Transcriptional profiling of Pseudomonas aeruginosa mature single- and dual-species biofilms in response to meropenem.

Pseudomonas aeruginosa is a Gram-negative opportunistic pathogen frequently isolated from chronic infections of the cystic fibrosis lung and burn wounds, and is a major cause of antimicrobial-resistant nosocomial infections. P. aeruginosa is frequently co-isolated with the opportunistic fungal pathogen Candida albicans, with the presence of C. albicans in dual-species biofilms promoting tolerance to meropenem. Here, transcription profiling of mature P. aeruginosa single- or dual-species biofilms was carried out to understand the molecular mechanism(s) by which C. albicans enhances meropenem tolerance. C. albicans appeared to have a mild impact on the transcriptome of P. aeruginosa mature biofilms, with most differentially regulated genes being involved in interkingdom interactions (i.e. quorum sensing and phenazine biosynthesis). The addition of meropenem to mature single- or dual-species biofilms resulted in a significant bacterial transcriptional response, including the induction of the beta-lactamase, ampC, genes involved in biofilm formation. P. aeruginosa elicited a similar transcriptional response to meropenem in the presence of C. albicans, but C. albicans promoted the expression of additional efflux pumps, which could play roles in increasing the tolerance of P. aeruginosa to meropenem.

Pseudomonas aeruginosa increases the susceptibility of Candida albicans to amphotericin B in dual-species biofilms.

BACKGROUND: Biofilms are the leading cause of nosocomial infections and are hard to eradicate due to their inherent antimicrobial resistance. Candida albicans is the leading cause of nosocomial fungal infections and is frequently co-isolated with the bacterium Pseudomonas aeruginosa from biofilms in the cystic fibrosis lung and severe burn wounds. The presence of C. albicans in multispecies biofilms is associated with enhanced antibacterial resistance, which is largely mediated through fungal extracellular carbohydrates sequestering the antibiotics. However, significantly less is known regarding the impact of polymicrobial biofilms on antifungal resistance. RESULTS: Here we show that, in dual-species biofilms, P. aeruginosa enhances the susceptibility of C. albicans to amphotericin B, an effect that was biofilm specific. Transcriptional analysis combined with gene ontology enrichment analysis identified several C. albicans processes associated with oxidative stress to be differentially regulated in dual-species biofilms, suggesting that P. aeruginosa exerts oxidative stress on C. albicans, likely through the secretion of phenazines. However, the mitochondrial superoxide dismutase SOD2 was significantly down-regulated in the presence of P. aeruginosa. Monospecies biofilms of the sod2Δ mutant were more susceptible to amphotericin B, and the susceptibility of these biofilms was further enhanced by exogenous phenazines. CONCLUSIONS: We propose that in dual-species biofilms, P. aeruginosa simultaneously induces mitochondrial oxidative stress, while down-regulating key detoxification enzymes, which prevents C. albicans mounting an appropriate oxidative stress response to amphotericin B, leading to fungal cell death. This work highlights the importance of understanding the impact of polymicrobial interactions on antimicrobial susceptibility.

Candida albicans enhances meropenem tolerance of Pseudomonas aeruginosa in a dual-species biofilm.

BACKGROUND: Pseudomonas aeruginosa is an opportunistic bacterium that infects the airways of cystic fibrosis patients, surfaces of surgical and burn wounds, and indwelling medical devices. Patients are prone to secondary fungal infections, with Candida albicans being commonly co-isolated with P. aeruginosa. Both P. aeruginosa and C. albicans are able to form extensive biofilms on the surfaces of mucosa and medical devices. OBJECTIVES: To determine whether the presence of C. albicans enhances antibiotic tolerance of P. aeruginosa in a dual-species biofilm. METHODS: Single- and dual-species biofilms were established in microtitre plates and the survival of each species was measured following treatment with clinically relevant antibiotics. Scanning electron microscopy and confocal microscopy were used to visualize biofilm structure. RESULTS: C. albicans enhances P. aeruginosa biofilm tolerance to meropenem at the clinically relevant concentration of 5 mg/L. This effect is specific to biofilm cultures and is dependent upon C. albicans extracellular matrix polysaccharides, mannan and glucan, with C. albicans cells deficient in glycosylation structures not enhancing P. aeruginosa tolerance to meropenem. CONCLUSIONS: We propose that fungal mannan and glucan secreted into the extracellular matrix of P. aeruginosa/C. albicans dual-species biofilms play a central role in enhancing P. aeruginosa tolerance to meropenem, which has direct implications for the treatment of coinfected patients.

