Microbial biofilms: e pluribus unum

Glycosyl transferase GT2 genes mediate the biosynthesis of an unusual (1,3;1,4)-β-glucan exopolysaccharide in the bacterium Sarcina ventriculi

Linear, unbranched (1,3;1,4)-β-glucans (mixed-linkage glucans or MLGs) are commonly found in the cell walls of grasses, but have also been detected in basal land plants, algae, fungi and bacteria. Here we show that two family GT2 glycosyltransferases from the Gram-positive bacterium Sarcina ventriculi are capable of synthesizing MLGs. Immunotransmission electron microscopy demonstrates that MLG is secreted as an exopolysaccharide, where it may play a role in organizing individual cells into packets that are characteristic of Sarcina species. Heterologous expression of these two genes shows that they are capable of producing MLGs in planta, including an MLG that is chemically identical to the MLG secreted from S. ventriculi cells but which has regularly spaced (1,3)-β-linkages in a structure not reported previously for MLGs. The tandemly arranged, paralogous pair of genes are designated SvBmlgs1 and SvBmlgs2. The data indicate that MLG synthases have evolved different enzymic mechanisms for the incorporation of (1,3)-β- and (1,4)-β-glucosyl residues into a single polysaccharide chain. Amino acid variants associated with the evolutionary switch from (1,4)-β-glucan (cellulose) to MLG synthesis have been identified in the active site regions of the enzymes. The presence of MLG synthesis in bacteria could prove valuable for large-scale production of MLG for medical, food and beverage applications.

Sticking to the Subject: Multifunctionality in Microbial Adhesins

Bacterial and fungal adhesins mediate microbial aggregation, biofilm formation, and adhesion to host. We divide these proteins into two major classes: professional adhesins and moonlighting adhesins that have a non-adhesive activity that is evolutionarily conserved. A fundamental difference between the two classes is the dissociation rate. Whereas moonlighters, including cytoplasmic enzymes and chaperones, can bind with high affinity, they usually dissociate quickly. Professional adhesins often have unusually long dissociation rates: minutes or hours. Each adhesin has at least three activities: cell surface association, binding to a ligand or adhesive partner protein, and as a microbial surface pattern for host recognition. We briefly discuss Bacillus subtilis TasA, pilin adhesins, gram positive MSCRAMMs, and yeast mating adhesins, lectins and flocculins, and Candida Awp and Als families. For these professional adhesins, multiple activities include binding to diverse ligands and binding partners, assembly into molecular complexes, maintenance of cell wall integrity, signaling for cellular differentiation in biofilms and in mating, surface amyloid formation, and anchorage of moonlighting adhesins. We summarize the structural features that lead to these diverse activities. We conclude that adhesins resemble other proteins with multiple activities, but they have unique structural features to facilitate multifunctionality.

Lactobacillus casei reduces the extracellular matrix components of fluconazole-susceptible Candida albicans biofilms

Fluconazole-sensitive (CaS) and -resistant (CaR) C. albicans were grown as single-species and dual-species biofilms with Lactobacillus casei (Lc) and Lactobacillus rhamnosus (Lr). Single-species Lc and Lr were also evaluated. Biofilm analysis included viable plate counts, the extracellular matrix components, biomass, and structural organization. Lc reduced the viability of CaS, water-soluble polysaccharides, and eDNA in CaS + Lc biofilm. Lc biofilm presented more eDNA than CaS. The total biomass of CaS + Lc biofilm was higher than the single-species biofilms. The viability of Lc and Lr was reduced by CaR dual-species biofilms. The total and insoluble biomass in CaS + Lr was higher than in single-species CaS biofilms. Lc hindered the growth of CaS, and their association hampered matrix components linked to the structural integrity of the biofilm. These findings allow understanding of how the implementation of probiotics influences the growth of C. albicans biofilms and thereby helps with the development of novel approaches to control these biofilms.

Revealing Candida glabrata biofilm matrix proteome: global characterization and pH response

Candida glabrata is a clinically relevant human pathogen with the ability to form high recalcitrant biofilms that contribute to the establishment and persistence of infection. A defining trait of biofilms is the auto-produced matrix, which is suggested to have structural, virulent and protective roles. Thus, elucidation of matrix components, their function and modulation by the host environment is crucial to disclose their role in C. glabrata pathogenesis. As a major step toward this end, this study aimed to reveal, for the first time, the matrix proteome of C. glabrata biofilms, to characterize it with bioinformatic tools and to study its modulation by the environmental pH (acidic and neutral). The results showed the presence of several pH-specific matrix proteins (51 acidic- and 206 neutral-specific) and also proteins commonly found at both pH conditions (236). Of note, several proteins related to mannan and β-glucan metabolism, which have a potential role in the delivery/organization of carbohydrates in the matrix, were found in both pH conditions but in much higher quantity under the neutral environment. Additionally, several virulence-related proteins, including epithelial adhesins, yapsins and moonlighting enzymes, were found among matrix proteins. Importantly, several proteins seem to have a non-canonical secretion pathway and Pdr1 was found to be a potential regulator of matrix proteome. Overall, this study indicates a relevant impact of environmental cues in the matrix proteome and provides a unique resource for further functional investigation of matrix proteins, contributing to the identification of potential targets for the development of new therapies against C. glabrata biofilms.

Biofilm of Candida albicans: formation, regulation and resistance

Candida albicans is the most common human fungal pathogen, causing infections that range from mucous membranes to systemic infections. The present article provides an overview of C. albicans, with the production of biofilms produced by this fungus, as well as reporting the classes of antifungals used to fight such infections, together with the resistance mechanisms to these drugs. Candida albicans is highly adaptable, enabling the transition from commensal to pathogen due to a repertoire of virulence factors. Specifically, the ability to change morphology and form biofilms is central to the pathogenesis of C. albicans. Indeed, most infections by this pathogen are associated with the formation of biofilms on surfaces of hosts or medical devices, causing high morbidity and mortality. Significantly, biofilms formed by C. albicans are inherently tolerant to antimicrobial therapy, so the susceptibility of C. albicans biofilms to current therapeutic agents remains low. Therefore, it is difficult to predict which molecules will emerge as new clinical antifungals. The biofilm formation of C. albicans has been causing impacts on susceptibility to antifungals, leading to resistance, which demonstrates the importance of research aimed at the prevention and control of these clinical microbial communities.

