Bacteria are prokaryotic, unicellular organisms. Due to the absence of nucleus or membrane bound organelles, bacteria's cell structure is much simpler than other organisms. Bacteria are found everywhere, they can thrive even in extreme conditions like hot springs, snow, deep ocean where other organisms face difficulty to survive.
Some of the bacteria's can be seen by naked eyes while the rest of the bacteria's are microscopic. As the resolution of the unaided human eye is about 200 microns, since many bacteria's are much smaller than this size, they are not visible with an unaided human eye.
The unit of measurement of bacteria is called (μ or μm). 1 micron is equal to 0.001 millimeters. The size of bacterias are constantly depends on the environment and growth conditions. Clinically important bacterias and pathogens typically range from 0.2 – 1.5 μm in diameter and 3-5μm in length.
This small size plays a major advantage in their survival and ability to inhabit several harsh environmental conditions. Since their small size bacterias can easily inhabit small spaces where it is not suitable for larger organisms.
In these harsh conditions competitions for the nutrition resource is relatively low allowing bacterias utilize the nutrients, which allows the steady growth and reproduction.
Bacterias smaller size gives them a higher surface area to volume ratio, meaning they have a relatively large cell surface through which nutrients can enter compared with the amount of cell material that needs those nutrients. This allows them to take up nutrients efficiently.
Bacterial cell wall is made up of peptidoglycan (murein), which is a polymer of sugars, alternating N-acetylglucosamine (NAG) linked to N-acetylmuramic acid (NAM) and amino acids peptide chain. Changes in the configuration of the polymer and its thickness is responsible for different morphology and shapes of a bacterial cell.
Although bacterias have many different shapes, each genus usually has only a few characteristic shapes.This strongly suggests bacterias shapes are not very random, they have evolved because they provide advantages in a particular environment.
Bacteria’s shapes constantly influence several properties like cell-surface area, nutrient absorption, attachment to the host or surface, motility, interaction with the surrounding environment.
As we read above bacterias are in different shapes which are influenced by adaptability and functionality. These shapes are primarily categorized into three main groups: cocci, bacilli, and spirilla.
1. Cocci (Spherical): Cocci are bacterias that are in a spherical or round shape. Cocci may occur in singles, pairs, in chains, in clusters or even in groups.
2. Bacilli (Rod-shaped): Bacilli are bacterias that are in rod-like shapes. These rod shapes can be short, little rounder or long. Bacilli bacterias can exist in singles, in pairs, or in chains.
3. Spirilla (Spiral or Helical): Spirilla are bacteria with a spiral or helical shape. These bacteria have a single, rigid spiral or a flexible corkscrew shape.
Cocci bacteria can be single or multiple in pairs, in groups of four, in chains, in clusters. Cocci bacteria can be round, oval or bean-shaped. The cells do not separate after cell division. According to the arrangement of cells, cocci bacterias are subdivided into various subtypes:
Monococcus: A monococcus is a bacteria that exists as a single spherical cell. The monococcus term comes from the Greek mono (single) and kokkos (berry).
Diplococcus: Diplococcus bacteria are arranged in pairs after cell division. After the cell division some of the cells might remain spherical while some might appear flattened, elongated, or bean-shaped. Examples of diplococcus bacteria are: Neisseria spp, Moraxella catarrhalis, Streptococcus pneumoniae.
Tetrads: Tetrads bacterias are arranged in a group of 4 cells. In tetrads cell division occurs in two different planes. Examples of tetrad bacteria are: Micrococcus spp, Pediococcus, Tetragenococcus.
Sarcinae: Sarcinae bacterias occur as a group of 8 cells. Sarcine shape happens because the cells divide in three planes and form a cuboidal cell arrangement. These bacterias are commonly found in the large intestine and skin and these are anaerobic gram-positive bacteria. Examples of sarcinae are: Sarcina aurantiaca, Sarcina lutea, Sarcina ventriculi.
