Candida biofilms and their role in infection

In vitro activity of 2-cyclohexylidenhydrazo-4-phenyl-thiazole compared with those of amphotericin B and fluconazole against clinical isolates of Candida spp. and fluconazole-resistant Candida albicans

OBJECTIVES

The aim of this study was to investigate the in vitro antifungal activity of an isothiosemicarbazone cyclic analogue against isolates of Candida spp. including fluconazole-resistant Candida albicans.

METHODS

We investigated the activity of 2-cyclohexylidenhydrazo-4-phenyl-thiazole (EM-01D2) against 114 clinical isolates of Candida spp., representing five different species, by microdilution, according to the NCCLS method 27-A. The activity against C. albicans biofilms was also investigated. Toxicity in vitro was evaluated by MTT reduction assay.

RESULTS

EM-01D2 demonstrated low toxicity, broad spectrum, fungicidal activity and was active against C. albicans and Candida krusei at concentrations lower than those shown by amphotericin B and fluconazole (P < 0.05). It maintained potent in vitro activity against fluconazole-resistant C. albicans isolates. Fungicidal activity occurred at concentrations 1-2 doubling dilutions greater than the corresponding MICs, and time-kill analysis indicated that a 99.9% loss of C. albicans viability occurred after 6 h of incubation in the presence of EM-01D2 at concentrations equal to four times the MIC. EM-01D2 was also active in inhibiting the growth of C. albicans ATCC 10231 biofilms, even though such inhibition occurred at concentrations higher than the MICs determined under planktonic growth conditions. However, when C. albicans biofilms were pre-exposed to subinhibitory concentrations of EM-01D2, a reduction of MIC50 of amphotericin B was observed.

CONCLUSIONS

Based on these results, EM-01D2 could represent a template for the development of novel fungicidal agents.

Comprehensive Proteomic Analysis of Saccharomyces cerevisiae Cell Walls

The cell wall of yeast contains proteins that are covalently bound to the glycan network. These cell wall proteins (CWPs) mediate cell-cell interactions and may be involved in cell wall biosynthesis. Using tandem mass spectrometry, we have identified 19 covalently bound CWPs of Saccharomyces cerevisiae. Twelve of them are shown for the first time to be covalently incorporated into the cell wall. The identified proteins include 12 predicted glycosylphosphatidylinositol-modified CWPs, all four members of the Pir protein family, and three additional proteins (Scw4p, Scw10p, and Tos1p) that are, like Pir proteins, connected to the cell wall glycan network via an alkali-sensitive linkage. However, Scw4p, Scw10p, and Tos1p do not contain internal repeat sequences shown to be essential for Pir protein incorporation and may represent a separate class of CWPs. Strikingly, seven of the identified proteins (Gas1p, Gas3p, Gas5p, Crh1p, Utr2p, Scw4p, and Scw10p) are classified as glycoside hydrolases. Phenotypic analysis of deletion mutants lacking the corresponding CWP-encoding genes indicated that most of them have altered cell wall properties, which reinforces the importance of the identified proteins for proper cell wall formation. In particular, gas1Delta and ecm33Delta were highly sensitive to Calcofluor White and high temperature, whereas gas1Delta, scw4Delta, and tos1Delta were highly resistant to incubation with beta-1,3-glucanase. The CWP identification method developed here relies on directly generating tryptic peptides from isolated cell walls and is independent of the nature of the covalent linkages between CWPs and cell wall glycans. Therefore, it will probably be equally effective in many other fungi.

Infections associated with medical devices: pathogenesis, management and prophylaxis