Characterizing the Mechanisms of Nonopsonic Uptake of Cryptococci by Macrophages.

The pathogenic fungus Cryptococcus enters the human host via inhalation into the lung and is able to reside in a niche environment that is serum- (opsonin) limiting. Little is known about the mechanism by which nonopsonic phagocytosis occurs via phagocytes in such situations. Using a combination of soluble inhibitors of phagocytic receptors and macrophages derived from knockout mice and human volunteers, we show that uptake of nonopsonized Cryptococcus neoformans and C. gattii via the mannose receptor is dependent on macrophage activation by cytokines. However, although uptake of C. neoformans is via both dectin-1 and dectin-2, C. gattii uptake occurs largely via dectin-1. Interestingly, dectin inhibitors also blocked phagocytosis of unopsonized Cryptococci in wax moth (Galleria mellonella) larvae and partially protected the larvae from infection by both fungi, supporting a key role for host phagocytes in augmenting early disease establishment. Finally, we demonstrated that internalization of nonopsonized Cryptococci is not accompanied by the nuclear translocation of NF-κB or its concomitant production of proinflammatory cytokines such as TNF-α. Thus, nonopsonized Cryptococci are recognized by mammalian phagocytes in a manner that minimizes proinflammatory cytokine production and potentially facilitates fungal pathogenesis.

Adaptation of Candida albicans to environmental pH induces cell wall remodelling and enhances innate immune recognition.

Candida albicans is able to proliferate in environments that vary dramatically in ambient pH, a trait required for colonising niches such as the stomach, vaginal mucosal and the GI tract. Here we show that growth in acidic environments involves cell wall remodelling which results in enhanced chitin and ß-glucan exposure at the cell wall periphery. Unmasking of the underlying immuno-stimulatory ß-glucan in acidic environments enhanced innate immune recognition of C. albicans by macrophages and neutrophils, and induced a stronger proinflammatory cytokine response, driven through the C-type lectin-like receptor, Dectin-1. This enhanced inflammatory response resulted in significant recruitment of neutrophils in an intraperitoneal model of infection, a hallmark of symptomatic vaginal colonisation. Enhanced chitin exposure resulted from reduced expression of the cell wall chitinase Cht2, via a Bcr1-Rim101 dependent signalling cascade, while increased ß-glucan exposure was regulated via a non-canonical signalling pathway. We propose that this "unmasking" of the cell wall may induce non-protective hyper activation of the immune system during growth in acidic niches, and may attribute to symptomatic vaginal infection.

Host Sensing by Pathogenic Fungi.

The ability to cause disease extends from the ability to grow within the host environment. The human host provides a dynamic environment to which fungal pathogens must adapt to in order to survive. The ability to grow under a particular condition (i.e., the ability to grow at mammalian body temperature) is considered a fitness attribute and is essential for growth within the human host. On the other hand, some environmental conditions activate signaling mechanisms resulting in the expression of virulence factors, which aid pathogenicity. Therefore, pathogenic fungi have evolved fitness and virulence attributes to enable them to colonize and infect humans. This review highlights how some of the major pathogenic fungi respond and adapt to key environmental signals within the human host.

Dressed to impress: impact of environmental adaptation on the Candida albicans cell wall.

