A bacterial cell is an incredibly small, single-celled organism that requires a high-powered microscope for visualization. These microorganisms typically measure only a few micrometers in size, far too small for the unaided human eye to perceive, making specialized techniques necessary. Describing a bacterium’s physical characteristics is the first step in its identification and classification. Under magnification, a bacterial cell’s appearance is defined by two primary features: its individual shape and how multiple cells group together after division.
The Primary Shapes of Bacteria
The first characteristic observed is the morphology, or shape, of the individual bacterial cell. Nearly all bacteria fall into one of three major shapes, providing immediate clues about the organism’s identity. The most common form is the coccus, which refers to any spherical or nearly round bacterium.
The rod-shaped bacteria are known as bacilli, appearing as elongated cylinders or pill-like structures. Bacilli vary in length and diameter, sometimes appearing as long, slender rods, or as short, stubby forms called coccobacilli, which are intermediate between spheres and rods. The ends of these rods can also differ, appearing squared off, rounded, or slightly club-shaped.
The third major category includes the spiral or curved bacteria, which present a more dynamic appearance. Within this group are the spirilla, which are rigid cells with one or more loose, wave-like twists. Separately, spirochetes are long, thin, and highly flexible cells that exhibit a distinct corkscrew or spring-like motion as they move.
How Bacteria Arrange Themselves
Beyond the shape of a single cell, bacteria often adhere to one another after cell division, forming characteristic groupings that serve as a secondary identifier. The cell’s division pattern determines the spatial arrangement, which is described using specific prefixes. When cells divide and remain attached in pairs, the arrangement is called diplo-, such as diplococci (two spherical cells) or diplobacilli (two rod-shaped cells).
Bacteria that continue to divide in the same plane create a linear formation known as strepto-, resulting in chains like streptococci or streptobacilli. A different division pattern leads to irregular, three-dimensional clusters called staphylo-, such as staphylococci, which resemble bunches of grapes.
Rod-shaped bacteria can also form a unique arrangement called a palisade, where the cells align themselves side-by-side. This stacking occurs when the rods bend at the point of division, resulting in a formation that looks like a picket fence or stacked logs.
Essential Preparation: The Role of Staining
Although a microscope is necessary, bacterial cells are naturally translucent and nearly invisible under a standard light microscope. Because they lack inherent color, staining is required to create contrast for visualization and differentiation. Staining involves applying colored dyes to the sample, which are absorbed by the cells, making their shape and arrangement clearly visible.
The most common technique is the Gram stain, a differential method that separates bacteria into two groups based on their cell wall structure. This procedure involves a series of dyes and a decolorizing step, resulting in two distinct color outcomes.
Bacteria with a thick layer of peptidoglycan in their cell wall retain the primary stain and appear purple or blue; these are classified as Gram-positive. Conversely, bacteria with a much thinner peptidoglycan layer lose the primary stain, absorb a counterstain, and appear pink or red, classified as Gram-negative.
The Gram stain reveals shape and arrangement while adding an essential color code foundational for guiding further identification. To clearly observe the stained cells, the specimen must be viewed at very high magnification, typically using a 100x objective lens combined with oil immersion, achieving a total magnification of 1000x.