Candida tropicalis RON1 is required for hyphal formation, biofilm development, and virulence but is dispensable for N-acetylglucosamine catabolism

NDT80-like family genes are highly conserved across a large group of fungi, but the functions of each Ndt80 protein are diverse and have evolved differently among yeasts and pathogens. The unique NDT80 gene in budding yeast is required for sexual reproduction, whereas three NDT80-like genes, namely, NDT80, REP1, and RON1, found in Candida albicans exhibit distinct functions. Notably, it was suggested that REP1, rather than RON1, is required for N-acetylglucosamine (GlcNAc) catabolism. Although Candida tropicalis, a widely dispersed fungal pathogen in tropical and subtropical areas, is closely related to Candida albicans, its phenotypic, pathogenic and environmental adaptation characteristics are remarkably divergent. In this study, we focused on the Ron1 transcription factor in C. tropicalis. Protein alignment showed that C. tropicalis Ron1 (CtRon1) shares 39.7% identity with C. albicans Ron1 (CaRon1). Compared to the wild-type strain, the C. tropicalis ron1Δ strains exhibited normal growth in different carbon sources and had similar expression levels of several GlcNAc catabolic genes during GlcNAc treatment. In contrast, C. tropicalis REP1 is responsible for GlcNAc catabolism and is involved in GlcNAc catabolic gene expressions, similar to C. albicans Rep1. However, REP1 deletion strains in C. tropicalis promote hyphal development in GlcNAc with low glucose content. Interestingly, CtRON1, but not CaRON1, deletion mutants exhibited significantly impaired hyphal growth and biofilm formation. As expected, CtRON1 was required for full virulence. Together, the results of this study showed divergent functions of CtRon1 compared to CaRon1; CtRon1 plays a key role in yeast-hyphal dimorphism, biofilm formation and virulence.

LAY ABSTRACT

In this study, we identified the role of RON1, an NDT80-like gene, in Candida tropicalis. Unlike the gene in Candida albicans, our studies showed that RON1 is a key regulator of hyphal formation, biofilm development and virulence but is dispensable for N-acetylglucosamine catabolism in C. tropicalis.

Fluconazole impacts the extracellular matrix of fluconazole-susceptible and -resistant Candida albicans and Candida glabrata biofilms

Background: Fluconazole (FLZ) is a drug commonly used for the treatment of Candida infections. However, β-glucans in the extracellular matrices (ECMs) hinder FLZ penetration into Candida biofilms, while extracellular DNA (eDNA) contributes to the biofilm architecture and resistance. Methods: This study characterized biofilms of FLZ-sensitive (S) and -resistant (R) Candida albicans and Candida glabrata in the presence or absence of FLZ focusing on the ECM traits. Biofilms of C. albicans American Type Culture Collection (ATCC) 90028 (CaS), C. albicans ATCC 96901 (CaR), C. glabrata ATCC 2001 (CgS), and C. glabrata ATCC 200918 (CgR) were grown in RPMI medium with or without FLZ at 5× the minimum inhibitory concentration (37°C/48 h). Biofilms were assessed by colony-forming unit (CFU)/mL, biomass, and ECM components (alkali-soluble polysaccharides [ASP], water-soluble polysaccharides [WSP], eDNA, and proteins). Scanning electron microscopy (SEM) was also performed. Data were analyzed by parametric and nonparametric tests (α  =  0.05). Results: In biofilms, FLZ reduced the CFU/mL of all strains (p < 0.001), except for CaS (p = 0.937). However, the ASP quantity in CaS was significantly reduced by FLZ (p = 0.034), while the drug had no effect on the ASP levels in other strains (p > 0.05). Total biomasses and WSP were significantly reduced by FLZ in the ECM of all yeasts (p < 0.001), but levels of eDNA and proteins were unaffected (p > 0.05). FLZ affected the cell morphology and biofilm structure by hindering hyphae formation in CaS and CaR biofilms, by decreasing the number of cells in CgS and CgR biofilms, and by yielding sparsely spaced cell agglomerates on the substrate. Conclusion: FLZ impacts biofilms of C. albicans and C. glabrata as evident by reduced biomass. This reduced biomass coincided with lowered cell numbers and quantity of WSPs. Hyphal production by C. albicans was also reduced.

What We Do Not Know about Fungal Cell Adhesion Molecules

There has been extensive research on structure and function of fungal cell adhesion molecules, but the most of the work has been about adhesins in Candida albicans and Saccharomyces cerevisiae. These yeasts are members of a single ascomycete order, and adhesion molecules from the six other fungal phyla are only sparsely described in the literature. In these other phyla, most of the research is at the cellular level, rather than at the molecular level, so there has been little characterization of the adhesion molecules themselves. A catalog of known adhesins shows some common features: high Ser/Thr content, tandem repeats, N- and O-glycosylations, GPI anchors, dibasic sequence motifs, and potential amyloid-forming sequences. However, none of these features is universal. Known ligands include proteins and glycans on homologous cells and host cells. Existing and novel tools can exploit the availability of genome sequences to identify and characterize new fungal adhesins. These include bioinformatics tools and well-established yeast surface display models, which could be coupled with an adhesion substrate array. Thus, new knowledge could be exploited to answer key questions in fungal ecology, animal and plant pathogenesis, and roles of biofilms in infection and biomass turnover.

Inactivation of genes TEC1 and EFG1 in Candida albicans influences extracellular matrix composition and biofilm morphology

Background: Infections caused by Candida spp. have been associated with formation of a biofilm, i.e. a complex microstructure of cells adhering to a surface and embedded within an extracellular matrix (ECM). Methods: The ECMs of a wild-type (WT, SN425) and two Candida albicans mutant strains, Δ/Δ tec1 (CJN2330) and Δ/Δ efg1 (CJN2302), were evaluated. Colony-forming units (cfu), total biomass (mg), water-soluble polysaccharides (WSPs), alkali-soluble polysaccharides (ASPs), proteins (insoluble part of biofilms and matrix proteins), and extracellular DNA (eDNA) were quantified. Variable-pressure scanning electron microscopy and confocal scanning laser microscopy were performed. The biovolume (μm3/μm2) and maximum thickness (μm) of the biofilms were quantified using COMSTAT2. Results: ASP content was highest in WT (mean ± SD: 74.5 ± 22.0 µg), followed by Δ/Δ tec1 (44.0 ± 24.1 µg) and Δ/Δ efg1 (14.7 ± 5.0 µg). The protein correlated with ASPs (r = 0.666) and with matrix proteins (r = 0.670) in the WT strain. The population in Δ/Δ efg1 correlated with the protein (r = 0.734) and its biofilms exhibited the lowest biomass and biovolume, and maximum thickness. In Δ/Δ tec1, ASP correlated with eDNA (r = 0.678). Conclusion: ASP production may be linked to C. albicans cell filamentous morphology.

