcriptional regulator MvfR, that controls a major network of acute virulence functions in P. aeruginosa through the control of its ligands, the 4-hydroxy-2-alkylquinolines (HAQs)—4-hydroxy-2-heptylquinoline (HHQ) and 3,4-dihydroxy-2-heptylquinoline (PQS). Though HHQ and PQS are produced in infected animals, their ratios differ from those in bacterial cultures. Because these molecules are critical for the potency of activation of acute virulence functions, here we investigated whether they are also produced during human P. aeruginosa acute wound infection and whether their ratio is similar to that observed in P. aeruginosa-infected mice. We found that a clinically relevant P. aeruginosa isolate produced detectable levels of HAQs with ratios of HHQ and PQS that were similar to those produced in burned and infected animals, and not resembling ratios in bacterial cultures. These molecules could be isolated from wound tissue as well as from drainage liquid. These results demonstrate for the first time that HAQs can be isolated and quantified from acute human wound infection sites and validate the relevance of previous studies conducted in mammalian models of infection.

Differential adaptation of microbial pathogens to airways of patients with cystic fibrosis and chronic obstructive pulmonary disease

Cystic fibrosis (CF), the most common autosomal recessive disorder in Caucasians, and chronic obstructive pulmonary disease (COPD), a disease of adults, are characterized by chronic lung inflammation, airflow obstruction and extensive tissue remodelling, which have a major impact on patients' morbidity and mortality. Airway inflammation is stimulated in CF by chronic bacterial infections and in COPD by environmental stimuli, particularly from smoking. Pseudomonas aeruginosa is the major bacterial pathogen in CF, while in COPD, Haemophilus influenzae is most frequently observed. Molecular studies indicate that during chronic pulmonary infection, P. aeruginosa clones genotypically and phenotypically adapt to the CF niche, resulting in a highly diverse bacterial community that is difficult to eradicate therapeutically. Pseudomonas aeruginosa clones from COPD patients remain within the airways only for limited time periods, do not adapt and are easily eradicated. However, in a subgroup of severely ill COPD patients, P. aeruginosa clones similar to those in CF persist. In this review, we will discuss the pathophysiology of lung disease in CF and COPD, the complex genotypic and phenotypic adaptation processes of the opportunistic bacterial pathogens and novel treatment options.

Quinolones: from antibiotics to autoinducers

Since quinine was first isolated, animals, plants and microorganisms producing a wide variety of quinolone compounds have been discovered, several of which possess medicinally interesting properties ranging from antiallergenic and anticancer to antimicrobial activities. Over the years, these have served in the development of many synthetic drugs, including the successful fluoroquinolone antibiotics. Pseudomonas aeruginosa and related bacteria produce a number of 2-alkyl-4(1H)-quinolones, some of which exhibit antimicrobial activity. However, quinolones such as the Pseudomonas quinolone signal and 2-heptyl-4-hydroxyquinoline act as quorum-sensing signal molecules, controlling the expression of many virulence genes as a function of cell population density. Here, we review selectively this extensive family of bicyclic compounds, from natural and synthetic antimicrobials to signalling molecules, with a special emphasis on the biology of P. aeruginosa. In particular, we review their nomenclature and biochemistry, their multiple properties as membrane-interacting compounds, inhibitors of the cytochrome bc(1) complex and iron chelators, as well as the regulation of their biosynthesis and their integration into the intricate quorum-sensing regulatory networks governing virulence and secondary metabolite gene expression.