Candida albicans is an opportunistic fungal pathogen of humans causing superficial mucosal infections and life-threatening systemic disease. The fungal cell wall is the first point of contact between the invading pathogen and the host innate immune system. As a result, the polysaccharides that comprise the cell wall act as pathogen associated molecular patterns, which govern the host-pathogen interaction. The cell wall is dynamic and responsive to changes in the external environment. Therefore, the host environment plays a critical role in regulating the host-pathogen interaction through modulation of the fungal cell wall. This review focuses on how environmental adaptation modulates the cell wall structure and composition, and the subsequent impact this has on the innate immune recognition of C. albicans.

Noisy neighbourhoods: quorum sensing in fungal-polymicrobial infections.

Quorum sensing was once considered a way in which a species was able to sense its cell density and regulate gene expression accordingly. However, it is now becoming apparent that multiple microbes can sense particular quorum-sensing molecules, enabling them to sense and respond to other microbes in their neighbourhood. Such interactions are significant within the context of polymicrobial disease, in which the competition or cooperation of microbes can alter disease progression. Fungi comprise a small but important component of the human microbiome and are in constant contact with bacteria and viruses. The discovery of quorum-sensing pathways in fungi has led to the characterization of a number of interkingdom quorum-sensing interactions. Here, we review the recent developments in quorum sensing in medically important fungi, and the implications these interactions have on the host's innate immune response.

The Mnn2 mannosyltransferase family modulates mannoprotein fibril length, immune recognition and virulence of Candida albicans.

The fungal cell wall is the first point of interaction between an invading fungal pathogen and the host immune system. The outer layer of the cell wall is comprised of GPI anchored proteins, which are post-translationally modified by both N- and O-linked glycans. These glycans are important pathogen associated molecular patterns (PAMPs) recognised by the innate immune system. Glycan synthesis is mediated by a series of glycosyl transferases, located in the endoplasmic reticulum and Golgi apparatus. Mnn2 is responsible for the addition of the initial α1,2-mannose residue onto the α1,6-mannose backbone, forming the N-mannan outer chain branches. In Candida albicans, the MNN2 gene family is comprised of six members (MNN2, MNN21, MNN22, MNN23, MNN24 and MNN26). Using a series of single, double, triple, quintuple and sextuple mutants, we show, for the first time, that addition of α1,2-mannose is required for stabilisation of the α1,6-mannose backbone and hence regulates mannan fibril length. Sequential deletion of members of the MNN2 gene family resulted in the synthesis of lower molecular weight, less complex and more uniform N-glycans, with the sextuple mutant displaying only un-substituted α1,6-mannose. TEM images confirmed that the sextuple mutant was completely devoid of the outer mannan fibril layer, while deletion of two MNN2 orthologues resulted in short mannan fibrils. These changes in cell wall architecture correlated with decreased proinflammatory cytokine induction from monocytes and a decrease in fungal virulence in two animal models. Therefore, α1,2-mannose of N-mannan is important for both immune recognition and virulence of C. albicans.

Mannosylation in Candida albicans: role in cell wall function and immune recognition.

The fungal cell wall is a dynamic organelle required for cell shape, protection against the environment and, in pathogenic species, recognition by the innate immune system. The outer layer of the cell wall is comprised of glycosylated mannoproteins with the majority of these post-translational modifications being the addition of O- and N-linked mannosides. These polysaccharides are exposed on the outer surface of the fungal cell wall and are, therefore, the first point of contact between the fungus and the host immune system. This review focuses on O- and N-linked mannan biosynthesis in the fungal pathogen Candida albicans and highlights new insights gained from the characterization of mannosylation mutants into the role of these cell wall components in host-fungus interactions. In addition, we discuss the use of fungal mannan as a diagnostic marker of fungal disease.

CO(2) acts as a signalling molecule in populations of the fungal pathogen Candida albicans.

