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Brogden KA, Guthmiller JM, editors. Polymicrobial Diseases. Washington (DC): ASM Press; 2002.

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Chapter 18Interactions between Candida Species and Bacteria in Mixed Infections

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Introduction

Candida albicans is an opportunistic fungal pathogen found as part of the normal microflora in the human digestive tract. It is just one of approximately 200 species in the genus Candida, but accounts for up to 75% of all candidal infections. In general, innate and acquired host defense mechanisms act in concert with the resident bacterial flora such that Candida organisms grow and survive as commensals. However, even a slight modification of the host defense system, or host ecological environment, can assist the transformation of C. albicans into a pathogen capable of causing infections that may be lethal. The most common body sites showing asymptomatic colonization by Candida are the oral cavity, rectum, and vagina. Oral swabs or rinses are positive for C. albicans in up to 40% of healthy adult subjects, while 20 to 25% of healthy women carry C. albicans in the vagina. Colonization by Candida is thought to occur at an early age, with the organisms being acquired during passage through the birth canal, during nursing, or from food. Long-term colonization is probably responsible for eliciting the circulating immunoglobulin G (IgG) and mucosal secretory immunoglobulin A (S-IgA) antibodies to C. albicans that are detectable in most healthy individuals. It is these acquired host responses, in conjunction with the anti-Candida activities of polymorphonuclear leukocytes and macrophages, that probably play a significant part in normally restricting C. albicans to superficial growth at mucosal sites.

C. albicans (and the closely related yeast Candida dubliniensis) is a dimorphic fungus, growing as an oval-shaped budding yeast (blastospore), or as pseudohyphae or true hyphae; both yeast and hyphal forms are usually found in infected tissues. The dimorphic transition is one of many characteristic properties of C. albicans associated with virulence. Other virulence factors include adhesins, which promote binding of Candida to host cells and tissues, hydrolytic enzymes such as proteinases that enhance adhesion and tissue destruction (32), and molecules such as CR3-like receptor and HSP 90 that modulate immune cell functions (31, 49). The development of pathogenicity is facilitated by endogenous physiological modifications of host immunity, by immunodeficiency diseases, or by iatrogenic factors. The latter include chemical and physical therapeutic techniques that weaken the body defenses at various levels and allow Candida to invade.

Although recent evidence suggests that some hospital-acquired (nosocomial) Candida infections may behave like minor epidemics with selection of more virulent strains (47), it is often the commensal (endogenous) organisms that are believed to be the initial sources of infection. However, it is important to recognize that C. albicans has the ability to live in harmony with the host, for a lifetime, within the resident complex microflora present on mucosal surfaces. In the oral cavity, C. albicans grows and survives by competing and cooperating with an estimated 300 or more species of bacteria. There is compelling evidence that C. albicans and C. dubliniensis form tight associations with specific oral bacterial species, and that these promote adhesion and colonization by mixed-species communities (41). Thus, when Candida infections arise, they often occur in association with bacteria. On the other hand, there is also strong evidence to suggest that components of the resident microflora, present in the oral cavity and at other mucosal sites, perform to check C. albicans growth. This is why factors that perturb the normal microflora, such as antibiotic therapy, or changes in hormonal or mucosal secretions, may encourage C. albicans overgrowth.

This chapter considers the etiology and pathology of some disease conditions that arise from, or involve directly, Candida interactions with bacteria. The clinical manifestations, and processes of adhesion and biofilm formation, are described for mixed-species infections, in particular those involving colonization of oral tissues and dental or medical prostheses by mixed communities of Candida and bacteria. Knowledge gained from studies of microbial colonization mechanisms in the laboratory and in vivo, and of disease mechanisms from model systems, should assist in the development of more effective methods for controlling or preventing Candida infections.

Types of Candida Infection

Infections caused by Candida may be superficial or systemic. Superficial infections of the cutaneous or mucocutaneous tissues include oropharyngeal candidiasis (involving the buccal mucosa, palate, and tongue), vaginitis, conjunctivitis, esophagitis, or gastrointestinal candidiasis. Systemic infections, which can be fatal and may involve multiple organs, include endocarditis, pyelonephritis, esophagitis, meningitis, and disseminated candidiasis (Candida septicemia).

Mucocutaneous candidiasis is observed in subjects with cellular immune deficiencies or who are immunosuppressed, have anemia or diabetes, or whose normal microflora is disturbed or suppressed. Oral thrush, a disease recognized in infants by Hippocrates, appears as soft creamy-colored plaques on the tongue and buccal mucosa. Thrush in the newborn or elderly may be related to inefficiency of the thymus, while adult males who develop thrush may be suspected of being infected with human immunodeficiency virus (HIV). Candida vulvovaginitis is often associated with pregnancy or contraceptive use and may be linked to modification of T-cell and neutrophil functions by progesterone. Primary or secondary defects in myeloid or lymphoid lineages generally facilitate development of Candida infections, while neutropenia is one of the main causes of systemic Candida proliferation.

