Fundamental Statements for this Learning Object:

1. There are three basic shapes of bacteria: coccus, bacillus, and spiral.
2. Based on planes of division, the coccus shape can appear in several distinct arrangements: diplococcus, streptococcus, tetrad, sarcina, and staphylococcus.

3. The bacillus shape can appear as a single bacillus, a streptobacillus, or a coccobacillus.
4. The spiral shape can appear in several forms: vibrio, spirillum, and spirochete.
5. The metric unit micrometer (1/1,000,000 or 10-6 of a meter) is used to measure bacterial size.



Bacteria are:

a. Prokaryotic.

b. Single-celled, microscopic organisms (Exceptions have been discovered that can reach sizes just visible to the naked eye. They include Epulopiscium fishelsoni, a bacillus-shaped bacterium that is typically 80 micrometers (µm) in diameter and 200-600 µm long, and Thiomargarita namibiensis, a spherical bacterium between 100 and 750 µm in diameter.)

c. Generally much smaller than eukaryotic cells (See Fig 1).

d. Very complex despite their small size. Even though bacteria are single-celled organisms, they are able to communicate with one another through a process called quorum sensing. In this way they can function as a multicellular population rather than as individual bacteria. This will be discussed in greater detail in Unit 2.


To view a nice interactive illustration comparing size of cells and microbes, see the Cell Size and Scale Resource at the University of Utah. (Genetic Science Learning Center. (2010, September 2) Cell Size and Scale. Retrieved October 19, 2017, from http://learn.genetics.utah.edu/content/cells/scale/ )

YouTube movie illustrating Size Comparison of Microorganisms created by Gracia Alvaro Montoya, MetaBallsStudios (MBS), United Kingdom, Nov., 2017


Most bacteria come in one of three basic shapes: coccus, rod or bacillus, and spiral.

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.




Flipped-class assignment
Read II. The Prokaryotic Cell: Bacteria: B. Prokaryotic Cell Structure, 1. The Cytoplasmic Membrane in your E-text and answer the 3 learning objectives for this section.

We will be doing a classroom group activity on this section so it is critical that you come prepared. I will be assuming that you have done this preparatory assignment.



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 at http://faculty.ccbcmd.edu/~gkaiser/index.html.

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Last updated: Feb., 2018
Please send comments and inquiries to Dr. Gary Kaiser