Last updated on June 21st, 2021

Bacterial cells have a wide variety of sizes, shapes, and arrangements. They are too small to see through the naked eye. The unit of measurement used for microorganisms is the micrometer (μm); 1 μm = 0.001 millimeter; 1 nanometer (nm) = 0.001 μm. Most bacteria are generally 0.2 to 2 µm in diameter and 1 to 6 µm in length. To illustrate how small a bacterium is, consider that 500 bacteria 1 μm in length could be placed end to end across the period at the end of this sentence.

Giant and Smallest Bacteria

Thiomargarita namibiensis, which means, “sulfur pearl of Namibia”, is the largest known prokaryote. This sulfur chemolithotroph can be 750 μm in diameter and nearly visible to the naked eye. This gram-negative coccid proteobacteria is about 100 times bigger than the average bacterial cell.

Epulopiscium fishelsoni is one another very large prokaryote, with cells longer than 600 μm (0.6 millimeters). This bacterium is phylogenetically related to the endospore-forming bacterium Clostridium and is found in the gut of the surgeonfish.

The smallest bacteria (Mycoplasma) are about the same size as the largest viruses (poxviruses) and are the smallest organisms capable of existing outside a host. The longest bacteria rods are the size of some yeasts and human red blood cells (7 μm).

Shape of Bacteria

The shape of a bacterium is determined by its rigid cell wall. The microscopic appearance of a bacterium is one of the most important criteria used in its identification. Bacteria exist in four basic morphologies: spherical cells, or cocci; rod-shaped cells, or bacilli; spiral-shaped cells, or spirilla; and comma-shaped cells, or vibrios. Some bacteria are variable in shape and are said to be pleomorphic (heterogeneous shape).

Morphological characteristics (size, shape, and arrangement of the bacterial cells) of pathogenic bacteria help in the preliminary identification of isolates.

Arrangements of Bacteria

In addition to their characteristic shapes, the arrangement of bacteria is important.

Cocci

Cocci appear in several characteristic arrangements such as diplococci (cocci in pairs), streptococci (cocci in chains), tetrads, staphylococci (cocci in grape-like clusters), and sarcinae). These arrangements are determined by whether the daughter cells stay together following division.

Arrangements of Gram-positive cocci

1. Staphylococci: Spherical, gram-positive cocci arranged in irregular, grape-like clusters.
2. Streptococci: Spherical, Gram-positive cocci arranged in chains or pairs.
3. Streptococcus pneumoniae (pneumococci): Gram-positive lancet-shaped cocci arranged in pairs (diplococci) or short chains.

Arrangements of Gram-negative cocci

The arrangement of rods and spirochetes are comparatively less important in medical microbiology. Rod-shaped organisms may be regular in morphology, maybe somewhat shorter (i.e., “coccobacillary”), or may appear club- or dumbbell-shaped (“coryneform”). Comma-shaped cells generally define a basic characteristic of certain species (e.g.,Vibrio species).

Bacilli

Bacilli are not arranged in patterns as complex as those of cocci, and most occur singly or in pairs (diplobacilli). But some species such as Bacillus subtilis, forms chains (streptobacilli); others such as Beggiatoa and Saprospira species, form trichomes (which are similar to chains but have a much larger area of contact between the adjacent cells). In some bacillus, the cells are lined side by side like matchsticks and at angles to one another. Such arrangement is called palisade arrangement and is found in Corynebacterium diphtheriae.

Streptomyces species form long, branched, multinucleate filaments called hyphae which collectively form a mycelium.

Arrangements of Gram-positive bacilli

1. Bacillus anthracis: Large, Gram-positive rod with square ends, frequently found in chains. Clostridium tetani: Gram-positive rod with characteristics “tennis racket shaped” or “Drum stick appearance” (because of terminal spore).
2. Clostridium perfringens: Large, Gram-positive, “boxcar” shaped rods.
3. Corynebacteria: Gram-positive rods that appear club-shaped and are arranged in palisades or in V or L shaped formations (“Chinese-Letter” appearance).
4. Listeria monocytogenes: It is a small Gram-positive rod arranged in V or L-shaped formations similar to corynebacteria.

Arrangements of Gram-negative bacilli

1. Vibrio cholerae: Comma “Curved” shaped Gram-negative rod.
2. Campylobacter:  Curved Gram-negative rods that appear either a comma or S-shaped.
3. H. Influenzae: Small, Gram-negative coccobacilli.
4. Fusobacterium spp: These pleomorphic, long Gram-negative rods often with tapered “pointy” ends.

Others

Curved bacteria are usually curved with a twist. Bacteria with less than one complete twist or turn have a vibriod shape, whereas those with one or more complete turns have a helical shape. Spirilla are rigid helical bacteria whereas spirochetes are highly flexible.

1. Spirochetes (Treponema, Leptospira, and Borrelia): Thin-walled, flexible, spiral rods  (Corkscrew shaped) seen only by darkfield microscope and generally not seen in a standard light microscope.
2. Borrelia: They are corkscrew-shaped and are larger than the Treponema, they can be viewed under a light microscope with Giemsa or Wright stains.

Other unique bacterial shapes ares;

• Pear-shaped cells e.g., Pasteuria
• Lobed spheres e.g., Sulfolobus
• Disks arranged like stacks of coins e.g., Caryophanon
• Rods with helically sculptured surfaces e.g., Seliberia

References and further readings

• Color Atlas and Textbook of Diagnostic Microbiology, Koneman, 5th edition
• Review of Medical Microbiology and Immunology, Warren E. Levinson, 15th edition