Soluble factors from biofilm of Candida albicans and Staphylococcus aureus promote cell death and inflammatory response

Background

The objective of this study was to better understand the effects of soluble factors from biofilm of single- and mixed-species Candida albicans (C. albicans) and methicillin-sensitive Staphylococcus aureus (MSSA) cultures after 36 h in culture on keratinocytes (NOK-si and HaCaT) and macrophages (J774A.1). Soluble factors from biofilms of C. albicans and MSSA were collected and incubated with keratinocytes and macrophages, which were subsequently evaluated by cell viability assays (MTT). Lactate dehydrogenase (LDH) enzyme release was measured to assess cell membrane damage to keratinocytes. Cells were analysed by brightfield microscopy after 2 and 24 h of exposure to the soluble factors from biofilm. Cell death was detected by labelling apoptotic cells with annexin V and necrotic cells with propidium iodide (PI) and was visualized via fluorescence microscopy. Soluble factors from biofilm were incubated with J774A.1 cells for 24 h; the subsequent production of NO and the cytokines IL-6 and TNF-α was measured by ELISA.

Results

The cell viability assays showed that the soluble factors of single-species C. albicans cultures were as toxic as the soluble factors from biofilm of mixed cultures, whereas the soluble factors of MSSA cultures were less toxic than those of C. albicans or mixed cultures. The soluble factors from biofilm of mixed cultures were the most toxic to the NOK-si and HaCaT cells, as confirmed by analyses of PI labelling and cell morphology. Soluble factors from biofilm of single-species MSSA and mixed-species cultures induced the production of IL-6, NO and TNF-α by J744A.1 macrophages. The production of IL-6 and NO induced by the soluble factors from biofilm of mixed cultures was lower than that induced by the soluble factors from biofilm of single-species MSSA cultures, whereas the soluble factors from biofilm of C. albicans cultures induced only low levels of NO.

Conclusions

Soluble factors from 36-h-old biofilm of C. albicans and MSSA cultures promoted cell death and inflammatory responses.

Co-immobilization of Palm and DNase I for the development of an effective anti-infective coating for catheter surfaces

Biomaterial-associated infections, in particular, catheter-associated infections (CAI) are a major problem in clinical practice due to their ability to resist antimicrobial treatment and the host immune system. This study aimed to co-immobilize the antimicrobial lipopeptide Palm and the enzyme DNase I to introduce both antimicrobial and anti-adhesive functionalities to polydimethylsiloxane (PDMS) material, using dopamine chemistry. Surface characterization confirmed the immobilization of both compounds and no leaching of Palm from the surfaces for up to 5days. Co-immobilization of both agents resulted in a bifunctional coating with excellent surface antimicrobial and anti-biofilm properties against both Staphylococcus aureus and Pseudomonas aeruginosa. The modified surfaces demonstrated superior biocompatibility. To better discriminate co-adhesion of both species on modified surfaces, PNA FISH (Fluorescence in situ hybridization using peptide nucleic acid probes) was employed, and results showed that P. aeruginosa was the dominant organism, with S. aureus adhering afterwards on P. aeruginosa agglomerates. Furthermore, Palm immobilization exhibited no propensity to develop bacterial resistance, as opposite to the immobilization of an antibiotic. The overall results highlighted that co-immobilization of Palm and DNase I holds great potential to be applied in the development of catheters.

STATEMENT OF SIGNIFICANCE

Catheter-associated infections (CAI) are the most common hospital-acquired infections worldwide. Several coating strategies have been proposed to fight these infections but most of them present some important limitations, including the emergence of resistant bacteria and toxicity concerns. The present work describes a two-step polydopamine-based surface modification strategy to successfully co-immobilize an antimicrobial peptide (Palm) and an enzyme targeting an important component of biofilm matrix (DNase I). This immobilization approach imparted polydimethylsiloxane surfaces with both anti-adhesive and antimicrobial properties against the adhesion of relevant bacteria as single and dual-species, with excellent stability and biocompatible and anti-biofilm properties, holding, therefore, great potential in the development of catheters able to prevent CAI.

Fungal morphogenetic changes inside the mammalian host

One of the main features of the majority of pathogenic fungi is the ability to switch between different types of morphological forms. These changes include the transition between cells of different shapes (such as the formation of pseudohyphae and hyphae), or the massive growth of the blastoconidia and formation of titan cells. Morphological changes occur during infection, and there is extensive evidence that they play a key role in processes required for disease, such as adhesion, invasion and dissemination, immune recognition evasion, and phagocytosis avoidance. In the present review, we will provide an overview of how morphological transitions contribute to the development of fungal disease, with special emphasis in two cases: Candida albicans as an example of yeast that switches between blastoconidia and filaments, and Cryptococcus neoformans as an example of a fungus that changes the size without modifying the shape of the cell.

The significance of Sarcina in routine surgical pathology practice

Sarcina was first described by Goodsir. The appearance of this bacterium is so characteristic that the diagnosis can be made on light microscopy. Although the original description of Sarcina was made more than 150 years ago, little is known about its role in various human diseases. This study was undertaken with the aim to analyze critically the reason for this sudden recent interest in human Sarcina infection. The results indicate that Sarcina is a histopathological marker of functional or anatomical causes of gastric stasis, and has a possible association with life-threatening emphysematous gastritis. Hence, its documentation in the final report is warranted as the patient might need further work-up.

Candida albicans biofilms: development, regulation, and molecular mechanisms

A major virulence attribute of Candida albicans is its ability to form biofilms, densely packed communities of cells adhered to a surface. These biofilms are intrinsically resistant to conventional antifungal therapeutics, the host immune system, and other environmental factors, making biofilm-associated infections a significant clinical challenge. Here, we review current knowledge on the development, regulation, and molecular mechanisms of C. albicans biofilms.

An expanded regulatory network temporally controls Candida albicans biofilm formation

Candida albicans biofilms are composed of highly adherent and densely arranged cells with properties distinct from those of free‐floating (planktonic) cells. These biofilms are a significant medical problem because they commonly form on implanted medical devices, are drug resistant and are difficult to remove. C. albicans biofilms are not static structures; rather they are dynamic and develop over time. Here we characterize gene expression in biofilms during their development, and by comparing them to multiple planktonic reference states, we identify patterns of gene expression relevant to biofilm formation. In particular, we document time‐dependent changes in genes involved in adhesion and metabolism, both of which are at the core of biofilm development. Additionally, we identify three new regulators of biofilm formation, Flo8, Gal4, and Rfx2, which play distinct roles during biofilm development over time. Flo8 is required for biofilm formation at all time points, and Gal4 and Rfx2 are needed for proper biofilm formation at intermediate time points.