Tomatidine inhibits replication of Staphylococcus aureus small-colony variants in cystic fibrosis airway epithelial cells

Small-colony variants (SCVs) often are associated with chronic Staphylococcus aureus infections, such as those encountered by cystic fibrosis (CF) patients. We report here that tomatidine, the aglycon form of the plant secondary metabolite tomatine, has a potent growth inhibitory activity against SCVs (MIC of 0.12 μg/ml), whereas the growth of normal S. aureus strains was not significantly altered by tomatidine (MIC, >16 μg/ml). The specific action of tomatidine was bacteriostatic for SCVs and was clearly associated with their dysfunctional electron transport system, as the presence of the electron transport inhibitor 4-hydroxy-2-heptylquinoline-N-oxide (HQNO) caused normal S. aureus strains to become susceptible to tomatidine. Inversely, the complementation of SCVs' respiratory deficiency conferred resistance to tomatidine. Tomatidine provoked a general reduction of macromolecular biosynthesis but more specifically affected the incorporation of radiolabeled leucine in proteins of HQNO-treated S. aureus at a concentration corresponding to the MIC against SCVs. Furthermore, tomatidine inhibited the intracellular replication of a clinical SCV in polarized CF-like epithelial cells. Our results suggest that tomatidine eventually will find some use in combination therapy with other traditional antibiotics to eliminate persistent forms of S. aureus.

Clinical significance of microbial infection and adaptation in cystic fibrosis

A select group of microorganisms inhabit the airways of individuals with cystic fibrosis. Once established within the pulmonary environment in these patients, many of these microbes adapt by altering aspects of their structure and physiology. Some of these microbes and adaptations are associated with more rapid deterioration in lung function and overall clinical status, whereas others appear to have little effect. Here we review current evidence supporting or refuting a role for the different microbes and their adaptations in contributing to poor clinical outcomes in cystic fibrosis.

Exit from dormancy in microbial organisms

Bacteria can exist in metabolically inactive states that allow them to survive conditions that are not conducive for growth. Such dormant cells may sense when conditions have improved and re-initiate growth, lest they be outcompeted by their neighbours. Growing bacteria turn over and release large quantities of their cell walls into the environment. Drawing from recent work on the germination of Bacillus subtilis spores, we propose that many microorganisms exit dormancy in response to cell wall muropeptides.

Parasitic growth of Pseudomonas aeruginosa in co-culture with the chitinolytic bacterium Aeromonas hydrophila

Polymer-degrading bacteria face exploitation by opportunistic bacteria that grow with the degradation products without investing energy into production of extracellular hydrolytic enzymes. This scenario was investigated with a co-culture of Aeromonas hydrophila and Pseudomonas aeruginosa with chitin as carbon, nitrogen and energy source. In single cultures, A. hydrophila could grow with chitin, while P. aeruginosa could not. Co-cultures with both strains had a biphasic course. In the first phase, P. aeruginosa grew along with A. hydrophila without affecting it. The second phase was initiated by a rapid inactivation of and a massive acetate release by A. hydrophila. Both processes coincided and were dependent on quorum sensing-regulated production of secondary metabolites by P. aeruginosa. Among these the redox-active phenazine compound pyocyanin caused the release of acetate by A. hydrophila by blocking the citric acid cycle through inhibition of aconitase. Thus, A. hydrophila was forced into an incomplete oxidation of chitin with acetate as end-product, which supported substantial growth of P. aeruginosa in the second phase of the co-culture. In conclusion, P. aeruginosa could profit from a substrate that was originally not bioavailable to it by influencing the metabolism and viability of A. hydrophila in a parasitic way.

Role of planktonic and sessile extracellular metabolic byproducts on Pseudomonas aeruginosa and Escherichia coli intra and interspecies relationships