When colonising host-niches or non-animated medical devices, individual cells of the fungal pathogen Candida albicans expand into significant biomasses. Here we show that within such biomasses, fungal metabolically generated CO(2) acts as a communication molecule promoting the switch from yeast to filamentous growth essential for C. albicans pathology. We find that CO(2)-mediated intra-colony signalling involves the adenylyl cyclase protein (Cyr1p), a multi-sensor recently found to coordinate fungal responses to serum and bacterial peptidoglycan. We further identify Lys 1373 as essential for CO(2)/bicarbonate regulation of Cyr1p. Disruption of the CO(2)/bicarbonate receptor-site interferes selectively with C. albicans filamentation within fungal biomasses. Comparisons between the Drosophila melanogaster infection model and the mouse model of disseminated candidiasis, suggest that metabolic CO(2) sensing may be important for initial colonisation and epithelial invasion. Our results reveal the existence of a gaseous Candida signalling pathway and its molecular mechanism and provide insights into an evolutionary conserved CO(2)-signalling system.

The quorum-sensing molecules farnesol/homoserine lactone and dodecanol operate via distinct modes of action in Candida albicans.

Living as a commensal, Candida albicans must adapt and respond to environmental cues generated by the mammalian host and by microbes comprising the natural flora. These signals have opposing effects on C. albicans, with host cues promoting the yeast-to-hyphal transition and bacteria-derived quorum-sensing molecules inhibiting hyphal development. Hyphal development is regulated through modulation of the cyclic AMP (cAMP)/protein kinase A (PKA) signaling pathway, and it has been postulated that quorum-sensing molecules can affect filamentation by inhibiting the cAMP pathway. Here, we show that both farnesol and 3-oxo-C(12)-homoserine lactone, a quorum-sensing molecule secreted by Pseudomonas aeruginosa, block hyphal development by affecting cAMP signaling; they both directly inhibited the activity of the Candida adenylyl cyclase, Cyr1p. In contrast, the 12-carbon alcohol dodecanol appeared to modulate hyphal development and the cAMP signaling pathway without directly affecting the activity of Cyr1p. Instead, we show that dodecanol exerted its effects through a mechanism involving the C. albicans hyphal repressor, Sfl1p. Deletion of SFL1 did not affect the response to farnesol but did interfere with the response to dodecanol. Therefore, quorum sensing in C. albicans is mediated via multiple mechanisms of action. Interestingly, our experiments raise the possibility that the Burkholderia cenocepacia diffusible signal factor, BDSF, also mediates its effects via Sfl1p, suggesting that dodecanol's mode of action, but not farnesol or 3-oxo-C(12)-homoserine lactone, may be used by other quorum-sensing molecules.

Molecular and Microscopic Methods of Quantifying Candida albicans Cell Wall PAMP Exposure.

The cell wall of Candida albicans is a multilayered structure consisting of polysaccharides and proteins. The inner cell wall layer is comprised of chitin and ß1-3 and ß1-6-glucan which contribute to the overall shape and structure of the cell, while the outer layer of highly glycosylated mannoproteins provides key functional traits such as cell adhesion required for virulence. However, the cell wall is not a static structure but is constantly being remodeled in response to the external environment. Given that all of the cell wall components act as pathogen-associated molecular patterns (PAMPs) that are recognized by a variety of receptors on the surface of innate immune cells, remodeling of the cell wall can have a dramatic impact on the host-pathogen interaction. For example, during growth in standard media, C. albicans shields its major cell wall PAMPs from the innate immune system, but during growth in acidic environments as encountered during colonization of the female reproductive tract, key PAMPs become exposed on the fungal cell surface initiating a strong pro-inflammatory innate immune response. The impact of environmental adaptation on fungal cell wall remodeling, and the subsequent impact this has on the host-pathogen interaction, has been the subject of much research. In this chapter, we outline techniques to assess cell wall components in both resting and environmentally adapted C. albicans cells.

Post-transcriptional control of fungal cell wall synthesis.

