1. Coccus (def)
The cocci are spherical or oval bacteria having one of several distinct arrangements
(see Fig. 2) based on their planes of division.
a. Division in one plane produces either a diplococcus or streptococcus arrangement.
diplococcus: cocci arranged in pairs; see Fig. 3.
- scanning electron micrograph of a Streptococcus pneumoniae, a diplococcus; see Fig. 4.
- transmission electron micrograph of Streptococcus pneumoniae, a diplococcus; see Fig. 5
streptococcus: cocci arranged in chains; see Fig. 6.
- scanning electron micrograph of a Streptococcus pyogenes, a streptococcus; see Fig. 7A.
- scanning Electron Micrograph of Enterococcus, a streptococcus; see Fig. 7B.
b. Division in two planes produces a tetrad arrangement.
tetrad: cocci arranged in squares of four; see Fig. 8.
- scanning electron micrograph of Micrococcus luteus showing several tetrads; see Fig. 9.
c. Division in three planes produces a sarcina arrangement.
sarcina: cocci in arranged cubes of eight; see Fig. 10).
d. Division in random planes produces a staphylococcus arrangement.
staphylococcus: cocci arranged in irregular, often grape-like clusters (see Fig. 11)
- negative image of Staphylococcus aureus; see Fig. 12.
- scanning electron micrograph of Staphylococcus aureus, a staphylococcus; see Fig. 13.
- Scanning electron micrograph of methicillin-resistant Staphylococcus aureus (MRSA); see Fig. 14.
An average coccus is about 0.5-1.0 micrometer (µm) in diameter. (A micrometer equals 1/1,000,000 of a meter.)
2. The rod or bacillus (def)
Bacilli are rod-shaped bacteria. Bacilli all divide in one plane producing a bacillus, streptobacillus, or coccobacillus arrangement (see Fig. 15).
a. bacillus: single bacilli; see Fig. 16.
- scanning electron micrograph of Pseudomonas aeruginosa, a bacillus; see Fig. 17.
- scanning electron micrograph of Escherichia coli O157H7, a bacillus; see Fig. 18.
b. streptobacillus: bacilli arranged in chains; see Fig. 19.
c. coccobacillus: oval and similar to a coccus; see Fig. 20 and Fig. 21.
An average bacillus is 0.5-1.0 µm wide by 1.0-4.0 µm long.
3. The spiral (def)
Spirals come in one of three forms, a vibrio, a spirillum, or a spirochete; see Fig. 22.
a. vibrio: a curved or comma-shaped rod; see Fig. 23.
- scanning electron micrograph of a Vibrio cholerae, a vibrio; see Fig. 24.
b. spirillum: a thick, rigid spiral; see Fig. 25.
c. spirochete: a thin, flexible spiral; see Fig. 26.
- scanning electron micrograph of the spirochete Leptospira; see Fig. 27.
- scanning electron micrograph of the spirochete Treponema pallidum; see Fig. 28.
Spirals range in size from 1 µm to over 100 µm in length.
In the center of a bacterium is a group of proteins called Fts (filamentous temperature sensitive) proteins that interact to form a Z-ring at the cell division plane. These proteins form the cell division apparatus known as the divisome and are directly involved in bacterial cell division by binary fission (see Fig. 29 and Fig. 30). The diameter of both coccus-shaped and bacillus-shaped bacteria is determined by this Z-ring. In bacillus-shaped and spiral-shaped bacteria, elongation and cell shape is determined primarily by a protein called MreB. MreB forms in arc-shaped patches around the interior of the cell just under the cytoplasmic membrane forming a simple cytoskeleton. Vibrios have a third protein called crescentin that forms along the inner curve of the vibrio to further determine shape. Bacillus-shaped bacteria that have an inactivated MreB gene become coccoid shaped, and coccus-shaped bacteria naturally lack the MreB gene.
4. Exceptions to the above shapes
There are exceptions to the three basic shapes of coccus, bacillus, and spiral. They include sheathed, stalked, filamentous, square, star-shaped, spindle-shaped, lobed, trichome-forming, and pleomorphic (def) bacteria.
5. Ultrasmall Bacteria: 150 could fit in a single Escherichia coli
Ultrasmall bacteria have been discovered in groundwater that was passed through a filter with a pore size of 0.2 micrometers µm). They showed an average length of only 323 nanometers (def) (nm) and an average width of 242 nm. They contain DNA, an average of 42 ribosomes per bacterium, and possessed pili (def). It is thought that they use these pili to attach to other bacteria from which they scavenge nutrients. Because the surface to volume ratio is even greater than in more traditional sized bacteria, they might be better designed to take up scarce nutrients from more nutrient-poor environments.
Gary E. Kaiser, Ph.D.
Professor of Microbiology
The Community College of Baltimore County, Catonsville Campus
This work is licensed under a Creative Commons Attribution 4.0 International License.
Based on a work The Grapes of Staph at https://cwoer.ccbcmd.edu/science/microbiology/index_gos.html.
Last updated: Feb., 2018
Please send comments and inquiries to Dr.
Gary Kaiser