Bacterial species are found primarily as residents of complex surface-associated communities, known as biofilms. Although these structures prevail in nature, bacteria still exist in planktonic lifestyle and differ from those in morphology, physiology, and metabolism. This study aimed to investigate the influence of physiological states of Pseudomonas aeruginosa and Escherichia coli in cell-to-cell interactions. Filtered supernatants obtained under planktonic and biofilm cultures of each single species were supplemented with tryptic soy broth (TSB) and used as the growth media (conditioned media) to planktonic and sessile growth of both single- and two-species cultures. Planktonic bacterial growth was examined through OD(640) measurement. One-day-old biofilms were evaluated in terms of biofilm biomass (CV), respiratory activity (XTT), and CFU number. Conditioned media obtained either in biofilm or in planktonic mode of life triggered a synergistic effect on planktonic growth, mainly for E. coli single cultures growing in P. aeruginosa supernatants. Biofilms grown in the presence of P. aeruginosa biofilms-derived metabolites presented less mass and activity. These events highlight that, when developed in biofilm, P. aeruginosa release signals or metabolites able to prejudice single and binary biofilm growth of others species and of their own species. However, products released by their planktonic counterparts did not impair biofilm growth or activity. E. coli, living as planktonic or sessile cultures, released signals and metabolites or removed un-beneficial compounds which promoted the growth and activity of all the species. Our findings revealed that inter and intraspecies behaviors depend on the involved bacteria and their adopted mode of life.

Revealing the dynamics of polymicrobial infections: implications for antibiotic therapy

As a new generation of culture-independent analytical strategies emerge, the amount of data on polymicrobial infections will increase dramatically. For these data to inform clinical thinking, and in turn to maximise benefits for patients, an appropriate framework for their interpretation is required. Here, we use cystic fibrosis (CF) lower airway infections as a model system to examine how conceptual and technological advances can address two clinical questions that are central to improved management of CF respiratory disease. Firstly, can markers of the microbial community be identified that predict a change in infection dynamics and clinical outcomes? Secondly, can these new strategies directly characterize the impact of antimicrobial therapies, allowing treatment efficacy to be both assessed and optimized?

Adhesion to and biofilm formation on IB3-1 bronchial cells by Stenotrophomonas maltophilia isolates from cystic fibrosis patients

Background

Stenotrophomonas maltophilia has recently gained considerable attention as an important emerging pathogen in cystic fibrosis (CF) patients. However, the role of this microorganism in the pathophysiology of CF lung disease remains largely unexplored. In the present study for the first time we assessed the ability of S. maltophilia CF isolates to adhere to and form biofilm in experimental infection experiments using the CF-derived bronchial epithelial IB3-1cell line. The role of flagella on the adhesiveness of S. maltophilia to IB3-1 cell monolayers was also assessed by using fliI mutant derivative strains.

Results

All S. maltophilia CF isolates tested in the present study were able, although at different levels, to adhere to and form biofilm on IB3-1 cell monolayers. Scanning electron and confocal microscopy revealed S. maltophilia structures typical of biofilm formation on bronchial IB3-1 cells. The loss of flagella significantly (P < 0.001) decreased bacterial adhesiveness, if compared to that of their parental flagellated strains. S. maltophilia CF isolates were also able to invade IB3-1 cells, albeit at a very low level (internalization rate ranged from 0.01 to 4.94%). Pre-exposure of IB3-1 cells to P. aeruginosa PAO1 significantly increased S. maltophilia adhesiveness. Further, the presence of S. maltophilia negatively influenced P. aeruginosa PAO1 adhesiveness.

Conclusions

The main contribution of the present study is the finding that S. maltophilia is able to form biofilm on and invade CF-derived IB3-1 bronchial epithelial cells, thus posing a rationale for the persistence and the systemic spread of this opportunistic pathogen in CF patients. Experiments using in vivo models which more closely mimic CF pulmonary tissues will certainly be needed to validate the relevance of our results.

A type VI secretion system of Pseudomonas aeruginosa targets a toxin to bacteria

The functional spectrum of a secretion system is defined by its substrates. Here we analyzed the secretomes of Pseudomonas aeruginosa mutants altered in regulation of the Hcp Secretion Island-I-encoded type VI secretion system (H1-T6SS). We identified three substrates of this system, proteins Tse1-3 (type six exported 1-3), which are coregulated with the secretory apparatus and secreted under tight posttranslational control. The Tse2 protein was found to be the toxin component of a toxin-immunity system and to arrest the growth of prokaryotic and eukaryotic cells when expressed intracellularly. In contrast, secreted Tse2 had no effect on eukaryotic cells; however, it provided a major growth advantage for P. aeruginosa strains, relative to those lacking immunity, in a manner dependent on cell contact and the H1-T6SS. This demonstration that the T6SS targets a toxin to bacteria helps reconcile the structural and evolutionary relationship between the T6SS and the bacteriophage tail and spike.