Predisposing Factors

The frequency of systemic Candida infections has grown steadily during the past decade, and C. albicans, together with a few closely related species such as C. tropicalis, C. glabrata, and C. parapsilosis, is now recognized as an important nosocomial pathogen. This is due, at least in part, to an increase in invasive surgical techniques, the growing use of prosthetic devices, particularly intravascular catheters, and the development of new drug therapies. Surgical procedures themselves, with their associated physical and psychological stresses, may promote breakthrough growth of Candida. Catheters and other prostheses provide novel sites for colonization by C. albicans and other microorganisms as biofilms. Corticosteroid therapy affects neutrophil, macrophage, and T-cell activities, chemotherapy with cytotoxic drugs leads to depletion of leukocytes, and polyantibiotic treatment perturbs or suppresses the mucosal microflora. All of these various drug regimens have the potential to result in Candida infections. On the other hand, surgical damage, catheterization or other prosthetic implantation, and the wearing of prosthetic appliances such as dentures, may lead to infections that are polymicrobial in nature, often comprising C. albicans in association with one or more defined species of endogenous host bacteria. In addition, it is apparent that many other oral disease conditions, including various forms of human periodontal disease afflicting the tooth roots, pulp, and gums (gingivitis), may involve mixed-species infections of C. albicans and bacteria.

Candida–Mixed-Species Infections

Denture-Induced Stomatitis

This condition is essentially a candidal infection of the oral mucosa that is promoted by a close-fitting upper denture. The upper denture cuts off the underlying mucosa from the normal lubricatory and protective functions of saliva. In susceptible patients, denture stomatitis is seen as a symptomless area of erythema, always sharply limited to the area of mucosa occluded by the upper denture (Color Plate 6A [see color insert]). Such inflammation is not seen under a more mobile lower denture because the salivary flow is less restricted. The clinical picture is usually quite clear and diagnosis is confirmed by microscopic analysis of smears taken from the inflamed mucosa or from the fitting surface of the denture. Gram-stained smears typically show Candida hyphal forms, as well as yeasts that have proliferated between the denture base and mucosa. Histologically, there is usually a mild chronic inflammatory infiltrate, probably in response to secreted C. albicans virulence factors such as phospholipases and proteinases. However, the presence of heavy oral bacterial growth on the palatal mucosa and fitting surface of the denture, and of bacteria not usually recognized as components of the oral microbiota, e.g., Staphylococcus aureus and Escherichia coli, are likely to support inflammatory reactions in the palatal mucosa.

Figure plate 6. Clinical presentation of oral candidiasis involving mixed infections of Candida and bacteria.

Figure plate 6

Clinical presentation of oral candidiasis involving mixed infections of Candida and bacteria. (A) Denture-induced stomatitis, showing erythema of the mucosa that has been occluded by an upper denture. (B) Angular cheilitis, showing cracking and erythema (more...)

Angular Cheilitis

Angular cheilitis is a disease frequently associated with denture-induced stomatitis and is caused by leakage of Candida-infected saliva at the angles of the mouth. It is also a characteristic sign of oral candidosis in general and systemic disorders, including HIV infections; diabetes mellitus and skin diseases are common among recurrent angular cheilitis patients. In elderly patients with dentures, local factors, such as skin creased as a result of sagging of the facial tissues with age, promote fungal and bacterial growth in saliva-contaminated skin folds. Clinically, mild inflammation occurs at the angles of the mouth with cracking and erythema at the commissure (Color Plate 6B). The microflora in angular cheilitis usually involves C. albicans or other Candida species, and S. aureus that may act to preserve the cheilitis. Different species of bacteria including hemolytic streptococci, enterococci, E. coli, Klebsiella, and Pseudomonas may also play a role in the pathogenesis and maintenance of this labial lesion.

Gingivitis

Inflammation of the gingival tissues is usually related to plaque accumulation on the tooth surfaces and gingival margins caused by inadequate oral hygiene. Chronic gingivitis is considered to be antecedent to the loss of periodontal attachment (periodontitis) and involves a shift in the plaque microflora from predominantly gram-positive bacteria to a complex microflora comprising up to 50% gram-negative bacteria including Fusobacterium, Prevotella, and Treponema species. Other forms of gingivitis, that may involve acute inflammation with ulceration (acute ulcerative gingivitis [AUG]), are promoted by heavy cigarette smoking, emotional stress, or immunosuppression, e.g., HIV infection. Gingival disorders are also associated with infections with S. aureus, enteric bacteria, Pseudomonas, and Candida, and are usually related to suppression of the normal subgingival microflora by antibiotics. In addition, systemic conditions, e.g., immunosuppression, or locally compromised conditions, e.g., osseointegrated implants and membranes for guided tissue regeneration, may be predisposing factors for the establishment of these microorganisms that are not usually present at healthy sites around the teeth.

