- Bacteria are a type of biological cell that is prokaryotic and single-celled. Due to the lack of a membrane-bound nucleus, these are simpler than other types of living organisms.
- Although only some of them can be seen with the naked eye, while the rest are microscopic, they show a wide range of shapes, sizes, and structures.
- The unit of measurement used in bacteriology is the micron (micrometer) which is one-thousandth of a millimeter.
- Bacteria are generally one-tenth of the size of the eukaryotic cell. On average, the size of the bacteria varies from 0.5 to 5 µm.
- However, they can be as small as 0.3 µm and as large as 0.7mm.
- The resolution limit to the naked eye is approximately 200 microns, and since many bacteria are smaller than this size, they are not visible to the naked eye.
Among the largest bacteria is Thiomargarita namibiensis, which is up to half a millimeter long, and Epulopiscium fishelsoni, which is 0.75 mm long.
The smallest bacteria are members of the Mycoplasma genus, which are only 0.3 µm, as small as the larger viruses.
- The size of common bacteria such as Escherichia coli varies in size from 1.1 to 1.5 µm in diameter.
- The size of bacteria has been found to play an important role in the survival of organisms.
- Due to their small size, they are able to survive and even thrive in various unlikely environments, such as vertical sediments in the marine environment.
- Since other organisms are absent in such an environment, bacteria can use available resources.
- In addition, the small size of the bacteria favors parasitism and the ability to survive in areas with low nutrition.
- The high surface area-volume ratio also allows bacteria to absorb all the nutrients necessary for survival while allowing for constant growth and reproduction.
- Most bacteria have a rigid cell wall that provides a defined shape for bacteria while protecting internal components.
- Although this characteristic is valid for most bacteria, they vary in a way that allows them to be classified into different groups according to their forms.
- This wide variety of forms is determined by the bacterial cell wall and the cytoskeleton.
- Despite the fact that bacteria have a wide variety of shapes, any genus generally exhibits a limited subset of morphologies, indicating that, with a universe of shapes to choose from, individual bacteria adopt only those that are adaptive.
- Bacteria with different shapes present different physical characteristics to the outside world, and these characteristics help cells cope with and adapt to external conditions.
- The bacterial form has been found to help measure survival value against the nutrient acquisition, cell division, predators, surface fixation, passive dispersal, active motility, and internal or external differentiation.
The common categories of bacteria based on their forms are:
- Bacteria that are oval or spherical in shape are included called cocci bacteria.
- These can remain alone or linked together in groups. They appear flattened when placed in groups.
- The coccoid forms are supposed to have been derived from rod-shaped organisms during evolutionary time.
- These are rod-shaped cells that also like coconuts, they stand-alone or attached to other cells.
- Bacilli bacteria are among the first bacteria to have emerged, and this form is said to be not as advantageous as other forms. This has been assumed by observing the behavior of E. coli filamentous cells which, although mobile and chemotactic, move slowly and cannot rotate to change direction.
- This group includes bacteria that are helical or curved (comma-shaped).
- The bacteria can vary from a slightly curved spiral to spiral.
Arrangements of Cocci
- Cocci bacteria can be organized individually, in pairs, in groups of four, in chains, in groups or cubes consisting of eight cells.
- These cells remain attached during cell division.
This group includes bacteria that are present as a single cell.
- This arrangement results when two bacterial cells occur as a pair (joined).
- Some of the cells in this arrangement may remain spherical, while others may appear flattened, elongated, or bean-shaped.
- Examples: streptococcal pneumonia, Moraxella catarrhalis, Enterococcus spp, Neisseria gonorrhea.
- The tetrad bacteria are arranged in a group of four cells that remain attached and grow in the attachment after cell division.
- This arrangement occurs when the cells are divided into two planes.
- Examples: Aerococcus, Pediococcus, and Tetragenococcus.
In this arrangement, the bacterial cells of a group of eight cells.
This happens when cells divide in a perpendicular plane.
The common feature associated with these organisms is being a strict anaerobe.
Examples: Sarcina aurantiaca, Sarcina lutea, Sarcina ventriculi.
- Here, the bacteria are arranged in long chains.
- These bacteria are present in the Streptococcaceae family, which is characterized by a lack of motility and Gram-positive bacteria.
- Examples: Streptococcus Pyogenes, Streptococcus pneumonia, Streptococcus mutants.
- This type includes bacteria that are arranged in grape-like clusters.
- This results from cell division on both planes and is characterized by immobile and Gram-positive organisms.
- Examples: Staphylococcus epidermidis, Staphylococcus haemolyticus, Staphylococcus aureus, Staphylococcus capitis.
Arrangement of Bacilli
- Bacilli are rod-shaped bacteria that are present as individual cells.
- These bacteria can form endospores and are facultative anaerobes.
- Examples: Salmonella enterica subsp, Bacillus cereus and Salmonella choleraesuis.
- Similar to Diplococci, Diplobacilli also exists in pairs.
- After cell division, the two cells do not divide and grow in an attached arrangement.
- Examples: Coxiella burnetii, Klebsiella rhinoscleromatis, Moraxella bovis.
- As the name suggests, cocobacilli resemble both coconuts and bacilli.
- These are shorter in size and therefore appear squat.
- Examples: Chlamydia trachomatis, Haemophilus influenza, Gardnerella vaginalis.
- Palisades are the type of bacillus bacteria that resemble a picket fence structure as a result of flexing at the point of division during cell division.
- They appear similar to Chinese letters.
- Example: Corynebacterium diphtheria that causes diphtheria.
Arrangement of Spiral
- These are slightly curved bacteria that resemble a coma.
- Examples: Vibrio mytili, Vibrio anguillarum, Vibrio parahaemolyticus, Vibrio cholera.
- Spirochetes are spiral bacteria that are helical in shape.
- These are flexible and have an axial filament that aids in motility. These filaments are essential distinguishing characters between spirochetes and other bacteria.
- These filaments run the length of the bacteria, and therefore help twist the movement of the bacteria.
- Examples: Leptospiraspecies (Leptospira interrogans), Treponema pallidum, Borrelia recurrentis.
Spirilla (Helical-shaped/Corkscrew form)
- These bacteria are similar in structure to spirochetes but are more rigid.
- They also have a flagellum but lack the endoflagella as in spirochetes.
- Examples: Campylobacter jejuni, Helicobacter pylori, Spirillum winogradskyi.
Other Shapes and Arrangements
- Bacteria that produce a unique structure such as pills or fimbriae are called adjunct bacteria.
- These bacteria are more virulent than other bacteria that do not form these appendages.
- Example: Neisseria gonorrheae, the agent of gonorrhea.
Box-shaped/ Rectangular Bacteria
- Box-shaped bacteria are rectangular in shape and look like a box.
- Example: Haloarcula marismortui.
Club-shaped Rod Bacteria
- These bacteria are thinner on one side than the other.
- One of the classic examples of this group is Corynebacterium.
- These are long, thin, filament-shaped bacteria.
- Sometimes they divide to form branches that resemble strands of hair or spaghetti called mycelium.
- Example: actinomycetes.
- This group includes bacteria that are triangular in shape.
- Example: Haloarcula.
- Bacteria that do not have a specific shape are included in this group.
- They can change their shape, but in pure culture they seem to have a definite shape.
- Examples: Mycoplasma pneumoniae, M. genitalium.
- These are the bacteria that have a stem at one end of the cell.
- Examples: Caulobacter crescentus.
- Bacteria that look like stars or are star-shaped are included in this group.
- Examples: Stella humosa.