A novel function for Hog1 stress-activated protein kinase in controlling white-opaque switching and mating in Candida albicans

Candida albicans is a commensal in heathy people but has the potential to become an opportunistic pathogen and is responsible for half of all clinical infections in immunocompromised patients. Central to understanding C. albicans behavior is the white-opaque phenotypic switch, in which cells can undergo an epigenetic transition between the white state and the opaque state. The phenotypic switch regulates multiple properties, including biofilm formation, virulence, mating, and fungus-host interactions. Switching between the white and opaque states is associated with many external stimuli, such as oxidative stress, pH, and N-acetylglucosamine, and is directly regulated by the Wor1 transcriptional circuit. The Hog1 stress-activated protein kinase (SAPK) pathway is recognized as the main pathway for adapting to environmental stress in C. albicans. In this work, we first show that loss of the HOG1 gene in A: / A: and α/α cells, but not A: /α cells, results in 100% white-to-opaque switching when cells are grown on synthetic medium, indicating that switching is repressed by the A1: /α2 heterodimer that represses WOR1 gene expression. Indeed, switching in the hog1Δ strain was dependent on the presence of WOR1, as a hog1Δ wor1Δ strain did not show switching to the opaque state. Deletion of PBS2 and SSK2 also resulted in C. albicans cells switching from white to opaque with 100% efficiency, indicating that the entire Hog1 SAPK pathway is involved in regulating this unique phenotypic transition. Interestingly, all Hog1 pathway mutants also caused defects in shmoo formation and mating efficiencies. Overall, this work reveals a novel role for the Hog1 SAPK pathway in regulating white-opaque switching and sexual behavior in C. albicans.

Methodologies to generate, extract, purify and fractionate yeast ECM for analytical use in proteomics and glycomics

BACKGROUND

In a multicellular organism, the extracellular matrix (ECM) provides a cell-supporting scaffold and helps maintaining the biophysical integrity of tissues and organs. At the same time it plays crucial roles in cellular communication and signalling, with implications in spatial organisation, motility and differentiation. Similarly, the presence of an ECM-like extracellular polymeric substance is known to support and protect bacterial and fungal multicellular aggregates, such as biofilms or colonies. However, the roles and composition of this microbial ECM are still poorly understood.

RESULTS

This work presents a protocol to produce S. cerevisiae and C. albicans ECM in an equally highly reproducible manner. Additionally, methodologies for the extraction and fractionation into protein and glycosidic analytical pure fractions were improved. These were subjected to analytical procedures, respectively SDS-PAGE, 2-DE, MALDI-TOF-MS and LC-MS/MS, and DAE and FPLC. Additional chemical methods were also used to test for uronic acids and sulphation.

CONCLUSIONS

The methodologies hereby presented were equally efficiently applied to extract high amounts of ECM material from S. cerevisiae and C. albicans mats, therefore showing their robustness and reproducibility for yECM molecular and structural characterization. yECM from S. cerevisiae and C. albicans displayed a different proteome and glycoside fractions. S. cerevisiae yECM presented two well-defined polysaccharides with different mass/charge, and C. albicans ECM presented a single different one. The chemical methods further suggested the presence of uronic acids, and chemical modification, possibly through sulphate substitution. All taken, the procedures herein described present the first sensible and concise approach to the molecular and chemical characterisation of the yeast ECM, opening the way to the in-depth study of the microbe multicellular aggregates structure and life-style.

From behavior to mechanisms: an integrative approach to the manipulation by a parasitic fungus (Ophiocordyceps unilateralis s.l.) of its host ants (Camponotus spp.)

Co-evolution of parasites and their hosts has led to certain parasites adaptively manipulating the behavior of their hosts. Although the number of examples from different taxa for this phenomenon is growing, the mechanisms underlying parasite-induced manipulation of hosts' behavior are still poorly understood. The development of laboratory infections integrating various disciplines within the life sciences is an important step in that direction. Here, we advocate for such an integrative approach using the parasitic fungi of the genus Ophiocordyceps that induce an adaptive biting behavior in Camponotus ants as an example. We emphasize the use of behavioral assays under controlled laboratory conditions, the importance of temporal aspects of the behavior (possibly involving the circadian clock), and the need to approach colonizing parasites as organizations with a division of labor.

Biological noise abatement: coordinating the responses of autonomous bacteria in a synthetic biofilm to a fluctuating environment using a stochastic bistable switch

Noise is inherent to single cell behavior. Its origins can be traced to the stochasticity associated with a few copies of genes and low concentrations of protein and ligands. We have studied the mechanisms by which the response of noisy elements can be entrained for biological signal processing. To elicit predictable biological function, we have engineered a gene environment that incorporates a gene regulatory network with the stringently controlled microenvironment found in a synthetic biofilm. The regulatory network leverages the positive feedback found in quorum-sensing regulatory components of the lux operon, which is used to coordinate cellular responses to environmental fluctuations. Accumulation of the Lux receptor in cells, resulting from autoregulation, confers a rapid response and enhanced sensitivity to the quorum-sensing molecule that is retained after cell division as epigenetic memory. The memory of the system channels stochastic noise into a coordinated response among quorum-sensing signal receivers in a synthetic biofilm in which the noise diminishes with repeated exposure to noisy transmitters on the input of a signaling cascade integrated into the same biofilm. Thus, gene expression in the receivers, which are autonomous and do not communicate with each other, is synchronized to fluctuations in the environment.

Mini-review: Antimicrobial peptides and enzymes as promising candidates to functionalize biomaterial surfaces

Biomaterial-associated infections remain a serious concern in modern healthcare. The development of materials that can resist or prevent bacterial attachment constitutes a promising approach to dealing with this problem. Antimicrobial peptides (AMPs) and enzymes have been recognized as promising candidates for the new generation of antimicrobial surfaces. AMPs have been the focus of great interest in recent years owing to a low propensity for developing bacterial resistance, broad-spectrum activity, high efficacy at very low concentrations, target specificity, and synergistic action with classical antibiotics. Biofilm-dispersing enzymes have been shown to inhibit biofilm formation, detach established biofilm, and increase biofilm susceptibility to other antimicrobials. This review critically examines the potential of these protein-like compounds for developing antibacterial coatings by reporting their immobilization into different substrata using different immobilization strategies.