Staphylococcus aureus sigma B-dependent emergence of small-colony variants and biofilm production following exposure to Pseudomonas aeruginosa 4-hydroxy-2-heptylquinoline-N-oxide

Background

Staphylococcus aureus and Pseudomonas aeruginosa are often found together in the airways of cystic fibrosis (CF) patients. It was previously shown that the P. aeruginosa exoproduct 4-hydroxy-2-heptylquinoline-N-oxide (HQNO) suppresses the growth of S. aureus and provokes the emergence of small-colony variants (SCVs). The presence of S. aureus SCVs as well as biofilms have both been associated with chronic infections in CF.

Results

We demonstrated that HQNO stimulates S. aureus to form a biofilm in association with the formation of SCVs. The emergence of SCVs and biofilm production under HQNO exposure was shown to be dependent on the activity of the stress- and colonization-related alternative sigma factor B (SigB). Analysis of gene expression revealed that exposure of a prototypical S. aureus strain to HQNO activates SigB, which was leading to an increase in the expression of the fibronectin-binding protein A and the biofilm-associated sarA genes. Conversely, the quorum sensing accessory gene regulator (agr) system and the α-hemolysin gene were repressed by HQNO. Experiments using culture supernatants from P. aeruginosa PAO1 and a double chamber co-culture model confirmed that P. aeruginosa stimulates biofilm formation and activates SigB in a S. aureus strain isolated from a CF patient. Furthermore, the supernatant from P. aeruginosa mutants unable to produce HQNO induced the production of biofilms by S. aureus to a lesser extent than the wild-type strain only in a S. aureus SigB-functional background.

Conclusions

These results suggest that S. aureus responds to HQNO from P. aeruginosa by forming SCVs and biofilms through SigB activation, a phenomenon that may contribute to the establishment of chronic infections in CF patients.

Nutrient availability as a mechanism for selection of antibiotic tolerant Pseudomonas aeruginosa within the CF airway

Microbes are subjected to selective pressures during chronic infections of host tissues. Pseudomonas aeruginosa isolates with inactivating mutations in the transcriptional regulator LasR are frequently selected within the airways of people with cystic fibrosis (CF), and infection with these isolates has been associated with poorer lung function outcomes. The mechanisms underlying selection for lasR mutation are unknown but have been postulated to involve the abundance of specific nutrients within CF airway secretions. We characterized lasR mutant P. aeruginosa strains and isolates to identify conditions found in CF airways that select for growth of lasR mutants. Relative to wild-type P. aeruginosa, lasR mutants exhibited a dramatic metabolic shift, including decreased oxygen consumption and increased nitrate utilization, that is predicted to confer increased fitness within the nutrient conditions known to occur in CF airways. This metabolic shift exhibited by lasR mutants conferred resistance to two antibiotics used frequently in CF care, tobramycin and ciprofloxacin, even under oxygen-dependent growth conditions, yet selection for these mutants in vitro did not require preceding antibiotic exposure. The selection for loss of LasR function in vivo, and the associated adverse clinical impact, could be due to increased bacterial growth in the oxygen-poor and nitrate-rich CF airway, and from the resulting resistance to therapeutic antibiotics. The metabolic similarities among diverse chronic infection-adapted bacteria suggest a common mode of adaptation and antibiotic resistance during chronic infection that is primarily driven by bacterial metabolic shifts in response to nutrient availability within host tissues.