The insertion or implantation of foreign bodies has become an indispensable part in almost all fields of medicine. However, medical devices are associated with a definitive risk of bacterial and fungal infections. Foreign body-related infections (FBRIs), particularly catheter-related infections, significantly contribute to the increasing problem of nosocomial infections. While a variety of micro-organisms may be involved as pathogens, staphylococci account for the majority of FBRIs. Their ability to adhere to materials and to promote formation of a biofilm is the most important feature of their pathogenicity. This biofilm on the surface of colonised foreign bodies is regarded as the biological correlative for the clinical experience with FBRI, that is, that the host defence mechanisms often seem to be unable to handle the infection and, in particular, to eliminate the micro-organisms from the infected device. Since antibacterial chemotherapy is also frequently not able to cure these infections despite the use of antibacterials with proven in vitro activity, removal of implanted devices is often inevitable and has been standard clinical practice. However, in specific circumstances, such as infections of implanted medical devices with coagulase-negative staphylococci, a trial of salvage of the device may be justified. All FBRIs should be treated with antibacterials to which the pathogens have been shown to be susceptible. In addition to systemic antibacterial therapy, an intraluminal application of antibacterial agents, referred to as the 'antibiotic-lock' technique, should be considered to circumvent the need for removal, especially in patients with implanted long-term catheters. To reduce the incidence of intravascular catheter-related bloodstream infections, specific guidelines comprising both technological and nontechnological strategies for prevention have been established. Quality assurance, continuing education, choice of the catheter insertion site, hand hygiene and aseptic techniques are aspects of particular interest. Furthermore, all steps in the pathogenesis of biofilm formation may represent targets against which prevention strategies may be directed. Alteration of the foreign body material surface may lead to a change in specific and nonspecific interactions with micro-organisms and, thus, to a reduced microbial adherence. Medical devices made out of a material that would be antiadhesive or at least colonisation resistant would be the most suitable candidates to avoid colonisation and subsequent infection. Another concept for the prevention of FBRIs involves the impregnation of devices with various substances such as antibacterials, antiseptics and/or metals. Finally, further studies are needed to translate the knowledge on the mechanisms of biofilm formation into applicable therapeutic and preventive strategies.

Transcriptional profiling of Saccharomyces cerevisiae cells under adhesion-inducing conditions

The ability to adhere to other cells is one of the most prominent determinants of fungal pathogenicity. Thus, adherence of fungi to human tissues or plastics triggers hospital-acquired fungal infections, which are an increasing clinical problem, especially in immunocompromised persons. In the model fungus Saccharomyces cerevisiae adhesion can be induced by starvation for amino acids, and depends on the transcriptional activator of the general amino acid control system, Gcn4p. However, not much is known about the transcriptional program that mediates adhesive growth under such conditions. In this study, we present a genome-wide transcriptional analysis of Sigma1278b yeast cells that were subjected to adhesion-inducing conditions imposed by amino acid starvation. Twenty-two novel genes were identified as inducible by amino acid starvation; 72 genes belonging to different functional groups, which were not previously known to be regulated by Gcn4p, require Gcn4p for full transcriptional induction under adhesion-inducing conditions. In addition, several genes were identified in Sigma1278b cells that were inducible by amino acid starvation in a Gcn4p-independent manner. Our data suggest that adhesion of yeast cells induced by amino acid starvation is regulated by a complex, Sigma1278b-specific transcriptional response.

A contact-activated kinase signals Candida albicans invasive growth and biofilm development

For mammalian cells, contact-dependent regulatory controls are crucially important for controlling cellular proliferation and preventing diseases such as cancer. Candida albicans, an opportunistic fungal pathogen that normally resides within a mammalian host, also exhibits contact-dependent cellular behaviors such as invasive hyphal growth and biofilm development. Results reported here demonstrate that, in C. albicans, physical contact results in activation of the mitogen-activated protein kinase Mkc1p. This kinase is part of the fungal cell integrity pathway, a signal transduction pathway known to be activated by cell wall stress. It is demonstrated here that Mkc1p is required for invasive hyphal growth and normal biofilm development. Therefore, Mkc1p signaling contributes to contact-dependent regulation. Because responding to contact appropriately allows coordinated cellular behavior in a metazoan, commensal C. albicans cells behave like a part of the host, using contact-activated signaling to regulate fungal behavior.

cDNA microarray analysis of differential gene expression in Candida albicans biofilm exposed to farnesol

Candida albicans biofilms are structured microbial communities with high levels of drug resistance. Farnesol, a quorum-sensing molecule that inhibits hyphal formation in C. albicans, has been found to prevent biofilm formation by C. albicans. There is limited information, however, about the molecular mechanism of farnesol against biofilm formation. We used cDNA microarray analysis to identify the changes in the gene expression profile of a C. albicans biofilm inhibited by farnesol. Confocal scanning laser microscopy was used to visualize and confirm normal and farnesol-inhibited biofilms. A total of 274 genes were identified as responsive, with 104 genes up-regulated and 170 genes down-regulated. Independent reverse transcription-PCR analysis was used to confirm the important changes detected by microarray analysis. In addition to hyphal formation-associated genes (e.g., TUP1, CRK1, and PDE2), a number of other genes with roles related to drug resistance (e.g., FCR1 and PDR16), cell wall maintenance (e.g., CHT2 and CHT3), and iron transport (e.g., FTR2) were responsive, as were several genes encoding heat shock proteins (e.g., HSP70, HSP90, HSP104, CaMSI3, and SSA2). Further study of these differentially regulated genes is warranted to evaluate how they may be involved in C. albicans biofilm formation. Consistent with the down-regulation of the cell surface hydrophobicity-associated gene (CSH1), the water-hydrocarbon two-phase assay showed a decrease in cell surface hydrophobicity in the farnesol-treated group compared to that in the control group. Our data provide new insight into the molecular mechanism of farnesol against C. albicans biofilm formation.

