Human saliva is a complex biological fluid produced by the salivary glands. While often viewed as a simple liquid, a drop of saliva examined under a microscope reveals a dynamic and crowded micro-ecosystem. This fluid acts as a suspension medium for a diverse population of microscopic entities, including cells shed from the body and a vast community of microorganisms. Analyzing these components provides a unique, non-invasive window into both local oral health and the overall state of the human host.
Host Cells: The Body’s Contribution
The human body contributes a significant cellular population to the salivary mix, primarily consisting of cells that line the mouth and immune system defenders. The most frequently observed human structures are the squamous epithelial cells, which are flat, large, and irregularly shaped, originating from the inner lining of the cheeks and gums. These cells are continually shed into the saliva as part of the natural mucosal turnover process.
Under a brightfield microscope, these epithelial cells appear as large, somewhat translucent structures with a relatively small, flattened nucleus located near the center. Their large size, often measuring around 50 to 100 micrometers in diameter, makes them easy to identify.
The second major cellular component is the leukocyte, or white blood cell. These cells are significantly smaller than the epithelial cells, typically measuring between 10 and 20 micrometers. Neutrophils are the predominant type of leukocyte found in saliva, having migrated from the underlying tissues or gingival crevicular fluid.
Neutrophils are recognizable by their lobed or segmented nuclei and granular cytoplasm when appropriately stained. A high concentration of these small, round cells often suggests an active inflammatory process, such as gingivitis or another localized infection.
The Oral Microbiome
The most numerous and diverse structures visible in human saliva are the microorganisms that constitute the oral microbiome. Saliva is constantly seeded with microbes sloughed off from all surfaces within the mouth, making it a rich suspension containing over 700 different species of bacteria, fungi, and other organisms.
Bacteria are the dominant life form, and their appearance under the microscope is categorized by their distinct shapes, known as morphology. Cocci, which are spherical or round bacteria, are extremely common and often appear in clusters or short chains, with genera like Streptococcus being highly prevalent. Rods, or bacilli, are elongated, cylinder-shaped organisms, which may be short and plump or long and thin, representing numerous genera.
A third key bacterial shape is the spirochete, a thin, spiral-shaped organism known for its corkscrew-like motility. When viewed in a fresh, unstained sample, some bacteria, especially those with flagella, may exhibit rapid, directed movement, which distinguishes them from non-motile debris.
Fungi are also part of this community, with yeasts from the genus Candida being the most common representative. These fungal cells are typically larger than bacteria, appearing as oval or budding cells. They can sometimes be seen forming pseudohyphae, which are elongated chains of cells that resemble filaments.
The Salivary Matrix and Non-Living Structures
Beyond the host cells and the vast microbial community, the background of a salivary sample is composed of a complex non-living matrix that provides structure and function to the fluid. This matrix is largely built upon mucins and glycoproteins, which are long, complex protein-sugar molecules secreted by the salivary glands. Mucin molecules interact to form a delicate, scaffold-like network that gives saliva its viscous, gel-like properties.
Under the microscope, this mucin network often appears as a faint, hazy background or as fine, translucent threads that crisscross the field of view. This network traps cells and microbes within the fluid.
Various non-cellular protein precipitates also float within this matrix. Salivary proteins are secreted to help regulate calcium and phosphate levels. Under certain conditions, these proteins can aggregate or combine with mineral constituents.
Occasionally, microscopic mineral crystals, often composed of calcium phosphate deposits, can be observed, especially in samples with a tendency toward calculus formation. These appear as small, geometrically shaped, bright structures, contrasting sharply with the organic material around them.
Techniques for Microscopic Viewing
To properly visualize the diverse components of saliva, specific preparation and microscopy techniques are required, as unstained samples often lack sufficient contrast. The simplest method is the wet mount, where a drop of fresh saliva is placed directly on a slide and covered with a coverslip. This quick preparation is particularly useful for observing motility, such as the rapid movement of certain bacteria, because the organisms are still alive and suspended in their natural fluid state.
For detailed structural analysis, however, a fixed and stained smear is necessary to enhance contrast and differentiate components. The specimen is first spread thinly across a slide and heat or chemically fixed to adhere the structures firmly. Staining then introduces color, making the transparent cells and microbes visible under a brightfield compound light microscope.
The Gram stain is the most common differential technique used in microbiology to classify bacteria based on their cell wall composition. This multi-step process colors Gram-positive bacteria, which have a thick peptidoglycan layer, a deep purple or blue with crystal violet. Gram-negative bacteria, which have a thinner wall, do not retain the purple stain and are counterstained pink or red with safranin.
To achieve the necessary magnification for viewing bacteria, which often requires a 100x objective lens, immersion oil must be used between the lens and the coverslip. Other simple stains, like methylene blue, can be used to quickly highlight the nuclei of human cells and the overall morphology of bacteria.