Between Amyloids and Aggregation Lies a Connection with Strength and Adhesion

We tell of a journey that led to discovery of amyloids formed by yeast cell adhesins and their importance in biofilms and host immunity. We begin with the identification of the adhesin functional amyloid-forming sequences that mediate fiber formation in vitro. Atomic force microscopy and confocal microscopy show 2-dimensional amyloid "nanodomains" on the surface of cells that are activated for adhesion. These nanodomains are arrays of adhesin molecules that bind multivalent ligands with high avidity. Nanodomains form when adhesin molecules are stretched in the AFM or under laminar flow. Treatment with antiamyloid perturbants or mutation of the amyloid sequence prevents adhesion nanodomain formation and activation. We are now discovering biological consequences. Adhesin nanodomains promote formation and maintenance of biofilms, which are microbial communities. Also, in abscesses within candidiasis patients, we find adhesin amyloids on the surface of the fungi. In both human infection and a Caenorhabditis elegans infection model, the presence of fungal surface amyloids elicits anti-inflammatory responses. Thus, this is a story of how fungal adhesins respond to extension forces through formation of cell surface amyloid nanodomains, with key consequences for biofilm formation and host responses.

Nonmammalian model systems to investigate fungal biofilms

Medical advances have resulted in an increase in the number of patients in immunocompromised states, vulnerable to infection, or individuals fitted with medical devices that form niches for microbial infections. These infections are difficult to treat and have significant morbidity and mortality rates. An important factor in the pathogenesis of fungal diseases is the development of biofilm-forming communities, enabling the invasion of host tissues and resistance to antimicrobial compounds. To investigate the genetic requirements for filamentation and seek compounds that inhibit the process, invertebrate hosts are employed as models of in vivo infection. The purpose of our review is to highlight methods that can be utilized to investigate fungal filamentation, an important step in the development of biofilms, in the invertebrate hosts Galleria mellonella, Caenorhabditis elegans, and Drosophila melanogaster.

Metabolomics reveals the heterogeneous secretome of two entomopathogenic fungi to ex vivo cultured insect tissues

Fungal entomopathogens rely on cellular heterogeneity during the different stages of insect host infection. Their pathogenicity is exhibited through the secretion of secondary metabolites, which implies that the infection life history of this group of environmentally important fungi can be revealed using metabolomics. Here metabolomic analysis in combination with ex vivo insect tissue culturing shows that two generalist isolates of the genus Metarhizium and Beauveria, commonly used as biological pesticides, employ significantly different arrays of secondary metabolites during infectious and saprophytic growth. It also reveals that both fungi exhibit tissue specific strategies by a distinguishable metabolite secretion on the insect tissues tested in this study. In addition to showing the important heterogeneous nature of these two entomopathogens, this study also resulted in the discovery of several novel destruxins and beauverolides that have not been described before, most likely because previous surveys did not use insect tissues as a culturing system. While Beauveria secreted these cyclic depsipeptides when encountering live insect tissues, Metarhizium employed them primarily on dead tissue. This implies that, while these fungi employ comparable strategies when it comes to entomopathogenesis, there are most certainly significant differences at the molecular level that deserve to be studied.

Genetic control of conventional and pheromone-stimulated biofilm formation in Candida albicans

Candida albicans can stochastically switch between two phenotypes, white and opaque. Opaque cells are the sexually competent form of C. albicans and therefore undergo efficient polarized growth and mating in the presence of pheromone. In contrast, white cells cannot mate, but are induced – under a specialized set of conditions – to form biofilms in response to pheromone. In this work, we compare the genetic regulation of such “pheromone-stimulated” biofilms with that of “conventional” C. albicans biofilms. In particular, we examined a network of six transcriptional regulators (Bcr1, Brg1, Efg1, Tec1, Ndt80, and Rob1) that mediate conventional biofilm formation for their potential roles in pheromone-stimulated biofilm formation. We show that four of the six transcription factors (Bcr1, Brg1, Rob1, and Tec1) promote formation of both conventional and pheromone-stimulated biofilms, indicating they play general roles in cell cohesion and biofilm development. In addition, we identify the master transcriptional regulator of pheromone-stimulated biofilms as C. albicans Cph1, ortholog of Saccharomyces cerevisiae Ste12. Cph1 regulates mating in C. albicans opaque cells, and here we show that Cph1 is also essential for pheromone-stimulated biofilm formation in white cells. In contrast, Cph1 is dispensable for the formation of conventional biofilms. The regulation of pheromone- stimulated biofilm formation was further investigated by transcriptional profiling and genetic analyses. These studies identified 196 genes that are induced by pheromone signaling during biofilm formation. One of these genes, HGC1, is shown to be required for both conventional and pheromone-stimulated biofilm formation. Taken together, these observations compare and contrast the regulation of conventional and pheromone-stimulated biofilm formation in C. albicans, and demonstrate that Cph1 is required for the latter, but not the former.

Candida albicans is the predominant fungal pathogen afflicting humans, where many infections arise due to its proclivity to form biofilms. Biofilms are complex multicellular communities in which cells exhibit distinct properties to those grown in suspension. They are particularly relevant in the development of device-associated infections, and thus understanding biofilm regulation and biofilm architecture is a priority. C. albicans has the ability to form different types of biofilms under different environmental conditions. Here, we compare the regulation of biofilm formation in conventional biofilms, for which a core transcriptional network has recently been identified, with pheromone-stimulated biofilms, which occur when C. albicans white cells are exposed to pheromone. Our studies show that several regulatory components control biofilm formation under both conditions, including the network transcriptional regulators Bcr1, Brg1, Rob1, and Tec1. However, other transcriptional regulators are specific to each model of biofilm development. In particular, we demonstrate that Cph1, the master regulator of the pheromone response during mating, is essential for pheromone-stimulated biofilm formation but is dispensable for conventional biofilms. These studies provide an in-depth analysis of the regulation of pheromone-stimulated biofilms, and demonstrate that both shared and unique components operate in different models of biofilm formation in this human pathogen.

Hyphal heterogeneity in Aspergillus oryzae is the result of dynamic closure of septa by Woronin bodies

Hyphae of higher fungi are compartmentalized by septa. These septa contain a central pore that allows for inter-compartmental and inter-hyphal cytoplasmic streaming. The cytoplasm within the mycelium is therefore considered to be a continuous system. In this study, however, we demonstrate by laser dissection that 40% of the apical septa of exploring hyphae of Aspergillus oryzae are closed. Closure of septa correlated with the presence of a peroxisome-derived organelle, known as Woronin body, near the septal pore. The location of Woronin bodies in the hyphae was dynamic and, as a result, plugging of the septal pore was reversible. Septal plugging was abolished in a ΔAohex1 strain that cannot form Woronin bodies. Notably, hyphal heterogeneity was also affected in the ΔAohex1 strain. Wild-type strains of A. oryzae showed heterogeneous distribution of GFP between neighbouring hyphae at the outer part of the colony when the reporter was expressed from the promoter of the glucoamylase gene glaA or the α-glucuronidase gene aguA. In contrast, GFP fluorescence showed a normal distribution in the case of the ΔAohex1 strain. Taken together, it is concluded that Woronin bodies maintain hyphal heterogeneity in a fungal mycelium by impeding cytoplasmic continuity.

