Candida albicans is a common fungus that resides in the human body, typically on mucosal surfaces like the mouth, gut, and vagina. It is classified as an opportunistic pathogen because, while it is often harmlessly present, a shift in host conditions can cause it to overgrow and lead to infection. This organism is highly adaptable, existing in multiple distinct physical forms, which is a trait known as dimorphism. For laboratory diagnostics and clinical management, identifying the specific appearance of C. albicans under a microscope is fundamental. The visual presentation of the fungus offers immediate clues about its reproductive state and its potential for invading host tissues.
The Basic Morphology of Candida Cells
The most frequently observed and non-invasive form of Candida albicans is the yeast phase, often referred to as blastoconidia. These cells appear microscopically as small, oval, or spherical structures, typically measuring between 2 and 4 micrometers in diameter. In this state, the fungus behaves as a commensal organism.
Reproduction in the yeast phase occurs asexually through a process called budding, where a smaller daughter cell grows directly out of the larger mother cell. A single yeast cell may display multiple instances of budding across its surface. When the new cell separates from the parent, it leaves behind a permanent, thickened area on the mother cell wall known as a “bud scar.” These scars are highly enriched with chitin, a major component of the fungal cell wall.
Visualizing these bud scars can sometimes indicate the cell’s history of division, though the scars are not always clearly visible in all preparations. The yeast form is the morphology typically seen when C. albicans is grown in routine laboratory culture or when sampled from non-invasive infections where tissue penetration has not yet occurred. The consistent size and shape of the blastoconidia aids in distinguishing C. albicans from bacteria or other fungal pathogens.
These single, ovoid cells are easily suspended in fluids, making them the ideal form for systemic dissemination if the fungus accesses the bloodstream. This unicellular state is considered the organism’s default mode of growth under normal physiological conditions. The thick polysaccharide cell wall surrounding the yeast cell provides structural integrity and is the target for many staining techniques.
Identifying the Invasive Filament Structures
A defining characteristic of C. albicans is its ability to transition from the single-celled yeast form to elongated, thread-like structures, a process known as dimorphism. This morphological shift is strongly associated with the fungus’s ability to penetrate and invade host tissues, contributing to its pathogenicity. The fungus can produce two distinct types of filaments: pseudohyphae and true hyphae, which must be differentiated under the microscope.
Pseudohyphae
Pseudohyphae are formed when the budding yeast cells elongate but fail to fully detach after division, remaining connected in a chain-like manner. Microscopically, these structures resemble a string of sausages because the junctions between individual cells show clear constrictions or “pinches” where the cells separate. The cells within the pseudohypha are typically elongated ellipses rather than tubes with parallel sides. Septation occurs at the site of the mother-bud neck.
True Hyphae
True hyphae represent the fully developed filamentous form and are structurally distinct from pseudohyphae, indicating a more aggressive tissue invasion. True hyphae appear as long, parallel-sided tubes that lack constrictions at the points of cell division, known as septa. The cell walls run smoothly and continuously, creating a unified, uninterrupted filament. The first nuclear division in a true hypha occurs within the initial germ tube, which differs from the division pattern observed in pseudohyphal cells.
These hyphal forms function in tissue penetration and anchoring the fungus to host surfaces. The visual distinction between the constricted septa of pseudohyphae and the smooth, unconstricted septa of true hyphae is a fundamental task for laboratory technicians.
Germ Tubes
A third, highly specific structure associated with C. albicans is the germ tube, used in the clinical laboratory for rapid identification. The germ tube is a short, filamentous outgrowth that emerges laterally from the yeast cell after a brief incubation in serum at body temperature. The germ tube lacks any constriction at its point of origin where it meets the yeast cell wall. This structure is the earliest stage of true hypha formation, appearing as a slender, non-septate extension that is typically about half the width of the parent yeast cell.
Chlamydospores
Under nutrient-limiting conditions, C. albicans can form chlamydospores, which are large, thick-walled, spherical cells. These structures are typically borne at the ends of hyphal suspensor cells or along the pseudohyphae. While they are a distinguishing feature of C. albicans in culture, they are rarely observed in specimens taken directly from infected tissues.
Preparation Methods for Microscopic Examination
The appearance of C. albicans is significantly influenced by the laboratory preparation method used to view the specimen. One of the simplest and most common techniques for rapidly screening clinical samples is the Potassium Hydroxide (KOH) preparation. This method involves mixing the specimen, such as a skin scraping or vaginal swab, with a solution of 10% to 20% KOH.
The strong alkaline nature of the potassium hydroxide dissolves the host cells, including epithelial cells, keratin, and background debris, which clarifies the field of view. Fungal elements like yeast cells and hyphae possess rigid cell walls and remain intact, standing out as translucent, refractile objects against the clear background. The KOH preparation is especially useful for visualizing filamentous structures in samples taken from skin, hair, and nails.
The Gram Stain is another routine technique, primarily used for bacterial identification but also informative for fungi. C. albicans is considered Gram-positive because its cell wall structure allows it to retain the crystal violet dye. Under the microscope, both the yeast cells and the filamentous forms appear as large, deep purple-staining ovoid or thread-like structures. This purple coloration contrasts sharply with Gram-negative bacteria, which appear pink, aiding in the differentiation of microorganisms in the sample.
For high sensitivity, especially when fungal loads are low, Specialized Fluorescent Stains are employed, with Calcofluor white (CFW) being a frequent choice. CFW is a colorless fluorescent dye that binds specifically to the chitin in the fungal cell wall. When viewed under a fluorescent microscope using ultraviolet light, the bound dye fluoresces, causing the fungal structures to glow a vivid apple-green or blue. This technique provides a high-contrast view, allowing for the rapid detection of fungal elements that might be missed using standard light microscopy.