Periodontal Disease

Adult periodontitis is a condition in which few or all teeth in a dentition may be involved; individual teeth can show vertical or horizontal bone loss, and associated gingivitis can range from very slight inflammation to severe bleeding with pus formation. Adult periodontitis lesions show a high proportion of anaerobic bacteria, mainly gram-negative organisms and spirochetes. Within these populations, specific species complexes, e.g., Porphyromonas gingivalis, Bacteroides forsythus, and Treponema denticola, show especially strong relationships to active periodontitis (61). Cigarette smoking is a potential risk factor associated with progression of periodontal disease, and smokers with early-onset periodontitis harbor a greater number of pathogenic microorganisms in periodontal pockets (24). In addition to the common major periodontal pathogens, an increased incidence of E. coli, S. aureus, C. albicans, and Aspergillus fumigatus underlines the potentially damaging effects of cigarette smoking on host defenses (39).

Approximately 10% of adult periodontitis patients experience continued loss of periodontal attachment despite periodontal therapy. A wide range of anaerobic bacteria can be recovered from these refractory periodontitis cases, as well as staphylococci and C. albicans. Therapy may be unsuccessful because it is difficult to access pathogenic organisms that have invaded gingival tissues or root structures. In addition, C. albicans is most frequently isolated in mixed infections with oral streptococci, Peptostreptococcus micros and Fusobacterium nucleatum from endodontic samples of root canals in persistent endodontic infections. Thus, Candida probably has an important role in therapy-resistant apical periodontitis (66) and in polymicrobial infections of root canals with pulp necrosis (42).

Periodontitis in HIV-infected patients often exhibits rapid onset and progression, particularly if patients have experienced AUG, with attachment loss within 9 months for up to 75% of patients. Leukemia patients on immunosuppressive therapies and broad spectrum antibiotics are susceptible to rapidly progressive periodontitis and colonization by staphylococci, enteric rods, Pseudomonas, and Candida. In patients with acute leukemia these agents may seed from periodontal pockets into the bloodstream and induce life-threatening septicemia.

Prosthetic Implant Infections

Several implant systems are now being used to replace missing teeth. Most implants osseointegrate without problems, and when surrounded by healthy tissue they carry microflora associated with periodontal health (44). However, implants may be lost because of excessive occlusal loading forces (traumatic failures) or as a result of microbial infections (infectious failures). Infectious failures are associated with complex microbial etiologies, either with periodontal pathogens such as P. micros, Campylobacter recta, and Prevotella spp., or with overgrowth of atypical periodontal microflora such as staphylococci, E. coli, Pseudomonas, and Candida, particularly after prolonged use of systemic antimicrobial agents or chlorhexidine mouth washes.

In patients with surgical laryngectomies, rehabilitation of the voice is accomplished with a voice prosthesis, which acts as a shunt valve between the trachea and esophagus (Fig. 1). Voice prostheses are usually made of medical-grade silicone rubber and, like other prostheses, are subject to microbial contamination. Indwelling voice prostheses often fail within several months of placement, because a polymicrobial biofilm forms on the esophageal side causing malfunction of the valve mechanism (65). The formation of a mixed biofilm is promoted because in laryngectomized patients salivary flow rates are often reduced (as a side effect of radiotherapy) and the antimicrobial properties of saliva are therefore subdued. The biofilms that form contain a variety of oral streptococci (e.g., S. gordonii, S. anginosus, S. salivarius), staphylococci (e.g., S. epidermidis), enterococci, and Candida spp., mainly C. albicans. Although adhesion of bacteria may be a prerequisite for Candida colonization, as it may also be for denture-induced stomatitis (51, 65), it is thought that Candida growing into the silicone rubber is responsible mainly for the deterioration of the prostheses.

Figure 1. Diagrammatic representation of positioning of partially implanted silicone rubber voice prosthesis (arrow).

Figure 1

Diagrammatic representation of positioning of partially implanted silicone rubber voice prosthesis (arrow). Diagram kindly provided by G. J. Elving.