Formation of lactic acid bacteria-yeasts communities on the olive surface during Spanish-style Manzanilla fermentations

This work examines the formation of poly-microbial communities adhered to the surface of Manzanilla olive fruits processed according to the Spanish style. The experimental design consisted of four pilot fermenters inoculated with four Lactobacillus pentosus strains, plus another fermenter which was not inoculated and fermented spontaneously. Lactic acid bacteria and yeasts were analysed in depth on olive epidermis throughout fermentation by plate count, molecular techniques and scanning electron microscopy. Data show that in all cases high population levels (above 8 log(10) CFU per olive) were reached for both groups of microorganisms at the second week of fermentation and that these counts never fell below 6 log(10) CFU per olive during the 3 months that fermenters were monitored. In situ observation of olive epidermis slices revealed a strong aggregation and adhesion between bacteria and yeasts by the formation of a matrix which embedded the microorganisms. Geotrichum candidum, Pichia galeiformis and Candida sorbosa were the main yeast species isolated from these biofilms at the end of fermentation (confirmed by RFLP analysis of the 5.8S-ITS region), while molecular characterization of lactobacilli isolates by means of RAPD-PCR with primer OPL(5) showed in many cases a high similarity in their banding profiles with the inoculated strains. Results obtained in this survey show the importance of studying the olive epidermis throughout fermentation, because ultimately, olives are ingested by consumers.

Biofilm formation on abiotic and biotic surfaces during Spanish style green table olive fermentation

In this work, the establishment of polymicrobial communities on the surfaces which come into contact with the brine during Spanish style Gordal cv. green olive fermentation when subjected to spontaneous or controlled processes (inoculated with Lactobacillus pentosus LPCO10 or 128/2) was studied. Scanning electron microscopy showed that L. pentosus and yeast populations were able to form mixed biofilms throughout the fermentation process on both abiotic (glass slide) and biotic (olive skin) surfaces. The biofilm architectures in both supports were completely different: on the glass slides only aggregates of L. pentosus and yeasts without any polymeric matrix surrounding them were found while on the skin of the fruits, true mature biofilms were observed. During fermentation, the lactic acid bacteria (LAB) population on the olives remained similar while that of yeasts increased progressively to reach similar levels at the end of the process (8-9 log CFU/cm(2)). Molecular analysis showed that different populations of L. pentosus and yeasts were the only microbial members of the biofilm formed during fermentation, regardless of inoculation. Hence, the green olive surface provides an appropriate environmental condition for the suitable development and formation of complex biofilms during controlled or natural table olive processing.

Single cell transcriptomics of neighboring hyphae of Aspergillus niger

Single cell profiling was performed to assess differences in RNA accumulation in neighboring hyphae of the fungus Aspergillus niger. A protocol was developed to isolate and amplify RNA from single hyphae or parts thereof. Microarray analysis resulted in a present call for 4 to 7% of the A. niger genes, of which 12% showed heterogeneous RNA levels. These genes belonged to a wide range of gene categories.

Alternative mating type configurations (a/α versus a/a or α/α) of Candida albicans result in alternative biofilms regulated by different pathways

Similar multicellular structures can evolve within the same organism that may have different evolutionary histories, be controlled by different regulatory pathways, and play similar but nonidentical roles. In the human fungal pathogen Candida albicans, a quite extraordinary example of this has occurred. Depending upon the configuration of the mating type locus (a/α versus a/a or α/α), C. albicans forms alternative biofilms that appear similar morphologically, but exhibit dramatically different characteristics and are regulated by distinctly different signal transduction pathways. Biofilms formed by a/α cells are impermeable to molecules in the size range of 300 Da to 140 kDa, are poorly penetrated by human polymorphonuclear leukocytes (PMNs), and are resistant to antifungals. In contrast, a/a or α/α biofilms are permeable to molecules in this size range, are readily penetrated by PMNs, and are susceptible to antifungals. By mutational analyses, a/α biofilms are demonstrated to be regulated by the Ras1/cAMP pathway that includes Ras1→Cdc35→cAMP(Pde2—|)→Tpk2(Tpk1)→Efg1→Tec1→Bcr1, and a/a biofilms by the MAP kinase pathway that includes Mfα→Ste2→ (Ste4, Ste18, Cag1)→Ste11→Hst7→Cek2(Cek1)→Tec1. These observations suggest the hypothesis that while the upstream portion of the newly evolved pathway regulating a/a and α/α cell biofilms was derived intact from the upstream portion of the conserved pheromone-regulated pathway for mating, the downstream portion was derived through modification of the downstream portion of the conserved pathway for a/α biofilm formation. C. albicans therefore forms two alternative biofilms depending upon mating configuration.

Single-celled microbes can form biofilms, or aggregates of cells that adhere to one another on a surface, in response to many environmental factors. Like many microbial pathogens, the yeast Candida albicans can form biofilms that normally provide protective environments against antifungals, antibodies, and white blood cells, thus ensuring higher rates of survival in response to assault by drugs or the human immune system. We report that while a majority (around 90%) of C. albicans strains form traditional biofilms that are impermeable to molecules of low and high molecular weight, and that are impenetrable to white blood cells, a minority (around 10%) form biofilms that are both permeable and penetrable. Formation of the minority-type alternative biofilms is dictated by a change at a single genetic locus, the mating type locus. Homozygous a/a or α/α cells are mating-competent, whereas the heterozygous a/α cells are mating-incompetent. Cells of the mating-incompetent a/α genotype form the impermeable, traditional biofilm, whereas the mating-competent a/a or α/α genotype forms the permeable biofilm. The characteristics of a/a and α/α biofilms are consistent with a suggested role in mating by facilitating the transfer of hormone signals through the permeable biofilm. The two types of biofilm are also regulated by different signal transduction pathways: the a/α form by the Ras1/cAMP pathway, and the a/a or α/α forms by the MAP kinase pathway. Components of the latter pathway suggest that its downstream portion evolved from the a/α pathway. C. albicans, therefore, forms two superficially similar biofilms, exhibiting very different permeability characteristics, regulated by different signal transduction pathways, dictated by different mating type locus configurations, and serving quite different purposes in its life history.

