Bacteria are prokaryotes, one of the three great classes of life. Their cells have no nuclei, and generally lack organelles, small sub-units bound in membranes. They are about a tenth the size of the eukaryotic cells found in plants and animals, on average, though a few giants among bacteria varieties are visible without a microscope, and some ultramicrobacteria are as small as viruses.
There are various ways of categorizing bacteria. They may be either gram positive or gram negative, depending upon how they react to staining. They may be considered aerobic or anaerobic, depending on whether they require oxygen or the absence of oxygen for growth. (Some prefer a precise percentage of oxygen, and some can tolerate either atmosphere.)
They may also be sorted by morphology, or form. The shapes that bacteria come in include cocci, bacilli, spirilli, or spheres, rods, and spirals. These are not the only forms bacteria take, but they are a general way of classifying them.
Cocci are spherical. Cocci may be solitary or joined in pairs. If they pair, they are called diplococci. Groups of four cocci are called tetrads. Some cocci form chains. Streptococci are cocci that form long chains. Staphylococci tend to gather in bunches, like grapes. The way these bacteria clump together has to do with the way they split when they reproduce. If the bacteria split along one axis, the result will be a pair or a chain. If they split along multiple axes the result will be clumps.
Staph and Strep are dreaded words for parents, because of the infections that some members of each genus cause. However, most species of staph and strep bacteria are harmless to human life, and a few are even helpful. For example, streptococci ferment Gruyere and Emmenthaler cheese.
Bacilli are rod shaped. There is a genus of bacteria, Bacillus, which refers to specific anaerobic bacteria that forms spherical spores. However, if the word bacillus is not capitalized and italicized, it refers to a sticklike shape. Rod-shaped bacillus are usually solitary, but can form pairs, as diplobacilli, chains, as streptobacilli, and palisades, "walls" of rods attached edge to edge. As with the cocci, the shapes of these aggregations depend upon the way that the bacilli divide and grow.
Spirilli and others
Spirilli are spirals. They can be described as spiral shaped rods. They may curl only slightly, with perhaps one helical turn. They may also curl tightly, and then they are described as spirochetes. Vibrio bacteria are bent into a comma or bean shape.
There are also other forms bacteria can take, like a form intermediate between rod and sphere, called coccobacillus, which is usually classed with the bacilli. Some bacteria are branched, and some, like Actinobacilli, grow in slender threads. Life is various. Some bacteria are triangular or squared.
Bacteria take these shapes because they are the most useful and best adapted for their environments. The sphere has the smallest surface area among all surfaces enclosing a given volume and it encloses the largest volume among all closed surfaces with a given surface area. Therefore, for its size, a spherical bacterium has the least surface area to defend against an environment that may be disagreeable. It also needs the least amount of "packaging material" to maintain its form
Rod shaped bacteria, on the other hand, are better navigators. They often have flagellae, whiplike extensions, which help them move to a more advantageous spot, reach food, or remain in place against a flow. The inside of a bacillus has relatively close access to the environment, which can be an advantage in some circumstances. Some rod shaped bacteria change shape in response to different conditions.
Certain extremophile bacteria grow in long rods that are star-shaped in cross-section. These live deep beneath the earth's surface, in conditions of high heat and pressure. Their particular shape helps them resist the pressure, while maintaining close access to food in an environment of scarcity.
Bacteria are a fascinating form of life. They take many forms, to suit their situations. Understanding the morphology of bacteria is one way to begin to learn about them.