Endotracheal tubes, nasogastric tubes, and urinary catheters are continually exposed to a range of microorganisms and thus, like oral and voice prostheses, tend to become colonized by mixed-species biofilms. These biofilms, containing different combinations of organisms and often encased within a mass of extracellular polymeric material, can be readily demonstrated by use of electron microscopy (48). Catheter-associated urinary tract infection is the most common nosocomial infection in hospitals and nursing homes; nosocomial bacteriuria or candiduria develops in up to 25% of patients requiring a urinary catheter for 7 days or more (46). However, in devices totally implanted into the body, such as prosthetic heart valves, cardiac pacemakers, and joint replacements (hip, knee, etc.), the risk of contamination occurs essentially at the time of surgical placement. Infections of such devices are usually by single microbial species, most frequently coagulase-negative staphylococci or S. aureus, although polymicrobial infections of orthopedic prostheses have been reported (11). The most commonly infected, surgically implanted device is the central venous catheter, which is used to administer fluids and nutrients as well as cytotoxic drugs. Infusion therapy carries a substantial risk of producing iatrogenic sepsis, either bacteremia or fungemia. Infections can arise in a variety of ways and at any time during the use of the catheter, which may be prolonged. Sometimes the infusion fluid itself or the catheter hub is contaminated. More commonly, organisms are introduced from the patient's skin microflora, from the hands of medical personnel or even from contaminated antiseptics used to swab the insertion site. The distal tip of the catheter may be contaminated at the time of insertion, or organisms may migrate down the interface between the catheter surface and the skin (i.e., down the catheter wound). Alternatively, if some other portal of entry exists, perhaps from the gut, then microorganisms may seed the catheter tip from the bloodstream. The range of microbes encountered in these infections is wide, including components of the skin microflora, some from the environment, and some of enteric origin. Most commonly, infections are caused by coagulase-negative staphylococci, S. aureus, enterococci, and Candida, predominantly involving only a single species. However, mixed-species infections of S. epidermidis and C. albicans, for example, have been reported (17). Overall, Candida spp. are currently responsible for 8% of all hospital-acquired infections and are the fourth most common cause of septicemia.

Mixed-Species Colonization

Microbial Colonization of the Oral Cavity

The oral cavity and nasopharynx harbor diverse and complex microbial communities. Microorganisms accumulate on the hard (dental) and softer (mucosal) tissues, or on prostheses, as sessile biofilms. These organisms engage the host in a cellular and molecular dialogue that usually serves to constrain the commensal microflora. However, under certain circumstances, components of the resident microflora become directly, or indirectly, responsible for disease. Microbial adhesion is the underlying process that drives oral colonization and ultimately disease progression.

The warm, moist, and generally nutrient-rich environment of the mouth favors microbial colonization of the available surfaces, but the mechanical shearing forces of salivary flow and tongue movement tend to dislodge and expel microorganisms. The importance of salivary flow in controlling colonization is well illustrated by the finding that individuals with xerostomia (dry mouth), who have reduced salivary flow for a variety of reasons, suffer from an overgrowth of dental plaque, a high incidence of dental decay, and an increased susceptibility to mucosal lesions. Successful colonizers are able to adhere to the surfaces available and resist, not only the innate host defense components present in saliva, but also the cleansing action of shear forces. In general, adhesion processes involve physicochemical (thermodynamic) forces providing surface-surface interactions, and higher-affinity adhesin-receptor interactions involving complementary molecules that act stereospecifically (41). This specificity of adhesion bestows specificity of colonization of the tissue site (15) and specificity of microbial community composition. Microorganisms that lack specific adhesive mechanisms are either lost from the oral cavity, or are found only at sites that are highly retentive.

Salivary fluid continually bathes and coats the oral tissues and microorganisms present. Molecules within saliva, such as lysozyme, lactoferrin, salivary peroxidase, and the host defense peptides of the histatin and cystatin families, all have the potential to inhibit microbial growth and metabolic activities of susceptible organisms. Other components such as salivary mucins, glycoprotein agglutinins, and S-IgA agglutinate microorganisms and facilitate their removal by expectoration or swallowing (35). These components, together with the proline-rich proteins (PRPs), statherin and α-amylase, also function as receptors for microbial adhesion when they are deposited onto oral surfaces. Thus, initial microbial adhesion to tooth or other hard surfaces, e.g., denture acrylic, and to a lesser extent epithelial cell surfaces, is mediated by interactions with deposited salivary molecules (Fig. 2). The epithelial surfaces support microflorae that are distinct from those formed on hard surfaces, partly because of retention effects (epithelia desquamate) but also because epithelial cells present unique receptor molecules including glycolipids and extracellular matrix proteins. Thus, it is logical to propose that the ability of microorganisms to grow and survive at colonizing sites depends primarily on their repertoire of functional adhesins. Adhesion favors community development within which nutritional interrelationships, and concerted resilience and resistance to innate and acquired host defenses, are fostered.

Figure 2. Candida multiple interactions with oral surfaces, salivary components, host components and bacteria in oral biofilms.

Figure 2

Candida multiple interactions with oral surfaces, salivary components, host components and bacteria in oral biofilms. Interactions designated A through E are as follows: A, adhesion of C. albicans to the surfaces of teeth or prostheses coated with an (more...)