Pathogenic fungi hide from their hosts by camouflage, obscuring immunogenic cell wall components such as beta-glucan with innocuous coverings such as mannoproteins and alpha-glucan that are less readily recognised by the host. Attempts to understand how such processes are regulated have met with varying success. Typically studies focus on understanding the transcriptional response of fungi to either their reservoir environment or the host. However, such approaches do not fully address this research question, due to the layers of post-transcriptional and post-translational regulation that occur within a cell. Although in animals the impact of post-transcriptional and post-translational regulation has been well characterised, our knowledge of these processes in the fungal kingdom is more limited. Mutations in RNA-binding proteins, like Ssd1 and Candida albicans Slr1, affect cell wall composition and fungal virulence indicating that post-transcriptional regulation plays a key role in these processes. Here, we review the current state of knowledge of fungal post-transcriptional regulation, and link this to potential mechanisms of immune evasion by drawing on studies from model yeast and plant pathogenic fungi. We highlight several RNA-binding proteins that regulate cell wall synthesis and could be involved in local translation of cell wall components. Expanding our knowledge on post-transcriptional regulation in human fungal pathogens is essential to fully comprehend fungal virulence strategies and for the design of novel antifungal therapies.

Estrogen promotes innate immune evasion of Candida albicans through inactivation of the alternative complement system.

Candida albicans is a commensal of the urogenital tract and the predominant cause of vulvovaginal candidiasis (VVC). Factors that increase circulatory estrogen levels such as pregnancy, the use of oral contraceptives, and hormone replacement therapy predispose women to VVC, but the reasons for this are largely unknown. Here, we investigate how adaptation of C. albicans to estrogen impacts the fungal host-pathogen interaction. Estrogen promotes fungal virulence by enabling C. albicans to avoid the actions of the innate immune system. Estrogen-induced innate immune evasion is mediated via inhibition of opsonophagocytosis through enhanced acquisition of the human complement regulatory protein, Factor H, on the fungal cell surface. Estrogen-induced accumulation of Factor H is dependent on the fungal cell surface protein Gpd2. The discovery of this hormone-sensing pathway might pave the way in explaining gender biases associated with fungal infections and may provide an alternative approach to improving women's health.

A multi-protein complex controls cAMP signalling and filamentation in the fungal pathogen Candida albicans.

Candida albicans is an opportunistic fungal pathogen of humans. The ability of the fungus to grow as both yeast and filamentous forms is essential for its pathogenicity. Morphogenesis of C. albicans is largely regulated through the secondary messenger cAMP, produced by the soluble adenylyl cyclase, Cyr1p. Recent evidence suggests that Cyr1p can be directly stimulated by environmental cues to increase cytoplasmic cAMP levels and thus promote hyphal development. In this issue of Molecular Microbiology, Zou et al. demonstrate that, in response to some environmental cues, Cyr1p functions as part of a tripartite complex additionally involving Cap1p and G-actin. All three proteins in the complex are required to raise cytosolic cAMP levels after stimulation with serum and bacterial peptidoglycan. The formation of such a complex highlights the importance of precise regulation of Cyr1p activity in response to host environmental cues.

Carbonic anhydrase inhibitors. Inhibition of the ß-class enzymes from the fungal pathogens Candida albicans and Cryptococcus neoformans with branched aliphatic/aromatic carboxylates and their derivatives.

The inhibition of the ß-carbonic anhydrases (CAs, EC 4.2.1.1) from the pathogenic fungi Cryptococcus neoformans (Can2) and Candida albicans (Nce103) with a series of 25 branched aliphatic and aromatic carboxylates has been investigated. Human isoforms hCA I and II were also included in the study for comparison. Aliphatic carboxylates were generally millimolar hCA I and II inhibitors and low micromolar/submicromolar ß-CA inhibitors. Aromatic carboxylates were micromolar inhibitors of the four enzymes but some of them showed low nanomolar activity against the fungal pathogenic enzymes. 4-Hydroxy- and 4-methoxy-benzoate inhibited Can2 with K(I)s of 9.5-9.9 nM. The methyl esters, hydroxamates, hydrazides and carboxamides of some of these derivatives were also effective inhibitors of the α- and ß-CAs investigated here.

Immune Sensing of Candida albicans.

