Cocci: Spherical Shapes
Spherical bacteria are classified based on how cells arrange relative to each other after the completion of cell division, which is determined by the number of division planes.
- Micrococci (Micrococcus): from the Greek word micros (small). Cells exist isolated, arranged singly.
- Diplococci (Diplococcus): from the Greek diploos (double). They form stable pairs and differ in shape: pneumococci (Streptococcus pneumoniae) are elongated like a candle flame (lanceolate) with blunt ends joined together, whereas gonococci (Neisseria gonorrhoeae) and meningococci (Neisseria meningitidis) resemble coffee beans apposed by their concave surfaces.
- Streptococci (Streptococcus): divide strictly in a single plane. Because cells maintain physical attachment at the division site, they line up in long chains (from Greek streptos).
- Tetrads: form characteristic groups of four cells.
- Sarcinae (Sarcina): division occurs in three mutually perpendicular planes, resulting in volumetric packets or "bales" (from Latin sarcina) containing eight or more structural units.
- Staphylococci (Staphylococcus): division planes occur randomly, forming irregular clusters resembling grape bunches (from Greek staphyle).
Rod-Shaped Bacteria
Rods vary widely in dimensions, with lengths from 1.0 to 10 µm and thicknesses from 0.5 to 2.0 µm. An important diagnostic criterion is the geometry of the bacterial cell ends:
- Straight (truncated): typical of the anthrax bacillus (Bacillus anthracis).
- Rounded: found in most regular microorganisms, such as Escherichia coli.
- Tapered/pointed: characteristic of fusobacteria.
- Club-shaped (swollen): a feature of irregular bacteria such as Corynebacterium diphtheriae.
The overwhelming majority of rods separate after replication; however, some species remain attached by cell wall remnants, forming chains (anthrax streptobacilli) or V-shapes at acute angles (corynebacteria).
Curved and Branching Forms
Curved microorganisms differ in the degree and pattern of spiraling. If a rod is only slightly bent, it is classified as a vibrio (a prominent example is Vibrio cholerae). Campylobacter and Helicobacter species have a characteristic bend visually resembling the wings of a flying gull. Bacteria with a pronounced spiral structure are called spirilla, while the thinnest and most tightly coiled forms are spirochetes.
A separate morphological group includes microorganisms capable of branching. These include actinomycetes, which form long intertwined filaments (true mycelium) demonstrating true branching. There are also rods bifurcated only at the poles, known as bifidobacteria.
Morphology and Gram Staining
The structural diversity of bacteria is closely related to their cell wall type:
- Gram-negative (Gram–) include bean-shaped diplococci, small veillonellae, classic rods, the entire spectrum of curved forms (from vibrios to spirochetes), and intracellular parasites (Chlamydia, Rickettsia).
- Gram-positive (Gram+) comprise staphylococci, streptococci, pneumococci, and branching microorganisms.
A special place among Gram+ bacteria is held by spore-forming rods—bacilli and clostridia. They synthesize protective endospores that can be located centrally, subterminally (displaced from the center), or terminally (at the very edge of the cell).