Bacteria are single-celled microorganisms found almost everywhere on Earth. Their physical form, known as bacterial morphology, is a primary characteristic used for identification. These shapes are maintained by the rigid bacterial cell wall and are the most fundamental way scientists categorize these organisms in a laboratory setting. This morphological classification simplifies the initial steps of identifying a bacterium, which is useful in clinical or environmental analysis.
Spherical Bacteria (Cocci)
Cocci (singular: coccus) are bacteria that are spherical or nearly spherical, sometimes appearing slightly oval or flattened. Their diameter typically falls within the range of 0.5 to 2.0 micrometers. What distinguishes this group is the way individual cells arrange themselves after division, which reflects the plane of cell division and whether the daughter cells separate completely.
The term “diplococci” describes cocci that remain paired after cell division, such as Neisseria gonorrhoeae. When the division occurs repeatedly in a single plane, the cells form long chains, a formation known as streptococci, exemplified by Streptococcus pyogenes. Conversely, if the cocci divide randomly in multiple planes, they stick together in irregular, grape-like clusters called staphylococci, a pattern seen in the common human pathogen Staphylococcus aureus.
Tetrads are groups of four cocci arranged in a square, resulting from division in two perpendicular planes. Sarcina form when cells divide in three perpendicular planes to create a cubical packet of eight cells. These arrangements provide immediate visual clues about a bacterium’s genus, making the coccus shape highly informative for initial microbial analysis.
Rod-Shaped Bacteria (Bacilli)
Rod-shaped bacteria, known as bacilli (singular: bacillus), are cylindrical and elongated. These bacteria are generally larger than cocci, with an average length of 1.0 to 4.0 micrometers and a width of 0.5 to 1.0 micrometers. There is considerable variation in how pronounced the rod appears, ranging from long, slender forms to very short, plump variations.
One variation is the coccobacillus, a rod so short and wide that it looks almost like a coccus, representing an intermediate shape. Examples of this short, stout form include Haemophilus influenzae and Chlamydia trachomatis. Like cocci, bacilli can also form characteristic arrangements after division, although these groupings tend to be simpler.
Bacilli that remain attached end-to-end after dividing form chains known as streptobacilli, a pattern observed in Bacillus anthracis. In some cases, dividing cells bend at the point of separation, creating a side-by-side stacking known as a palisade arrangement, typical of the genus Corynebacterium. Most bacilli, such as Escherichia coli, exist as single, independent rods.
Spiral and Curved Bacteria
This morphological group includes bacteria that are curved, wavy, or helical structures. This category is subdivided into three distinct forms based on their degree of curvature and flexibility. The least curved of these are the vibrios, which are short, curved rods with a single bend, often described as comma-shaped.
Vibrio cholerae, which causes cholera, is a classic example of a vibrio. Next are the spirilla (singular: spirillum), which are rigid, helical bacteria with one or more twists that often possess external flagella for mobility. These rigid spirals are distinct from the final, most coiled form, the spirochetes.
Spirochetes are thin, long, and highly flexible, resembling a tightly coiled corkscrew. They employ unique internal flagella, called axial filaments, located between the cell wall and an outer sheath, allowing them to move with a characteristic rotating motion. This flexibility is advantageous for moving through dense tissues, exemplified by Treponema pallidum, the causative agent of syphilis.