Chronic infections are distinguished from many other infections in that they are difficult to eradicate with antibiotics. Thus, the microbes that cause chronic infections persist within host tissues for long periods despite our best treatment efforts. During the course of these chronic infections, the causative microbes often change genetically. For example, a bacterium that commonly infects the lungs of people with the genetic disease cystic fibrosis (CF) undergoes several known changes that affect the growth of this pathogen. However, the causes and clinical impact of the changes undergone by this and other chronically infecting microbes are unclear. We show that a common, early mutation found in bacteria isolated from chronically infected CF airways renders these bacteria better able to grow in the nutrients found in CF lung secretions. Interestingly, these same changes also confer resistance to several antibiotics used commonly to treat CF patients. Many of the characteristics conferred by this mutation are exhibited by other microbes found in chronic infections, suggesting that adaptation of these microbes to host tissue nutrient environments may be a common mechanism of antibiotic resistance in chronic infections.

Studying bacterial infections through culture-independent approaches

The ability to characterize accurately the cause of infection is fundamental to effective treatment. The impact of any antimicrobial agents used to treat infection will, however, always be constrained by both the appropriateness of their use and our ability to determine their effectiveness. Traditional culture-based diagnostic microbiology is, in many cases, unable to provide this information. Molecular microbiological approaches that assess the content of clinical samples in a culture-independent manner promise to change dramatically the types of data that are obtained routinely from clinical samples. We argue that, in addition to the technical advance that these methodologies offer, a conceptual advance in the way that we reflect on the information generated is also required. Through the development of both of these advances, our understanding of infection, as well as the ways in which infections can be treated, may be improved. In the analysis of the microbiological content of certain clinical samples, such as blood, cerebrospinal fluid, brain and bone biopsy, culture-independent approaches have been well documented. Herein, we discuss how extensions to such studies can shape our understanding of infection at the many sites of the human body where a mixed flora, or in more ecological terms, a community of microbes, is present. To do this, we consider the underlying principles that underpin diagnostic systems, describe the ways in which these systems can be applied to community characterization, and discuss the significance of the data generated. We propose that at all locations within the human body where infection is routinely initiated within the context of a community of microbes, the same principles will apply. To consider this further, we take insights from areas such as the gut, oral cavity and skin. The main focus here is understanding respiratory tract infection, and specifically the infections of the cystic fibrosis lung. The impact that the use of culture-independent, molecular analyses will have on the way we approach the treatment of infections is also considered.

Interspecies chemical communication in bacterial development

Our view of bacteria, from the earliest observations through the heyday of antibiotic discovery, has shifted dramatically. We recognize communities of bacteria as integral and functionally important components of diverse habitats, ranging from soil collectives to the human microbiome. To function as productive communities, bacteria coordinate metabolic functions, often requiring shifts in growth and development. The hallmark of cellular development, which we characterize as physiological change in response to environmental stimuli, is a defining feature of many bacterial interspecies interactions. Bacterial communities rely on chemical exchanges to provide the cues for developmental change. Traditional methods in microbiology focus on isolation and characterization of bacteria in monoculture, separating the organisms from the surroundings in which interspecies chemical communication has relevance. Developing multispecies experimental systems that incorporate knowledge of bacterial physiology and metabolism with insights from biodiversity and metagenomics shows great promise for understanding interspecies chemical communication in the microbial world.

Defining the mode of action of tetramic acid antibacterials derived from Pseudomonas aeruginosa quorum sensing signals

In nature, bacteria rarely exist as single, isolated entities, but rather as communities comprised of many other species including higher host organisms. To survive in these competitive environments, microorganisms have developed elaborate tactics such as the formation of biofilms and the production of antimicrobial toxins. Recently, it was discovered that the gram-negative bacterium Pseudomonas aeruginosa , an opportunistic human pathogen, produces an antibiotic, 3-(1-hydroxydecylidene)-5-(2-hydroxyethyl)pyrrolidine-2,4-dione (C(12)-TA), derived from one of its quorum sensing molecules. Here, we present a comprehensive study of the expanded spectrum of C(12)-TA antibacterial activity against microbial competitors encountered by P. aeruginosa in nature as well as significant human pathogens. The mechanism of action of C(12)-TA was also elucidated, and C(12)-TA was found to dissipate both the membrane potential and the pH gradient of Gram-positive bacteria, correlating well with cell death. Notably, in stark contrast to its parent molecule 3-oxo-dodecanoyl homoserine lactone (3-oxo-C(12)-HSL), neither activation of cellular stress pathways nor cytotoxicity was observed in human cells treated with C(12)-TA. Our results suggest that the QS machinery of P. aeruginosa has evolved for a dual-function, both to signal others of the same species and also to defend against host immunity and competing bacteria. Because of the broad-spectrum antibacterial activity, established mode of action, lack of rapid resistance development, and tolerance by human cells, the C(12)-TA scaffold may also serve as a new lead compound for the development of antimicrobial therapeutics.