Candida albicans infections range from superficial to systemic and are one of the leading causes of fungus-associated nosocomial infections. The innate immune responses during these various infection types differ, suggesting that the host environment plays a key role in modulating the host-pathogen interaction. In addition, C. albicans is able to remodel its cell wall in response to environmental conditions to evade host clearance mechanisms and establish infection in niches, such as the oral and vaginal mucosa. Phagocytes play a key role in clearing C. albicans, which is primarily mediated by Pathogen Associated Molecular Pattern (PAMP)-Pattern Recognition Receptor (PRR) interactions. PRRs such as Dectin-1, DC-SIGN, and TLR2 and TLR4 interact with PAMPs such as ß-glucans, N-mannan and O-mannan, respectively, to trigger the activation of innate immune cells. Innate immune cells exhibit distinct yet overlapping repertoires of PAMPs, resulting in the preferential recognition of particular Candida morphotypes by them. The role of phagocytes in the context of individual infection types also differs, with neutrophils playing a prominent role in kidney infections, and dendritic cells playing a prominent role in skin infections. In this review, we provide an overview of the key receptors involved in the detection of C. albicans and discuss the differential innate immune responses to C. albicans seen in different infection types such as vulvovaginal candidiasis (VVC) and oral candidiasis.

Carbonic anhydrase inhibitors. The beta-carbonic anhydrases from the fungal pathogens Cryptococcus neoformans and Candida albicans are strongly inhibited by substituted-phenyl-1H-indole-5-sulfonamides.

A series of 2-(hydrazinocarbonyl)-3-substituted-phenyl-1H-indole-5-sulfonamides and 1-({[5-(aminosulfonyl)-3-phenyl-1H-indol-2-yl]carbonyl}amino)-2,4,6 trimethylpyridinium perchlorates possessing various 2-, 3- or 4-substituted phenyl groups with methyl-, halogeno- and methoxy-functionalities, as well as the perfluorophenyl moiety, have been evaluated as inhibitors of the beta-carbonic anhydrases (CAs, EC 4.2.1.1) from the pathogenic fungi Cryptococcus neoformans (Can2) and Candida albicans (CaNce103). Both enzymes were potently inhibited by these sulfonamides, K(I)s in the range of 4.4-118 nM against Can2, and of 5.1-128 against CaNce103, respectively. Minor structural changes in the 3-substituted phenyl moiety contribute significantly to the inhibitory activity. Some of the investigated sulfonamides showed promising selectivity ratios for inhibiting Can2 over the host, human enzymes CA I and II.

Carbonic anhydrase activators: activation of the beta-carbonic anhydrases from the pathogenic fungi Candida albicans and Cryptococcus neoformans with amines and amino acids.

The proteins encoded by the Nce103 genes of Candida albicans and Cryptococcus neoformans are catalytically active beta-carbonic anhydrases (CAs, EC 4.2.1.1) playing various roles in the life cycle of these fungal pathogens, such as CO(2) sensing, regulation of capsule biosynthesis, filamentation, and adaptation of the organism to various pH and CO(2) conditions in various niches where the fungi grow. Here, we report the first activation study of these two enzymes, CaNce103 and Can2, respectively, with amines and amino acids. The C. albicans enzyme, CaNce103 was activated by amino acids such as L-/D-His, L-D-Trp, L-Tyr with K(A)s in the range of 19.5-46 microM. More effective activators were some amines such as histamine, dopamine, 2-aminoethyl-piperazine, and L-adrenaline (K(A)s of 13.2-18.5 microM). The best CaNce103 activators were L- and D-Dopa, with K(A)s of 0.96-2.5 microM. The C. neoformans enzyme, Can2, showed much lower propensity to be activated by all these amino acids and amines, which had activation constants in the range of 28.7-47.2 microM. The best Can2 activator was L-Trp. This study may help to better understand the catalytic/activation mechanisms of the beta-CAs and eventually to design CA activity modulators of such widespread enzymes in pathogenic fungi.