Use of green fluorescent protein and reverse transcription-PCR to monitor Candida albicans agglutinin-like sequence gene expression in a murine model of disseminated candidiasis

Candida albicans PALS-green fluorescent protein (GFP) reporter strains were inoculated into mice in a disseminated candidiasis model, and GFP production was monitored by immunohistochemistry and reverse transcription-PCR (RT-PCR). GFP production from the ALS1 and ALS3 promoters was detected immunohistochemically. ALS1, ALS2, ALS3, ALS4, and ALS9 transcription was detected by RT-PCR, further identifying ALS genes expressed in this model.

High frequency of Candida parapsilosis on the hands of healthy hosts

The presence of yeasts on the hands of 86 healthy hosts (62 hospital workers and 24 healthy members of the community with no hospital exposure) was investigated. A high rate of colonization was found (59.3%). Candida parapsilosis was the most frequently isolated species (51%), independent of the origin of the samples. The potential virulence and resistance to antifungals of the 26 C. parapsilosis isolates were determined. All were proteinase producers and formed biofilms. The haemolytic activity was variable, with a predominance of total haemolysis of sheep erythrocytes. All isolates were susceptible to amphotericin B but two showed reduced susceptibility to fluconazole. Healthy people may be colonized by a species of yeast with a high capacity for adhesion to plastic surfaces, providing an infection risk to susceptible individuals.

Catheter-Related Bloodstream Infections in the Intensive Care Unit Population

Catheter-related bloodstream infections (CRBIs) are a cause of significant morbidity and mortality in intensive care unit patients. Development of CRBIs may occur by several mechanisms. The role of fibrin and biofilm development and their impact on therapy are described. Multiple preventative strategies related to the insertion and maintenance of the catheter site have been identified. Topical antisepsis and use of antibiotic-impregnated catheters are also beneficial for reducing the incidence of CRBI. Antibiotic lock solutionsmay be effective in preventing CRBI for select patients with longterm catheter use. Treatment of CRBIs should be based on suspected organisms, whichmost commonly include staphylococci, gram-negatives, including Pseudomonas aeruginosa andCandida species. Duration of treatment and decisions regarding catheter removal are controversial. Evaluation and treatment of potential complications of CRBIs, including endocarditis, infected thrombophlebitis, osteomyelitis, and endophthalmitis, are essential.

Candida albicans biofilms: a developmental state associated with specific and stable gene expression patterns

Like many bacteria, yeast species can form biofilms on several surfaces. Candida albicans colonizes the surfaces of catheters, prostheses, and epithelia, forming biofilms that are extremely resistant to antifungal drugs. We have used transcript profiling to investigate the specific properties of C. albicans biofilms. Biofilm and planktonic cultures produced under different conditions of nutrient flow, aerobiosis, or glucose concentration were compared by overall gene expression correlation. Correlation was much higher between biofilms than planktonic populations irrespective of the growth conditions, indicating that biofilm populations formed in different environments display very similar and specific transcript profiles. A first cluster of 325 differentially expressed genes was identified. In agreement with the overrepresentation of amino acid biosynthesis genes in this cluster, Gcn4p, a regulator of amino acid metabolism, was shown to be required for normal biofilm growth. To identify biofilm-related genes that are independent of mycelial development, we studied the transcriptome of biofilms produced by a wild-type, hypha-producing strain and a cph1/cph1 efg1/efg1 strain defective for hypha production. This analysis identified a cluster of 317 genes expressed independently of hypha formation, whereas 86 genes were dependent on mycelial development. Both sets revealed the activation of the sulfur-amino acid biosynthesis pathway as a feature of C. albicans biofilms.