Small-colony variant selection as a survival strategy for Staphylococcus aureus in the presence of Pseudomonas aeruginosa

Previously it has been demonstrated that Staphylococcus aureus is sensitive toward Pseudomonas-secreted exotoxins, which preferentially target the electron transport chain in staphylococci. Here it is shown that a subpopulation of S. aureus survives these respiratory toxins of Pseudomonas aeruginosa by selection of the small-colony variant (SCV) phenotype. Purified pyocyanin alone causes the same effect. A hemB mutant of S. aureus survives cocultivation with P. aeruginosa without a decrease in CFU.

Quorum sensing and environmental adaptation in Pseudomonas aeruginosa: a tale of regulatory networks and multifunctional signal molecules

Bacteria employ sophisticated cell-to-cell communication or 'quorum sensing' (QS) systems for promoting collective behaviours that depend on the actions of one or more chemically distinct diffusible signal molecules. As determinants of cell population density, multiple QS systems are often integrated with each other and within global regulatory networks and subject to the prevailing environmental conditions as well as the presence and activities of other organisms. QS signal molecules, although largely considered as effectors of QS-dependent gene expression are also emerging as multifunctional molecules that influence life, development and death in single and mixed microbial populations and impact significantly the outcome of host-pathogen interactions.

New developments in microbial interspecies signaling

There is a growing appreciation that in addition to well-documented intraspecies quorum sensing systems, small molecules act as signals between microbes of different species. This review will focus on how bacterial small molecules modulate these interspecies interactions. We will particularly emphasize complex relationships such as those between microbes and insects, interactions resulting in non-antagonistic outcomes (i.e. developmental and morphological processes), how co-culture can lead to the discovery of new small molecules, and the use of known compounds to evoke unexpected responses and mediate crosstalk between microbes.

The fruit fly as a meeting place for microbes

Many infectious diseases of humans are caused by polymicrobial communities, but there are few in vivo models to study such communities. In a recent issue of PLoS Pathogens, Sibley and colleagues (Sibley et al., 2008a) report the development of a fruit fly infection model to investigate polymicrobial interactions and their effects on the host.

Influence of the Pseudomonas quinolone signal on denitrification in Pseudomonas aeruginosa

Denitrification is a well-studied respiratory system that is also important in the biogeochemical nitrogen cycle. Environmental signals such as oxygen and N-oxides have been demonstrated to regulate denitrification, though how denitrification is regulated in a bacterial community remains obscure. Pseudomonas aeruginosa is a ubiquitous bacterium that controls numerous genes through cell-to-cell signals. The bacterium possesses at least two N-acyl-L-homoserine lactone (AHL) signals. In our previous study, these quorum-sensing signals controlled denitrification in P. aeruginosa. In addition to the AHL signals, a third cell-to-cell communication signal, 2-heptyl-3-hydroxy-4-quinolone, referred to as the Pseudomonas quinolone signal (PQS), has been characterized. In this study, we examined the effect of PQS on denitrification to obtain more insight into the respiratory regulation in a bacterial community. Denitrification in P. aeruginosa was repressed by PQS, which was partially mediated by PqsR and PqsE. Measuring the denitrifying enzyme activities indicated that nitrite reductase activity was increased by PQS, whereas PQS inhibited nitric oxide reductase and the nitrate-respiratory chain activities. This is the first report to demonstrate that PQS influences enzyme activities, suggesting this effect is not specific to P. aeruginosa. Furthermore, when iron was supplied to the PQS-added medium, denitrifying activity was almost restored, indicating that the iron chelating property of PQS affected denitrification. Thus, our data indicate that PQS regulates denitrification primarily through iron chelation. The PQS effect on denitrification was relevant in a condition where oxygen was limited and denitrification was induced, suggesting its role in controlling denitrification where oxygen is present.