Bacterial lipopolysaccharides variably modulate in vitro biofilm formation of Candida species

The objective of this study was to evaluate the effect of the bacterial endotoxin LPS on Candida biofilm formation in vitro. The effect of the LPS of Pseudomonas aeruginosa, Klebsiella pneumoniae, Serratia marcescens and Salmonella typhimurium on six different species of Candida, comprising Candida albicans ATCC 90028, Candida glabrata ATCC 90030, Candida krusei ATCC 6258, Candida tropicalis ATCC 13803, Candida parapsilosis ATCC 22019 and Candida dubliniensis MYA 646, was studied using a standard biofilm assay. The metabolic activity of in vitro Candida biofilms treated with LPS at 90 min, 24 h and 48 h was quantified by XTT reduction assay. Viable biofilm-forming cells were qualitatively analysed using confocal laser scanning microscopy (CLSM), while scanning electron microscopy (SEM) was employed to visualize the biofilm structure. Initially, adhesion of C. albicans was significantly stimulated by Pseudomonas and Klebsiella LPS. A significant inhibition of Candida adhesion was noted for the following combinations: C. glabrata with Pseudomonas LPS, C. tropicalis with Serratia LPS, and C. glabrata, C. parapsilosis or C. dubliniensis with Salmonella LPS (P<0.05). After 24 h of incubation, a significant stimulation of initial colonization was noted for the following combinations: C. albicans/C. glabrata with Klebsiella LPS, C. glabrata/C. tropicalis/C. krusei with Salmonella LPS. In contrast, a significant inhibition of biofilm formation was observed in C. glabrata/C. dubliniensis/C. krusei with Pseudomonas LPS, C. krusei with Serratia LPS, C. dubliniensis with Klebsiella LPS and C. parapsilosis/C. dubliniensis /C. krusei with Salmonella LPS (P<0.05). On further incubation for 48 h, a significant enhancement of biofilm maturation was noted for the following combinations: C. glabrata/C. tropicalis with Serratia LPS, C. dubliniensis with Klebsiella LPS and C. glabrata with Salmonella LPS, and a significant retardation was noted for C. parapsilosis/C. dubliniensis/C. krusei with Pseudomonas LPS, C. tropicalis with Serratia LPS, C. glabrata/C. parapsilosis/C. dubliniensis with Klebsiella LPS and C. dubliniensis with Salmonella LPS (P<0.05). These findings were confirmed by SEM and CLSM analyses. In general, the inhibition of the biofilm development of LPS-treated Candida spp. was accompanied by a scanty architecture with a reduced numbers of cells compared with the profuse and densely colonized control biofilms. These data are indicative that bacterial LPSs modulate in vitro Candida biofilm formation in a species-specific and time-dependent manner. The clinical and the biological relevance of these findings have yet to be explored.

Structure and function of glycosylated tandem repeats from Candida albicans Als adhesins

Tandem repeat (TR) regions are common in yeast adhesins, but their structures are unknown, and their activities are poorly understood. TR regions in Candida albicans Als proteins are conserved glycosylated 36-residue sequences with cell-cell aggregation activity (J. M. Rauceo, R. De Armond, H. Otoo, P. C. Kahn, S. A. Klotz, N. K. Gaur, and P. N. Lipke, Eukaryot. Cell 5:1664-1673, 2006). Ab initio modeling with either Rosetta or LINUS generated consistent structures of three-stranded antiparallel beta-sheet domains, whereas randomly shuffled sequences with the same composition generated various structures with consistently higher energies. O- and N-glycosylation patterns showed that each TR domain had exposed hydrophobic surfaces surrounded by glycosylation sites. These structures are consistent with domain dimensions and stability measurements by atomic force microscopy (D. Alsteen, V. Dupres, S. A. Klotz, N. K. Gaur, P. N. Lipke, and Y. F. Dufrene, ACS Nano 3:1677-1682, 2009) and with circular dichroism determination of secondary structure and thermal stability. Functional assays showed that the hydrophobic surfaces of TR domains supported binding to polystyrene surfaces and other TR domains, leading to nonsaturable homophilic binding. The domain structures are like "classic" subunit interaction surfaces and can explain previously observed patterns of promiscuous interactions between TR domains in any Als proteins or between TR domains and surfaces of other proteins. Together, the modeling techniques and the supporting data lead to an approach that relates structure and function in many kinds of repeat domains in fungal adhesins.

The uronic acids assay: a method for the determination of chemical activity on biofilm EPS

In this work, the uronic acids assay was evaluated for its potential to function as a bioassay to screen for antagonistic activity against the production of microbial biofilm exopolysaccharide (EPS). The assay was first applied to biofilms produced in the presence of two universal disinfectants (sodium hypochlorite and sodium dodecyl sulfate) known to inhibit microbial growth and biofilm formation. The performance of the assay was then characterized through statistical assessment of threshold concentrations for disinfection efficiency and consistency relative to values reported in the literature. The assay was then evaluated for its utility in screening for enzymatic or chemical inhibitors of biofilm formation (eg glycosidases, halogenated furanones, and semi-crude fractions extracted from minimally fouled marine plants) and its ability to distinguish between true anti-biofilm activity and simple disinfection. Activity was characterized as (i) no effect, (ii) a true positive effect (ie increased biofilm EPS), (iii) anti-bacterial activity (ie decreased biofilm EPS and analogous decrease in planktonic growth), and (iv) anti-biofilm EPS activity (ie decreased biofilm EPS, without analogous decrease in planktonic growth). Results demonstrate that the uronic acids assay can augment existing biofilm characterization methods by providing a quantitative measure of biofilm EPS.

Presence of extracellular DNA in the Candida albicans biofilm matrix and its contribution to biofilms

DNA has been described as a structural component of the extracellular matrix (ECM) in bacterial biofilms. In Candida albicans, there is a scarce knowledge concerning the contribution of extracellular DNA (eDNA) to biofilm matrix and overall structure. This work examined the presence and quantified the amount of eDNA in C. albicans biofilm ECM and the effect of DNase treatment and the addition of exogenous DNA on C. albicans biofilm development as indicators of a role for eDNA in biofilm development. We were able to detect the accumulation of eDNA in biofilm ECM extracted from C. albicans biofilms formed under conditions of flow, although the quantity of eDNA detected differed according to growth conditions, in particular with regards to the medium used to grow the biofilms. Experiments with C. albicans biofilms formed statically using a microtiter plate model indicated that the addition of exogenous DNA (>160 ng/ml) increases biofilm biomass and, conversely, DNase treatment (>0.03 mg/ml) decreases biofilm biomass at later time points of biofilm development. We present evidence for the role of eDNA in C. albicans biofilm structure and formation, consistent with eDNA being a key element of the ECM in mature C. albicans biofilms and playing a predominant role in biofilm structural integrity and maintenance.