Mechanisms of Bacterial and Candidal Adhesion

In general, it is accepted that the buildup of plaque in the oral cavity starts with the adsorption of salivary proteins to a surface, thus forming an acquired pellicle. Bacteria that have high affinities of adhesion to salivary pellicle, such as species of Streptococcus and Actinomyces, are primary colonizers of teeth. Their deposition onto a surface provides a linking film onto which further attachment and accumulation of cells can occur. Every new organism that binds to the linking film presents a new surface and therefore forms a basis for the accretion of defined microbial groupings. Oral microorganisms such as Fusobacterium, Porphyromonas, and Candida, are not generally considered to be primary colonizers, but they do have the ability to bind avidly to experimental salivary pellicles. These organisms may therefore have become adapted to bind preferentially to salivary components that have become deposited on primary colonizers. While specific components of saliva are recognized as receptors by microorganisms, the nature of the components deposited depends on the physicochemical properties of the surface. Thus, acidic PRPs and statherin become readily deposited onto enamel and provide high-affinity receptors for S. gordonii and A. naeslundii (19, 20). In addition, human salivary mucin MG1, α-amylase, and S-IgA may also be deposited onto denture acrylic surfaces and provide additional receptors for a range of microorganisms. In some instances the molecular mechanisms of cell adhesion are well understood. Adhesion of various oral streptococci to salivary glycoprotein agglutinin gp-340 (56) is mediated by high-molecular-mass cell wall-anchored proteins of the antigen I/II family (34). These are Ca2+-dependent proteins with lectin-like activities recognizing glycosidic structures present within host glycoproteins and glycolipids. Adhesion of A. naeslundii and P. gingivalis to PRPs and statherin is mediated by adhesins located on the fimbriae of these organisms (20, 40). However, the adhesins present on the C. albicans cell surface that mediate binding to various salivary pellicle components, such as statherin (14, 38) and basic PRPs (54), have not yet been identified.

Multiple adherent interactions occur between microbial cells and salivary pellicle, epithelial cells, other microbial cells, host matrix proteins such as fibronectin and collagen, and platelets. Such a plethora of adhesion mechanisms involve production by the microbial cells of a wide range of adhesins. Multimodal adhesion facilitates diversity in colonization, enabling organisms to attach to a variety of surfaces presenting different receptors, and increases the avidity of binding to individual substrates. C. albicans exhibits a wide range of adhesion capabilities (Fig. 2) and is especially predisposed to forming mixed-species communities with bacteria. In addition to its high capacity to bind to salivary pellicle components adsorbed to enamel or acrylic surfaces, it also exhibits electrostatic and hydrophobic properties that enhance adhesion of cells to charged or hydrophobic surfaces (60), and that are positively correlated with adhesion levels to host tissues. Candida binds to a variety of host-cell receptors through lectin-like or protein-protein-type interactions, including galactosyl (37) or fucosyl (12, 64) receptors, also to fibronectin, laminin, fibrinogen, and collagen (reviewed in reference 13) (see Fig. 2). Because many species of oral bacteria bind similar components, they therefore compete with C. albicans for these receptors (52). A special adhesive mechanism of C. albicans involves hypha-specific protein Hwp1. This serves as a substrate for epithelial cell transglutaminase which effectively cross-links Candida to the epithelial cell surface (62). However, of particular relevance to the formation of mixed-species biofilms is the property of C. albicans (and C. dubliniensis) to bind a range of bacteria. It is this property, together with the spectrum of other adhesive interactions as portrayed in Fig. 2, that enables Candida to persist so successfully as a commensal on mucosal surfaces, and within denture plaque biofilms that act as protective reservoirs (1).

Mechanisms of Mixed Community Development

Coaggregation and coadhesion reactions of microorganisms are significant colonization factors because they enable development, stabilization, and maintenance of complex communities. The ability of an organism, such as Candida, to adhere to preattached organisms is an obvious advantage if it is not present in sufficiently high numbers, or lacks a sufficiently high affinity for adhesion sites, to compete with the primary colonizers. Coaggregation, or intermicrobial adhesion reactions, involve principally protein-carbohydrate or protein-protein interactions between complementary molecules present on the microbial cell surfaces. C. albicans has the ability to coaggregate with several oral streptococcal species (36), with S. gordonii having one of the highest affinities for C. albicans in suspension. Scanning electron micrographs of in vitro coaggregates show that streptococci attach singly, or in short chains, to yeast cells, effectively forming cross-bridges (Fig. 3A). When streptococcal cells are deposited onto a surface, C. albicans cells appear to localize specifically onto the streptococci (Fig. 3B). This interaction depends on at least three adhesin-receptor pairs (30): the recognition by an unidentified C. albicans surface protein of streptococcal cell wall linear polysaccharide; the binding activities of streptococcal antigen I/II proteins SspA and SspB to a C. albicans receptor; and the activity of S. gordonii high-molecular-mass cell wall adhesin CshA, that confers surface hydrophobicity. The fluid phase coaggregation reaction is blocked effectively by addition of soluble streptococcal polysaccharide (29), but the multimodal adhesion of C. albicans to immobilized streptococcal cells is not. Candida coadhesion with streptococci is thus a complex multimodal interaction by which mixed-species colonization would be promoted. C. albicans also coaggregates with A. naeslundii (23), while C. albicans and C. dubliniensis both coaggregate with Fusobacterium species (22, 33). These latter interactions, which are inhibitable by mannose, are thought to involve a Fusobacterium adhesin binding to a Candida cell surface mannan receptor (33). These kinds of intergeneric associations are believed to be responsible, at least in part, for the development of defined communities of yeast and oral bacteria (Fig. 4). The ability of Candida to interact with streptococci, actinomyces, and fusobacteria enables them to become enmeshed within more complex communities at different oral cavity sites. These interplays are consistent with the findings of C. albicans in associations with P. gingivalis and T. denticola in periodontitis, with F. nucleatum in gingivitis, and with streptococci in plaque formed on teeth or dentures. Evidence shows that in groups with higher susceptibility to caries there is a higher incidence of Candida (63). The ability of C. albicans to cocolonize with streptococci (10, 28) and to grow and survive at low pH (<4.5) suggests that active carious lesions may harbor C. albicans.