Burkholderia pseudomallei, B. thailandensis, and B. ambifaria produce 4-hydroxy-2-alkylquinoline analogues with a methyl group at the 3 position that is required for quorum-sensing regulation

4-Hydroxy-2-alkylquinolines (HAQs), especially 3,4-dihydroxy-2-heptylquinoline (Pseudomonas quinolone signal) and its precursor, 4-hydroxy-2-heptylquinoline, are attracting much attention, mainly because of their role as signaling molecules in Pseudomonas aeruginosa. The pqsABCDE operon is centrally involved in their biosynthesis. The presence of a homologous operon in Burkholderia pseudomallei and B. thailandensis was recently reported. Thus, we have investigated the abilities of 11 Burkholderia species to produce HAQ-like molecules by liquid chromatography/mass spectrometry. We have identified 29 different HAQ derivatives produced by the only three Burkholderia species where a pqsABCDE homologue was found among available sequenced Burkholderia species genomes, including B. ambifaria, a member of the Burkholderia cepacia complex. In contrast with those of P. aeruginosa, Burkholderia HAQs typically bear a methyl group, hence their designation as 4-hydroxy-3-methyl-2-alkylquinolines (HMAQs). We identified three families of HMAQs with a saturated or unsaturated alkyl chain at the 2' position, in contrast with the 1' position of P. aeruginosa, including one with an N-oxide group. Furthermore, the operon in these species contains two more genes downstream of the pqsE homologue, resulting in the hmqABCDEFG operon. While the inactivation of hmqA inhibits the production of HMAQs, the methylation of the quinoline ring requires a putative methyltransferase encoded by hmqG. Interestingly, hmqA or hmqG mutations increase the production of acyl homoserine lactones and, consequently, phenotypes under the control of quorum sensing in B. ambifaria: antifungal activity, siderophore production, and proteolytic activity. These results indicate that only HAQs bearing a methyl group (HMAQs) are involved in quorum-sensing regulation.

Quorum sensing by 2-alkyl-4-quinolones in Pseudomonas aeruginosa and other bacterial species

Pseudomonas aeruginosa produces the cell-to-cell signal molecule 2-heptyl-3-hydroxy-4-quinolone (The Pseudomonas quinolone signal; PQS), which is integrated within a complicated quorum sensing signaling system. PQS belongs to the family of 2-alkyl-4-quinolones (AQs), which have been previously described for their antimicrobial activities. PQS is synthesized via the pqsABCDE operon which is responsible for generating multiple AQs including 2-heptyl-4-quinolone (HHQ), the immediate PQS precursor. In addition, PQS signaling plays an important role in P. aeruginosa pathogenesis because it regulates the production of diverse virulence factors including elastase, pyocyanin and LecA lectin in addition to affecting biofilm formation. Here, we summarize the most recent findings on the biosynthesis and regulation of PQS and other AQs including the discovery of AQs in other bacterial species.

The social behaviours of bacterial pathogens

INTRODUCTION

The term quorum sensing (QS) is used to describe communication between bacterial cells, whereby a coordinated population response is controlled by diffusible signal molecules produced by individuals.

SOURCES OF DATA

Studies on QS-mediated signalling processes in bacteria have revealed the existence of intricate regulatory networks to enable bacterial populations to fine tune their responses to environmental changes and increase their chances of survival, using complex signalling pathways.