Escherichia coli and its lipopolysaccharide modulate in vitro Candida biofilm formation

Demystification of microbial behaviour in mixed biofilms could have a major impact on our understanding of infectious diseases. The objectives of this study were to evaluate in vitro the interactions of six different Candida species and a Gram-negative coliform, Escherichia coli, in dual-species biofilms, and to assess the effect of E. coli LPS on Candida biofilm formation. A single isolate of E. coli ATCC 25922 and six different species of Candida, Candida albicans ATCC 90028, Candida glabrata ATCC 90030, Candida krusei ATCC 6258, Candida tropicalis ATCC 13803, Candida parapsilosis ATCC 22019 and Candida dubliniensis MYA-646, were studied using a standard biofilm assay. Each Candida species was co-cultured with E. coli on a polystyrene surface and biofilm formation was quantified by a c.f.u. assay. The biofilm was then analysed by Live/Dead staining and fluorescence microscopy (confocal laser-scanning microscopy, CLSM), whilst scanning electron microscopy (SEM) was employed to visualize the biofilm architecture. The effect of E. coli LPS on Candida biofilm cell activity at defined time intervals was assessed with an XTT reduction assay. A significant quantitative reduction in c.f.u. counts of C. tropicalis (after 90 min), C. parapsilosis (after 90 min and 24 h), C. krusei (after 24 h) and C. dubliniensis (after 24 and 48 h) was noted on incubation with E. coli in comparison with their monospecies biofilm counterparts (P <0.05). On the other hand, a simultaneous and significant reduction in E. coli cell numbers occurred on co-culture with C. albicans (after 90 min), and an elevation of E. coli cell numbers followed co-culture with C. tropicalis (after 24 h) and C. dubliniensis (after 24 h and 48 h) (P <0.05). All quantitative findings were confirmed by SEM and CLSM analyses. By SEM observation, dual-species biofilms demonstrated scanty architecture with reduced visible cell counts at all stages of biofilm development, despite profuse growth and dense colonization in their single-species counterparts. Significantly elevated metabolic activity, as assessed by XTT readings, was observed in E. coli LPS-treated C. tropicalis and C. parapsilosis biofilms (after 48 h), whilst this had the opposite effect for C. dubliniensis (after 24 h) (P <0.05). These data indicate that E. coli and Candida species in a mixed-species environment mutually modulate biofilm development, both quantitatively and qualitatively, and that E. coli LPS appears to be a key component in mediating these outcomes.

Biofilm matrix regulation by Candida albicans Zap1

A biofilm is a surface-associated population of microorganisms embedded in a matrix of extracellular polymeric substances. Biofilms are a major natural growth form of microorganisms and the cause of pervasive device-associated infection. This report focuses on the biofilm matrix of Candida albicans, the major fungal pathogen of humans. We report here that the C. albicans zinc-response transcription factor Zap1 is a negative regulator of a major matrix component, soluble beta-1,3 glucan, in both in vitro and in vivo biofilm models. To understand the mechanistic relationship between Zap1 and matrix, we identified Zap1 target genes through expression profiling and full genome chromatin immunoprecipitation. On the basis of these results, we designed additional experiments showing that two glucoamylases, Gca1 and Gca2, have positive roles in matrix production and may function through hydrolysis of insoluble beta-1,3 glucan chains. We also show that a group of alcohol dehydrogenases Adh5, Csh1, and Ifd6 have roles in matrix production: Adh5 acts positively, and Csh1 and Ifd6, negatively. We propose that these alcohol dehydrogenases generate quorum-sensing aryl and acyl alcohols that in turn govern multiple events in biofilm maturation. Our findings define a novel regulatory circuit and its mechanism of control of a process central to infection.

Effect of tunicamycin on Candida albicans biofilm formation and maintenance

BACKGROUND

Candida albicans is a common opportunistic pathogen of the human body and is the frequent causative agent of candidiasis. Typically, these infections are associated with the formation of biofilms on both host tissues and implanted biomaterials. As a result of the intrinsic resistance of C. albicans biofilms to most antifungal agents, new strategies are needed to combat these infections.

METHODS

Here we have used a 96-well microtitre plate model of C. albicans biofilm formation to study the inhibitory effect of tunicamycin, a nucleoside antibiotic that inhibits N-linked glycosylation affecting cell wall and secreted proteins, on C. albicans biofilm formation. A proteomic approach was used to study the effect of tunicamycin on levels of glycosylation of key secreted mannoproteins in the biofilm matrix.

RESULTS

Our results revealed that physiological concentrations of tunicamycin displayed significant inhibitory effects on biofilm development and maintenance, while not affecting overall cell growth or morphology. However, tunicamycin exerted a minimal effect on fully mature, pre-formed C. albicans biofilms.

CONCLUSIONS

The effect of tunicamycin on the C. albicans biofilm mode of growth demonstrates the importance of N-linked glycosylation in the developmental stages of biofilm formation. In addition, our results indicate that N-linked glycosylation represents an attractive target for the development of alternative strategies for the prevention of biofilm formation by this important pathogenic fungus.

Complementary adhesin function in C. albicans biofilm formation

BACKGROUND

Biofilms are surface-associated microbial communities with significant environmental and medical impact. Here, we focus on an adherence mechanism that permits biofilm formation by Candida albicans, the major invasive fungal pathogen of humans.

RESULTS

The Als surface-protein family has been implicated in biofilm formation, and we show that Als1 and Als3 have critical but redundant roles. Overexpression of several other Als proteins permits biofilm formation in a biofilm-defective als1/als1 als3/als3 strain, thus arguing that the function of Als proteins in this process is governed by their respective expression levels. The surface protein Hwp1 is also required for biofilm formation, and we find that a mixture of biofilm-defective hwp1/hwp1 and als1/als1 als3/als3 strains can form a hybrid biofilm both in vitro and in vivo in a catheter infection model. Complementary function of Hwp1 and Als1 and 3 seems to reflect their interaction because expression of Hwp1 in the heterologous host S. cerevisiae permits adherence to wild-type C. albicans, but not to an als1/als1 als3/als3 strain.

CONCLUSIONS

The complementary roles of Hwp1 and Als1 and Als3 in biofilm formation are analogous to the roles of sexual agglutinins in mating reactions. This analogy suggests that biofilm-adhesin complementarity may promote formation of monospecies biofilms.