Figure 3. Coaggregation and coadhesion of C.

Figure 3

Coaggregation and coadhesion of C. albicans and oral streptococci. (A) Coaggregation of C. albicans in suspension with Streptococcus gordonii. (B) Adhesion of C. albicans cells to S. gordonii cells immobilized onto polystyrene. Bars, 5 μm. Reproduced (more...)

Figure 4. Intergeneric coaggregation reactions and coadhesion of Candida with oral bacteria in mixed-species biofilms.

Figure 4

Intergeneric coaggregation reactions and coadhesion of Candida with oral bacteria in mixed-species biofilms. Interactions between Candida, Streptococcus, Actinomyces, and Fusobacterium involve multiple adhesin-receptor reactions and promote retention (more...)

The formation and maintenance of these Candida-bacteria communities are subject to modification by extrinsic factors. Interactions of Candida with bacterial biofilms may be modified by environmental parameters such as pH and nutrient supply (7), and by host factors such as salivary components (28, 51). The adsorption of salivary proteins, such as basic PRPs, to the S. gordonii cell surface promotes adhesion of C. albicans (55), while S-IgA and salivary mucin MG2 might act to destabilize such associations (45, 50). Understanding the complex mechanisms by which Candida and bacteria cocolonize will assist development of new protocols to block adhesive reactions and control the formation of biofilms.

Structure and Properties of Candida Biofilms Formed In Vitro

As an alternative to adhesion and coaggregation studies, Candida biofilms can be grown on plastic surfaces in vitro, either in the presence or absence of bacteria. Although investigation of mixed-species biofilms is still at an early stage, several model systems have been used to characterize the properties and drug susceptibilities of single-species Candida biofilms (4). The simplest system involves growing adherent populations on the surfaces of small discs cut from catheters. Growth is monitored quantitatively by a colorimetric assay that depends on the reduction of a tetrazolium salt, or by [3H]leucine incorporation; both methods give excellent correlation with biofilm dry weight (25). Biofilm growth by different Candida species and strains, and on different catheter materials, can be compared with this protocol. A recent modification of the method to study C. albicans biofilm formation on strips of denture acrylic (polymethylmethacrylate) has shown that biofilm growth, as measured by the tetrazolium reduction procedure, is stimulated by pretreating the acrylic strips with saliva (16). Similar results have been reported (53) with use of a bioluminescent ATP assay based on the firefly luciferase-luciferin system.

Scanning electron microscopy has revealed that C. albicans biofilms on catheter discs, or denture acrylic, consist of yeasts, hyphae, and pseudohyphae. The organisms are arranged in a bilayer structure (5); there is a dense, basal yeast layer which appears to anchor the biofilm to the surface, and an overlying but more open, hyphal layer (Fig. 5A). When appropriate preparative techniques are used, a matrix of extracellular polymeric material can be visualized around the biofilm cells. The synthesis of this material increases markedly if developing biofilms are subjected to a liquid flow (27). Production of an extracellular matrix has also been reported for C. albicans biofilms formed on human premolar teeth (58).

Figure 5. C.

Figure 5

C. albicans and mixed-species biofilms formed in vitro. (A) C. albicans biofilm showing bilayer structure of germ-tube-forming cells and hyphae growing above layers of yeast cells (blastospores). (B) Mixed-species biofilm of C. albicans (yeast and hyphal (more...)

Candida biofilms are, like bacterial biofilms, more resistant to the action of antimicrobial agents, including clinically important antifungal drugs such as amphotericin B, fluconazole, and nystatin, as well as the antiseptic chlorhexidine that is used in the treatment of patients with denture stomatitis (16, 26). The mechanisms of resistance are not known but may include drug exclusion by the biofilm matrix, phenotypic changes resulting from nutrient limitation or a low growth rate, or surface-induced gene expression. Drug exclusion seems unlikely in view of experiments in which susceptibility profiles of biofilms incubated statically (which have relatively little matrix) were compared with those for biofilms incubated with gentle shaking (which produce much greater amounts of matrix material). Biofilms grown with or without shaking failed to exhibit significant differences in susceptibility to any of the drugs tested, suggesting that drug resistance is unrelated to the extent of matrix formation (6). To investigate possible phenotypic changes resulting from decreased growth rate, C. albicans biofilms were generated at different growth rates in a perfused biofilm fermentor, and susceptibility of biofilm cells to amphotericin B was compared with that of planktonic organisms grown at the same growth rate in a chemostat. The results showed that biofilms were resistant to the drug at all growth rates tested, whereas planktonic cells were resistant only at low growth rates (2). A separate study using a different model system demonstrated that glucose- and iron-limited biofilms grown at the same low rate were equally resistant to amphotericin B (3).

