Shapes of Bacteria: Cocci, Bacilli, and Spirochetes

The shape of a bacterium is determined by its rigid cell wall. Bacteria that lack a cell wall (Mycoplasma and L-forms) display a great diversity of unusual shapes. Bacteria having various shapes are said to be pleomorphic (heterogeneous shape or many-shaped). 

Bacteria are classified by shape into three primary groups: cocci, bacilli, and spiral-shaped. The cocci are round, the bacilli are rods, and the spiral-shaped bacteria are either rigid (spirilla) or flexible (spirochetes). 

These three primary bacteria shapes determine the bacteria’s morphological characteristics. This is one of the most important criteria used in identifying organisms. 

1. Spherical or oval bacteria are called cocci (singular: bacillus)
2. Rods are called bacilli (singular: bacillus)
   1. Very short rods that can sometimes almost be mistaken for cocci are called cocobacilli (singular: coccobacillus).
   2. Rod-shaped bacteria having tapered ends are called fusiform, whereas others are characteristically club-shaped and may be curved or comma-shaped (Vibrios) or bent.
3. Spiral-shaped bacteria are called spirilla if the cells are rigid and spirochetes if they are more flexible and undulating.

In addition to their characteristic shapes, the arrangement of bacteria is essential. For example, certain cocci occur in pairs (diplococci), some in chains (streptococci), and others in grapelike clusters (staphylococci). These arrangements are determined by the bacteria’s orientation and degree of attachment at the time of cell division. The arrangement of rods and spirochetes is medically less important.

Characteristics shape	Examples
Cocci in clusters	Staphylococcus
Cocci in chains	Streptococcus
Cocci in pairs with pointed ends	Streptococcus pneumoniae
Cocci in pairs with a kidney bean shape	Neisseria
Rods (bacilli) with square ends	Bacillus
Rods with rounded ends	Salmonella
Club-shaped rods	Corynebacterium diphtheriae
Fusiform	Fusobacterium
Comma shaped	Vibrio
Spirochetes (relaxed coil)	Borrelia
Spirochetes (tightly coiled)	Treponema

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. 

Cocci can be found in pairs, chains, squares of four, cubes of eight, or grapelike clusters.

Diplococci

Diplococci arise when cocci divide and remain together to form pairs. Examples, Neisseria gonorrhoeae, Neisseria meningitidis, Streptococcus pneumoniae, and Moraxella catarrhalis.

Streptococci

Long chains of cocci (streptococci) result when cells adhere after repeated divisions in one plane; this pattern is seen in the genera Streptococcus agalactiae, Streptococcus pyogenes, Enterococcus faecalis, and Lactococcus.

Staphylococci

Staphylococcus divides into random planes to generate irregular grapelike clumps. Divisions in two or three planes can produce symmetrical clusters of cocci. Examples, Staphylococcus aureus, Staphylococcus saprophyticus, Staphylococcus epidermidis, etc.

Tetrad

Members of the genus Micrococcus often divide into two planes to form square groups of four cells called tetrads.

Sarcina

In the genus Sarcina, cocci divide into three planes producing cubical packets of eight cells.

Name of the Bacteria	Characteristics Shape
Staphylococcus	Spherical, Gram-positive cocci arranged in irregular, grape-like clusters.
Streptococcus	Spherical, Gram-positive cocci arranged in chains or pairs
Streptococcus pneumoniae	Gram-positive lancet-shaped cocci arranged in pairs (diplococci) or short chains.
Merisopedia	Cocci arranged in a tetrad, formed by division into two planes
Sarcinae lutea	Cocci arranged in a sarcina, formed by a division in three planes.
Neisseria meningitidis (meningococci)	Gram-negative diplococci, lens-shaped or kidney-bean shaped.
Neisseria gonorrhoeae (gonococci)	Gram-negative diplococci, lens-shaped or kidney-bean shaped.

Bacilli 

These are not arranged in patterns as complex as cocci;  most occur singly or in pairs (diplobacilli). But some species, such as Bacillus subtilis, form 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).

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).

Name of the Bacteria	Characteristics Shape
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” due to presence of terminal spore.
Clostridium perfringens	Large, Gram-positive, “boxcar” shaped bacilli.
Corynebacteria	Gram-positive rods are club-shaped and arranged in palisades or V or L-shaped formations, also called “Chinese-Letter” appearance.
Bacillus megaterium	Gram-positive, streptobacilli (bacillus arranged in chains).
Listeria monocytogenes	Small Gram-positive rods arranged in V or L-shaped formations similar to corynebacteria.
Vibrio cholerae	Comma-shaped, curved, Gram-negative bacilli
Campylobacter	Curved Gram-negative rods that appear as comma or S-shaped
Haemophilus influenzae	Small, Gram-negative coccobacilli
Fusobacterium spp.	These pleomorphic, long Gram-negative rods, often with tapered “pointy” ends.

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. 

Spiral-Shaped Bacteria 

Spiral bacteria have a variety of curved shapes. 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. Spirilla (singular: spirillum) are rigid, wavy-shaped curved bacteria, and spirochete is curved corkscrew-shaped bacteria. 

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. 

Pleomorphic Bacteria

Some bacteria are variable in shape and are said to be pleomorphic (heterogeneous shape). Some common pleomorphic pathogenic bacteria are;

Name of the bacteria	Gram-staining characteristics
Corynebacterium spp.	Gram positive  rods
Haemophilus influenzae	Gram-negative coccobacilli
Francisella tularensis	Gram-negative rod
Bartonella henselae	Gram-negative rod
Cardiobacterium hominis	Gram-negative rod
Propionibacterium acnes	Gram-negative rod

Other unusual shapes of bacteria 

Some bacteria do not fit in any of the above-mentioned categories and have spindle or irregular, lobed shapes. Some of these bacterial shapes are;

Bacillus arranged in a rosette attached by stalks to a substrate, for example, Caulobacter.

Star-shaped bacteria, for example, Stella

Square-shaped bacteria, for example, Haloarcula (a salt-loving member of the Archaea)

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

1. Color Atlas and Textbook of Diagnostic Microbiology, Koneman, 5th edition
2. Review of Medical Microbiology and Immunology, Warren E. Levinson, 15th edition
3. Microbiology: An Introduction. Gerald J. Tortora, Berdell R. Funke, and Christine L.Case. Pearson Education.