Efficacy of antiseptics and disinfectants on clinical and environmental yeast isolates in planktonic and biofilm conditions

The aim of this study was to evaluate the efficacy of five antiseptics, three surface disinfectants and UV radiation against a wide range of clinical and environmental yeast isolates. Their efficacy against pure cultures, yeast mixtures and biofilms (prepared by culturing yeasts in Sabouraud broth containing a final concentration of 8% glucose) was tested. Three clinical isolates of Candida albicans, Cryptococcus neoformans and Rhodotorula rubra, and two environmental isolates of Candida albicans and Cryptococcus uniguttulatus were selected. For seven out of eight biocides tested (Betadine, Dermacide, Chlorhexidine, Dosisepsine, hydrogen peroxide, sodium hypochlorite, 70% alcohol, 0.5% Ecodiol) and for UV radiation, susceptibility did not differ according to genus, species or origin. Hydrogen peroxide, 0.25% Ecodiol and UV radiation were ineffective against the five isolates tested. On pure planktonic cultures, and, to a lesser extent, on free-living yeast mixtures, the other products were active and were then tested against biofilms: eight out of nine biocides were ineffective. Chlorhexidine at 0.5% was the only fungicidal agent on pure cultures, yeast mixtures and biofilms. The importance of the test method, including agent concentration, is discussed.

The Candida albicans CaACE2 gene affects morphogenesis, adherence and virulence

Morphogenesis between yeast and hyphal growth is a characteristic associated with virulence in Candida albicans and involves changes in the cell wall. In Saccharomyces cerevisiae, the transcription factor pair Ace2p and Swi5p are key regulators of cell wall metabolism. Here, we have characterized the CaACE2 gene, which encodes the only C. albicans homologue of S. cerevisiae ACE2 and SWI5. Deleting CaACE2 results in a defect in cell separation, increased invasion of solid agar medium and inappropriate pseudohyphal growth, even in the absence of external inducers. The mutant cells have reduced adherence to plastic surfaces and generate biofilms with distinctly different morphology from wild-type cells. They are also avirulent in a mouse model. Deleting CaACE2 has no effect on expression of the chitinase gene CHT2, but expression of CHT3 and the putative cell wall genes CaDSE1 and CaSCW11 is reduced in both yeast and hyphal forms. The CaAce2 protein is localized to the daughter nucleus of large budded cells at the end of mitosis. C. albicans Ace2p therefore plays a major role in morphogenesis and adherence and resembles S. cerevisiae Ace2p in function.

Adherence of Candida albicans to silicone induces immediate enhanced tolerance to fluconazole

Wild-type and efflux pump-deficient cells of Candida albicans adhering to silicone were compared with planktonic cells by flow cytometry for their relative resistance to fluconazole (FCZ). Flow cytometry data on cells carrying a fusion of green fluorescent protein to efflux pump promoters confirmed that enhanced tolerance of attached cells to FCZ was due in part to increased expression of CaMDR1 and CDR1 promoters. Within 2 h of their attachment to silicone, the adherent cells demonstrated levels of FCZ tolerance shown by cells from 24-h biofilms. Following their mechanical detachment, this subset of cells retained a four- to eightfold increase in tolerance compared with the tolerance of planktonic cells for at least two generations. Enhanced efflux pump tolerance to FCZ appeared to be induced within the initial 15 min of attachment in a subset of cells that were firmly attached to the substrata.

Penetration of Candida biofilms by antifungal agents

A filter disk assay was used to investigate the penetration of antifungal agents through biofilms containing single and mixed-species biofilms containing Candida. Fluconazole permeated all single-species Candida biofilms more rapidly than flucytosine. The rates of diffusion of either drug through biofilms of three strains of Candida albicans were similar. However, the rates of drug diffusion through biofilms of C. glabrata or C. krusei were faster than those through biofilms of C. parapsilosis or C. tropicalis. In all cases, after 3 to 6 h the drug concentration at the distal edge of the biofilm was very high (many times the MIC). Nevertheless, drug penetration failed to produce complete killing of biofilm cells. These results indicate that poor antifungal penetration is not a major drug resistance mechanism for Candida biofilms. The abilities of flucytosine, fluconazole, amphotericin B, and voriconazole to penetrate mixed-species biofilms containing C. albicans and Staphylococcus epidermidis (a slime-producing wild-type strain, RP62A, and a slime-negative mutant, M7) were also investigated. All four antifungal agents diffused very slowly through these mixed-species biofilms. In most cases, diffusion was slower with biofilms containing S. epidermidis RP62A, but amphotericin B penetrated biofilms containing the M7 mutant more slowly. However, the drug concentrations reaching the distal edges of the biofilms always substantially exceeded the MIC. Thus, although the presence of bacteria and bacterial matrix material undoubtedly retarded the diffusion of the antifungal agents, poor penetration does not account for the drug resistance of Candida biofilm cells, even in these mixed-species biofilms.