Overall, it seems unlikely that drug exclusion or decreased growth rate are factors of major importance in determining the drug resistance of Candida biofilms. In support of the notion that Candida responds to a surface-growth-associated signal, evidence has been obtained that biofilms formed on two different types of polyvinyl chloride catheter, produced by different manufacturers, show significant differences in susceptibilities to amphotericin B (6). This suggests that decreased antifungal drug sensitivity may be related to highly specific, surface-induced gene expression. Such a mechanism would be directly analogous to bacterial biofilm systems in which contact-induced gene expression is now well established (57).

Mixed Candida-Bacterial Biofilms

There is mounting interest in the study of Candida-bacteria biofilms formed in vitro. Preliminary work indicates the likelihood of extensive interspecies interactions in these adherent populations. For example, the catheter disc model system has been used to investigate mixed-species biofilms consisting of C. albicans and Staphylococcus epidermidis, a common agent of catheter-related infections. Two strains of S. epidermidis were used: a slime-producing wild type and a slime-negative mutant derived from it by chemical mutagenesis. Scanning electron microscopy revealed numerous physical interactions between the staphylococci and both yeasts and hyphae in these mixed species biofilms (B. Adam, G. S. Baillie, and L. J. Douglas, unpublished results). Drug susceptibility studies further indicated that fungal cells may modulate the action of antibiotics and that, conversely, bacteria can affect antifungal activity. For example, the presence of Candida in a biofilm increased the resistance of slime-negative staphylococci to vancomycin. On the other hand, Candida resistance to fluconazole was enhanced in the presence of slime-producing staphylococci, but was unaffected by the presence of the slime-negative mutant (Adam et al., unpublished).

Similar observations have been made with mixed-species biofilms consisting of C. albicans and oral streptococci on denture acrylic. Physical interactions between yeasts, hyphae, and either S. gordonii or S. salivarius were apparent by the use of scanning electron microscopy (Fig. 5B and C). Moreover, in results analogous to those obtained with Candida-Staphylococcus biofilms, the presence of oral streptococci enhanced the resistance of C. albicans to amphotericin B and nystatin (N. J. S. Henderson, B. Adam, and L. J. Douglas, unpublished results). Clearly, understanding the molecular basis of this decreased drug sensitivity will assist in the future development of more powerful ways to eliminate Candida from biofilm-related infections.

Treatment and Management

No effective means is yet available of simply preventing adhesion of C. albicans and bacteria to plastic materials in situ. Good hygiene measures applied to teeth and dentures, and removal of pseudomembranous and plaque layers on mucosal membranes, are fundamental to antimicrobial prophylaxis and treatment of denture stomatitis. It is important to first reduce the numbers of biofilm microorganisms by mechanical measures, and then to administer an effective concentration of antimicrobial agent at the site of the lesion. C. albicans adheres tenaciously with oral bacteria to methyl-methacrylate denture base, and so provides a reservoir of organisms that may continually detach and reinfect the mucosa. Elimination of the microbial biofilm from the denture base is thus crucial and can be achieved by careful scrubbing and then cleansing by soaking the denture in 0.1% hypochlorite solution. Another means of inhibiting growth and retention of C. albicans on the denture is to coat the fitting surface of the denture, while it is being worn, with a gel containing an antifungal compound, usually miconazole. It takes between 1 and 2 weeks for C. albicans to be eliminated and for the inflammation to clear. The inflamed mucosa responds to topical application of antifungal drugs, but agents such as nystatin and amphotericin can only gain access to the palate mucosa if the patient removes the denture while the antifungal tablets are allowed to dissolve in the mouth. Combinations of Candida and staphylococci, or enteric bacteria, are usually adequately controlled by starting patient treatment with an antifungal agent. Lack of response to the antifungal treatment may be due to reduced antifungal sensitivity in the mixed-species biofilm, poor patient compliance, or an underlying disorder such as iron or immune deficiency. If there is immune deficiency, the candidal infection is usually florid or associated with patches of thrush, in which case a blood analysis is recommended. Resistant cases may be treated with oral itraconazole or fluconazole, both of which have systemic effects. Antifungal treatment of patients with intraoral Candida infection often causes an associated angular cheilitis condition to resolve. However, since angular cheilitis may result from a mixed infection of Candida with S. aureus or enteric bacteria, local application of fusidic acid or metronidazole creams may also be required to eliminate this condition.