AREAS OF AGREEMENT

A population of bacteria invading a host may benefit from the coordinated release of virulence determinants and in vitro studies have shown that QS regulates virulence factor production in many species of bacteria.

AREAS OF CONTROVERSY

However, the role of QS in vivo is less well understood, but has been demonstrated to be important in several pathogenic organisms.

GROWING POINTS AND AREAS TIMELY FOR DEVELOPING RESEARCH

There is a growing interest in blocking bacterial cell-cell communication as a means to control infections. This review discusses QS from a pathogenic perspective and discusses the potential of QS as an anti-pathogenic target.

Towards understanding Pseudomonas aeruginosa burn wound infections by profiling gene expression

Pseudomonas aeruginosa is a key opportunistic pathogen causing severe acute and chronic nosocomial infections in immunocompromised or catheterized patients. It is prevalent in burn wound infections and it is generally multi-drug resistant. Understanding the genetic programs underlying infection is essential to develop highly needed new strategies for prevention and therapy. This work reviews expression profiling efforts conducted worldwide towards gaining insights into pathogenesis by P. aeruginosa, in particular in burn wounds. Work on various infection models, including the burned mouse model, has identified several direct virulence factors and elucidated their mode of action. In vivo gene expression experiments using In vivo Expression Technology (IVET) ascertained distinct regulatory circuits and traits that have helped explain P. aeruginosa s success as a general pathogen. The sequencing of the whole genome from a number of P. aeruginosa strains and the construction of genome-wide microarrays have paved the road to the several insightful studies on the (interacting) traits underlying infection. A series of in vitro and initial in vivo gene expression studies revealed specific traits pivotal for infection, such as quorum sensing systems, iron acquisition and oxidative stress responses, and toxin production among others. The data sets obtained from global transcriptional profiling provide insights that will be essential for the development of new targets and options for prevention and intervention.

Update on the epidemiology and management of Staphylococcus aureus, including methicillin-resistant Staphylococcus aureus, in patients with cystic fibrosis

PURPOSE OF REVIEW

Staphylococcus aureus is one of the first and most common pathogens to be isolated from the respiratory tract of patients with cystic fibrosis. The prevalence of respiratory tract colonization/infection with both methicillin-susceptible and methicillin-resistant S. aureus has increased over the past decade. The clinical significance of colonization/infection with these pathogens is variable, leading to numerous therapeutic strategies: primary prophylaxis, eradication, treatment of cystic fiboris pulmonary exacerbations, and treatment of methicillin-resistant S. aureus.

RECENT FINDINGS

Studies have demonstrated increased prevalence of S. aureus in clinical laboratories that use selective media. Additionally, small colony variant S. aureus has been associated with persistent infection, co-infection with Pseudomonas aeruginosa, and frequent courses of antibiotics, but this phenotype may be difficult to identify in clinical laboratories. Increased prevalence of methicillin-resistant S. aureus has led to use of oral and inhaled antibiotics in attempts to eradicate this pathogen; these studies have yielded variable results.

SUMMARY

The epidemiology of S. aureus in cystic fibrosis has changed. Studies are needed to assess the clinical significance of the increased prevalence of both methicillin-susceptible and methicillin-resistant S. aureus, and whether primary prophylaxis or new treatment/eradication protocols are effective.

PqsA is required for the biosynthesis of 2,4-dihydroxyquinoline (DHQ), a newly identified metabolite produced by Pseudomonas aeruginosa and Burkholderia thailandensis

A new metabolite, 2,4-dihydroxyquinoline (DHQ), was identified in cultures of the bacteria Pseudomonas aeruginosa and Burkholderia thailandensis. We found that the biosynthesis of DHQ correlates with the presence of a functional PqsA, which is a product of the pqsABCDE operon responsible for the synthesis of 4-hydroxy-2-alkylquinolines (HAQs) in P. aeruginosa. However, DHQ is not a degradation product or precursor of HAQs. This finding sheds some light on the poorly understood biosynthesis pathway of HAQs, which includes important communication signals regulating the expression of virulence factors.