Developing animal models for polymicrobial diseases

Although polymicrobial diseases are not a new concept for microbiologists, they are experiencing a resurgence of interest owing to the development of suitable animal models and new molecular techniques that allow these diseases to be studied effectively. This broad review provides an excellent introduction to this fascinating topic.

Examples are included of each type of polymicrobial disease and the animal models that are used to study these diseases are discussed. In many instances, schematics for the animal model are presented.

Viral co-infections including bovine viral diarrhoeal viruses, porcine reproductive and respiratory syndrome, mixed hepatitis virus infections and HIV co-infection with hepatitis virus are discussed, together with attempts to model these diseases in animals.

Viral and bacterial co-infections are reviewed with a special focus on otitis media and the rodent models that have been used to probe this important childhood illness.

Of the polybacterial diseases, periodontitis is one of the best understood and a clinically relevant rodent model is now available. This model, and the role of biofilm formation in periodontitis are examined.

Fungal infections of humans are often referred to as 'opportunistic' but in fact these infections are often fungal co-infections with viruses such as HIV and fungal mixed co-infections. The roles of these infections in disease and the rodent models used to study them are discussed.

Parasite co-infections are thought to have a role in the severity of malaria and the severity of Lyme arthritis. These diseases and attempts to model them are evaluated.

Finally, co-infections that are associated with virus-induced immunosuppression are discussed, together with their animal models.

Polymicrobial diseases involve two or more microorganisms that act synergistically, or in succession, to mediate complex disease processes. Although polymicrobial diseases in animals and humans can be caused by similar organisms, these diseases are often also caused by organisms from different kingdoms, genera, species, strains, substrains and even by phenotypic variants of a single species. Animal models are often required to understand the mechanisms of pathogenesis, and to develop therapies and prevention regimes. However, reproducing polymicrobial diseases of humans in animal hosts presents significant challenges.

Antifungal combinations against Candida albicans biofilms in vitro

Candida biofilms display increased resistance to most antifungal agents. We have evaluated the efficacy of combinations of fluconazole (FLC), amphotericin B, and caspofungin (CSP) against Candida albicans biofilms in vitro. Indifference was observed for all the combinations of paired antifungal agents when a checkerboard titration method was used. Time-kill experiments revealed an antagonistic effect of high FLC doses with CSP.

Denture stomatitis: a role for Candida biofilms

OBJECTIVE

To assess the contribution of Candida biofilms to the etiology of denture stomatitis.

STUDY DESIGN

Samples of denture acrylic were retrieved from patients with denture stomatitis and subjected to scanning electron microscopy (SEM) analysis. Oral swab and swish samples were taken from the same group of patients and representative C albicans isolates recovered were used to investigate the kinetics of biofilm development in vitro.

RESULTS

Candida biofilms could be visualized by SEM directly from denture samples from patients with denture stomatitis. These biofilms showed a propensity to adhere along cracks and imperfections of the denture acrylic. C albicans clinical isolates were able to form biofilms in vitro, although differences in the extent of biofilm formation were observed for different isolates recovered from the same patient. Susceptibility testing indicated that the resulting biofilms showed increased resistance to antifungal treatment. Presence of serum and saliva conditioning films increased the initial adherence of selected isolates but had little effect in overall biofilm formation.

CONCLUSIONS

Candida biofilms play a role in denture stomatitis.

Candida infections of medical devices

The number of indwelling medical devices is escalating, and an increasing proportion of device-related infections are being caused by Candida spp. Candida spp. produce biofilms on synthetic materials, which facilitates adhesion of the organisms to devices and renders them relatively refractory to medical therapy. Management of device-related Candida infections can be challenging. Removal of the infected device is generally needed to establish cure of Candida infections of medical devices. However, since the pathogenesis of Candida bloodstream infection is complicated, more studies are necessary to determine the role of catheter exchange in patients with both gastrointestinal tract mucositis and indwelling catheters. The medical and economic impact of these infections is enormous.

Fungal biofilms and drug resistance

Candida species, including the novel opportunistic pathogen Candida dubliniensis, are now emerging as major agents of nosocomial infections. Many such manifestations of infections associated with the formation of Candida biofilms include those occurring on devices such as indwelling intravascular catheters. Fungal biofilm-associated infections are frequently refractory to conventional therapy because of resistance to antimicrobial agents. This resistance could be in part due to the surface-induced upregulation of drug efflux pumps. Biofilm-associated Candida show uniform resistance to a wide spectrum of the currently available conventional antifungal agents, which implies that antimicrobial drugs that specifically target biofilm-associated infections are needed. The novel classes of antifungal agents, the lipid formulation of amphotericins, and the echinocandins have demonstrated unique antifungal activity against the resistant Candida biofilms, providing a breakthrough in the treatment of life-threatening invasive systemic mycoses. The use of drugs effective in combating biofilm-associated infections could lead to major developments in the treatment of fungal implant infections.