While polyene and azole antifungals are still the principal compounds used to treat patients with Candida infections, widespread use of azoles has resulted in an increase in the prevalence of azole-resistant C. albicans and C. dubliniensis, and a shift to infections caused by other species such as C. krusei. It is imperative, therefore, that new anti-Candida agents be developed. Accordingly, there is much interest at present in the family of histidine-rich proteins, designated histatins, which are usually found in human saliva and are capable of killing and inhibiting the germination of a wide range of microorganisms, especially Candida (43). The major members of this family, constituting about 80% of all histatins present in parotid and sub-mandibular secretions, are histatins 1, 3, and 5, comprising 38, 32, and 24 amino acid residues, respectively. Histatin 5 appears to be the most effective at killing Candida. The mechanism of killing by histatins is not fully understood (67), but uptake of histatin into the cytoplasm appears to be necessary. Histatins, being host-derived antimicrobial peptides, are unlikely to promote the emergence of resistant Candida strains or provoke adverse host reactions, so they may be very suitable for treatment of fungal infections. Although it should be feasible to use histatin preparations in local delivery, it is also suggested that to augment salivary histatin concentrations in patients with refractory candidiasis, it may be possible to use gene therapy. Experiments have shown that delivery of the HIS2 gene (encoding histatin 3) into salivary gland cells results in functional overexpression of histatin 3 (8). The efficiency of histatin expression in animal models of oropharyngeal candidiasis is currently being tested as a prelude to possible human applications.

Salivary antimicrobial peptides, such as histatins, or synthetic antimicrobial peptides with sufficiently broad specificity to cover a range of microorganisms are suggested as possible agents for controlling biofilm formation on voice prostheses (18). Interestingly though, the application of probiotic microorganisms in combination with a modified diet may assist in the control of Candida on voice box prostheses (65). Thus, strains of Lactobacillus fermentum and L. acidophilus have been found to produce antifungal substances, hydrogen peroxide, and antiadhesive biosurfactants that may all interfere with colonization of silicone rubber prostheses by Candida. For other surgically implanted prostheses, impregnation of the materials with antiseptic agents may render them less likely to be colonized by bacteria (9). For example, gram-positive cocci and fungi are more likely to colonize central venous catheters made of standard polyurethane than catheters impregnated with antiseptic (59).

Summary and Conclusions

Roughly 40% of the healthy adult population carries C. albicans in their oral cavities, with the organism being present in dental plaque and on mucosal surfaces, the tongue, and denture surfaces. It may be isolated from healthy sites, but often greater numbers are isolated from diseased sites, such as those associated with gingivitis, periodontitis, infected root canals, and denture stomatitis. In normal circumstances the yeast is found in relatively low numbers within commensal communities, but if changes in host physiology or environmental conditions favor its numerical dominance, it may then be able to outcompete the resident bacterial flora (Fig. 6). Specific perturbations that promote Candida outgrowth include disruption of the bacterial components of the commensal microflora (e.g., by the application of broad-spectrum antibiotics), dampening of the host innate defenses (e.g., reduction of salivary flow), and compromising the function of the cellular immune system (e.g., through viral infection or pharmaceutical intervention). The interactions between Candida and bacteria are especially important in the establishment of oral microbial communities and in the etiology of candidiasis. There are few other examples of cooperative interactions between bacteria and unicellular eukaryotic microorganisms.

Figure 6. The critical effect of perturbation of host environmental conditions in the shift from commensal to pathogenic microflora.

Figure 6

The critical effect of perturbation of host environmental conditions in the shift from commensal to pathogenic microflora. Candida cells (black) are present in low numbers within the oral microflora or are acquired from other individuals. A major disruption (more...)

Adhesion processes are the defining events in establishing these polymicrobial biofilms. As the physiological and molecular processes that occur during biofilm formation are studied in greater detail, it is becoming apparent that organisms within biofilms acquire properties that are quite different from those of their free-living counterparts. Biofilm formation involves activation of genes, the products of which are essential for survival and confer biofilm-related properties, such as increased resistance to immune cell detection, to environmental trauma, and to antimicrobial compounds. The key mechanisms and molecules that are involved in biofilm formation and in enhanced resistance will provide novel targets for strategies designed to control and prevent Candida biofilmassociated diseases.

Antifungal therapy may be effective in controlling an individual Candida infection, but it does not, of course, provide immunity. It is also vitally important to improve the understanding of the immunological mechanisms that promote susceptibility to Candida infection. It will only be by enhancing the immune status of subjects at risk, as well as by introducing new antifungal therapies, that the incidence of candidal infections will be reduced.

Acknowledgments

We thank J. Luker for helpful comments on the manuscript. We also acknowledge the invaluable contributions of G. Baillie, R. Cannon, S. Hawser, A. Holmes, and J. O'Sullivan to research in our laboratories.

Our research is financially supported by the Wellcome Trust, Sir Jules Thorn Charitable Trust, Health Research Council of New Zealand, and New Zealand Dental Research Foundation.

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