Bacterial shape and arrangement vary with species to species. Bacteria exist in many forms, but the most common types are bacilli, cocci and Spirilla. Some bacteria are pleomorphic, i.e. they live in variable shapes. Bacteria are prokaryotic cells as they have a primitive nucleus, i.e. bacteria devoid a nucleus and a nuclear membrane that surrounds the genetic material. Bacteria are the unicellular structures, i.e. they are minute enough, that we could not see them through the naked eye.
Therefore, there are many methods like culturing and staining that helps us to visualize the bacterial shape and arrangement under the microscope. Through microscopic observation, we can determine the morphological or physiological characteristics of bacteria based on bacterial shape and arrangement.
Content: Bacterial Shape and Arrangement
The bacterial size ranges from 0.5-1.0 µm in size as you can observe in the image given below. The spherical shaped bacteria have an average diameter ranging from 0.5-2.0 µm, whereas, the rod shaped bacteria have an average diameter of 0.25-1.0 µm.
Some bacteria are having a diameter of 80 µm and a length of 600 µm, which can be seen through the naked eyes. For example, Epulosiscium fishelsoni.
In the most diverse group of bacteria, Mycoplasma is a type of bacteria which is considered as the smallest organism, measures a size of 0.25 µm. The largest bacteria known so far is Thiomargarita namibiensis which measures a size of 0.1-0.75nm.
Being a prokaryotic cell, the size of bacteria is smaller than the eukaryotic cells like fungi. Due to the small size of bacteria, its structure becomes more complicated.
Even though of small size, bacteria can communicate with the other microorganisms through a mechanism refers as “Quorum sensing”. Bacteria can also function as a multicellular population rather than an individual organism.
The rigid cell wall decides the shape of bacteria. Generally, there are three common shapes of bacteria, namely, cocci, bacilli and Spirilla.
Cocci is a plural form, which represents a large number of bacteria which possess a spherical shape whereas coccus is a singular form, which represents a single bacterial cell.
Cocci are small, round and slightly flattened in shape. These can appear singly or in pairs, chains, groups, clusters etc.
Examples: Staphylococcus aureus, Streptococcus hominis, Neisseria gonorrhoea etc.
Bacilli is a plural form, which represents a bacterial population that possess a rod shape, whereas coccus is a singular form, which represents a single bacterial cell.
Bacilli are small, cylindrical and elongated in shape. These can appear either singly or in pairs, groups etc.
Examples: Lactobacillus sp, Salmonella sp, Escherichia coli etc.
Spirilla is a plural form, which represents a large number of bacteria that possess a helical shape, whereas spirillum is a singular form, which represents a single bacterial cell.
Spirilla are small, curved and coiled in appearance. These usually appear singly.
Examples: Treponema pallidum, Spirillum minus, Chlamydia trachomatis etc.
Other forms of Bacteria
This group of bacteria do not possess a characteristic shape and size and comprises of variable shapes throughout the life cycle.
Examples: Mycoplasma pneumoniae, Deinococcus radiodurans, Mycoplasma genitalium etc.
Some bacteria are filamentous in structure by having a long and tubular shape. The filamentous bacteria are sometimes branched, which resembles the filamentous structure of fungi, i.e. Mycelium.
Examples: Candidatus savagella, Nocardia amarae, Alcaligenes paemba, Thiothrix sp. etc.
Stella is a type of bacteria which resembles a star.
It also refers as “Square shaped bacteria” which are having flat or straight edges. Box-shaped bacteria appear as a thin, flexible sheet of bacteria with a smooth surface. Small cells are square shaped, and the large cells are rectangular.
Box-shaped bacteria adhere to each other after the cell division. In 1980, box-shaped bacteria were isolated from the natural salt ponds.
Examples: Haloarcula vallismortis, Haloarcula marismortui etc.
This group of bacteria possess an extension or outgrowth in the form of stalk, hypha or buds.
The arrangement of bacteria can define as the position and organisation of bacteria, which strongly depends upon the type of cell division. The bacteria classify into four types based on bacterial cell arrangement and cell number.
Arrangement of Cocci
The arrangement of cocci is more complicated than the bacilli. Cocci cells can remain attached after the cell division. In cocci, the cell division can occur in many ways like in one plane, two planes, three planes or randomly. These exist in various forms based on the arrangement and cell number:
Diplococci: These cells appear in pairs, and the cell division occurs in one plane.
Examples: Neisseria gonorrhoea, Diplococcus pneumoniae etc.
Streptococci: These cells form a chain by adhering with each other. The cell multiplication occurs in one plane.
Examples: Streptococcus pyogenes, Streptococcus mutans, Streptococcus mitis etc.
Tetracocci: These cells form a group of four cells and remain joined with each other. In Tetracocci, the cell division occurs in two planes.
Example: Gaffkya tetragena etc.
Sarcinae: These cells appear as a group where the cell division occurs in three planes but in a regular pattern.
Examples: Sarcina aurantiaca, Sarcina lutea, Sarcina ventriculi etc.
Staphylococcus: These cells appear as a grape-like cluster where the cell division occurs in three planes with an irregular pattern.
Examples: Staphylococcus aureus, Staphylococcus epidermis, Staphylococcus auricularis etc.
Arrangement of Bacilli
Bacilli cells can remain attached after the cell division. In cocci, the cell division can occur in many ways like in one plane. These exist in various forms based on the arrangement and cell number:
Coccobacilli: These are oval, rod-shaped bacteria, where the division occurs in one plane.
Examples: Acinetobacter lwofii, Brucella canis etc.
Diplobacilli: These are the rod-shaped bacteria which appears in pair after the cell division. In Diplobacilli, the cell division occurs in only one plane.
Examples: Coxiella burnetii, Moraxella bovis etc.
Streptobacilli: These are the rod-shaped bacteria which appear in the form of a long chain. In Streptobacilli, cell division occurs in one plane.
Example: Streptobacillus moniliformis etc.
Palisades: These are the rod-shaped bacteria which are arranged in a side by side sequence which resembles the matchsticks.
Example: Corynebacterium diphtheria
Trichomes: These are the rod-shaped bacteria which appears in the form of chain that resembles the string of Streptobacilli. But in contrast to Streptobacilli, Trichomes have a much larger area of contact between the adjacent cells.
Examples: Spirulina sp , Oscillotoria sp etc.
Arrangement of Spirilla
Spirilla have a rigid spiral structure and usually exists singly. In the spiral bacterial cells, twisting occurs within the cell, which makes it helical in shape. These are of two types, based on arrangement and number:
Vibrio: These are comma-shaped bacteria, which forms after one complete twist.
Examples: Vibrio cholera, Vibrio vulnificus etc.
Spirochetes: It is another type, which forms after two or three complete twists. Spirochetes are highly flexible in structure.
Examples: Spirochaeta sp , Treponema sp , Borrelia sp etc.