Effects of aspirin and other nonsteroidal anti-inflammatory drugs on biofilms and planktonic cells of Candida albicans

Prostaglandins are now known to be produced by Candida albicans and may play an important role in fungal colonization. Their synthesis in mammalian cells is decreased by inhibitors of the cyclooxygenase isoenzymes required for prostaglandin formation. In the present study, a catheter disk model system was used to investigate the effects of nonsteroidal anti-inflammatory drugs (all cyclooxygenase inhibitors) on biofilm formation by three strains of C. albicans. Seven of nine drugs tested at a concentration of 1 mM inhibited biofilm formation. Aspirin, etodolac, and diclofenac produced the greatest effects, with aspirin causing up to 95% inhibition. Celecoxib, nimesulide, ibuprofen, and meloxicam also inhibited biofilm formation, but to a lesser extent. Aspirin was active against growing and fully mature (48-h) biofilms; its effect was dose related, and it produced significant inhibition (20 to 80%) at pharmacological concentrations. Simultaneous addition of prostaglandin E(2) abolished the inhibitory effect of 25 or 50 micro M aspirin. At 1 mM, aspirin reduced the viability of biofilm organisms to 1.9% of that of controls. Surviving cells had a wrinkled appearance, as judged by scanning electron microscopy, and consisted of both yeasts and hyphae. Treatment with other cyclooxygenase inhibitors, such as etodolac, resulted in biofilms that consisted almost entirely of yeast cells. In conventional assays for germ tube formation, these drugs produced significant inhibition, whereas aspirin had little effect. Our findings suggest that cyclooxygenase-dependent synthesis of fungal prostaglandin(s) is important for both biofilm development and morphogenesis in C. albicans and may act as a regulator in these physiological processes. Our results also demonstrate that aspirin possesses potent antibiofilm activity in vitro and could be useful in combined therapy with conventional antifungal agents in the management of some biofilm-associated Candida infections.

Influence of antifungal polyenes on the adhesion of Candida albicans and Candida glabrata to human epithelial cells in vitro

Candidal adherence to mucosal surfaces is considered as the first step in the pathogenesis of oral candidiasis. We examined the effect of antifungal polyenes, amphotericin B, nystatin and natamycin, at sublethal and minimum inhibitory concentrations (MICs) on the adherence of Candida albicans and Candida glabrata to HeLa cervical carcinoma and HSC-3 oral squamous cell carcinoma cells. A total of six oral Candida isolates were used throughout the study. Two Candida strains, C. albicans (44990) and C. glabrata (MYA-275) were obtained from ATCC. Four Candida strains, C. albicans 19 and 24 and C. glabrata 15 and 21, were isolated from patients with documented Candida-associated denture stomatitis. Cells were either incubated with Candida in the presence of the drug, or pre-incubated with yeasts and exposed subsequently to the drug. In the drug-free controls, the mean number of C. albicans yeasts associated with HeLa cells obtained from all experiments (130.1+/-10.1 yeasts/mm(2)) was significantly greater than that for HSC-3 cells (114.7+/-10.1 yeasts/mm(2); P<0.025). For C. glabrata, the mean adherence to HeLa and HSC-3 cells was 84.4+/-5.5 and 84.4+/-3.3 yeasts/mm(2), respectively, and these values were not statistically different (P>0.4). Candidal adherence was significantly reduced when the tested polyenes were present during the "adherence phase". The obtained values were significantly different from the controls, except for the effect of nystatin at the MIC on the adherence of C. glabrata strain MYA-275 to HeLa cells (P<0.375). Amphotericin B had the highest effect against both Candida species, reducing adherence by approximately 50 and approximately 60%, at the MIC and sublethal concentrations, respectively. The susceptibility of cell-associated Candida to polyenes was decreased markedly and the treatment did not result in significant detachment of adherent yeasts. The reduction in adherence was between 2 and 10%, when compared to the drug-free controls. These findings suggest that sub-therapeutic levels of polyenes that are likely to persist in the oral cavity following topical treatment may modulate candidal colonization when present during the "adherence phase".