Streptococcus: Cocci divide in one plane and remain attached after division from a chain-like arrangement. These cocci’s are non-motile, aerobic and gram-positive bacteria which causes many diseases. Examples of streptococcus are Streptococcus pyogenes, Streptococcus pneumonia.
Staphylococcus: In this type bacteria cells are arranged in an irregular cluster, which looks like a grapes. This is due to the cell division in three planes. These types of bacterias are immotile and Gram-positive. Examples of staphylococcus are staphylococcus aureus, Staphylococcus haemolyticus.
Bacilli are bacteria with rod-like shape, these bacterias can be short or long. Based on the arrangement of rod shaped cells, bacilli are classified into various types:
Bacillus: Bacillus are rod shaped bacteria and present as single cells. These are endospore forming bacteria. These bacterias aerobic or facultatively anaerobic. Examples of bacillus are; Bacillus cereus, Salmonella enterica.
Diplobacilli: Diplobacilli are two rod cells attached to each other and appear in pairs after cell division. Examples of Diplobacilli are: Moraxella bovis, Klebsiella rhinoscleromatis, Coxiella burnetii.
Streptobacilli: This type of bacilli are arranged in chains, as the cells divide in one plane. Streptobacilli bacteria are gram-negative, aerobic or facultative anaerobic bacteria. Examples are Streptobacillus moniliformis, Streptobacillus felis, Streptobacillus moniliformis, Streptobacillus Levaditi.
Coccobacilli: Compared to other bacilli, these are shorter and oval in shape, they appear more like a coccus. Examples are Chlamydia trachomatis, Haemophilus influenzae, Chlamydia trachomatis, Haemophilus influenza.
Palisades: Palisades are types of bacilli bacteria that fence like structure. This is a result of bent at the point of division during cell division. An example of palisades is Corynebacterium diphtheria.
Spirilla types of bacterias are spiral or helical in shape. Based on the thickness of the cell wall, flexibility and motility of cell they are further divided into three types;
Vibrio: These are slightly curved bacteria and appear more like a comma. Vibrio are most commonly gram-negative bacterias. These bacterias are commonly known for spreading food and water borne diseases. Examples: Vibrio mytili, Vibrio cholera.
Spirillum: Spirillum bacterias are rigid spiral shaped bacteria. These types of bacterias have an external flagella, which helps in their motility. Examples are Spirillum, Campylobacter jejuni, Helicobacter pylori, Spirillum winogradskyi.
Spirochet: Spirochetes are spiral, thin and flexible. They have an axial filament which runs throughout the length of the bacteria, helping in twisting motion of bacteria. These are one of the most pathogenic species that causes a lot of serious health issues. Examples are Leptospira, Treponema pallidum, Borrelia recurrentis.
Filamentous: Filamentous bacterias have a very thin long filamentous structure. Some of the bacteria's branching elements result in a multiple or network of filaments called mycelium.
Appendaged: This type has a very unique structure like pillus or fimbriae. Due to their structure they are also known as budding bacteria. These bacterias are more virulent than other bacterial types. These types can be motile with flagella or non-motile.
Rectangular: These are box or rectangular shaped bacterias. Example: Haloarcula marismortui, Haloarcula vallismortis.
Pleomorphic: The bacteria that do not have a specified shape. These have the ability to change their shape and size due to various external factors like pressure, environmental stress, etc. Examples: Mycoplasma pneumoniae, M. genitalium.
Stalked: These are the bacteria that possess a stalk on one end of the cell. A stalk develops at one end of bacteria due to asymmetrical cell division.
Star-shaped: The bacteria that resemble a star. or are star-shaped are included in this group. Example: Stella humosa, Stella vacuolata.
Size of Typical Bacteria
Some of the bacteria's can be seen by naked eyes while the rest of the bacteria's are microscopic. As the resolution of the unaided human eye is about 200 microns, since many bacteria's are much smaller than this size, they are not visible with an unaided human eye.
The unit of measurement of bacteria is called (μ or μm). 1 micron is equal to 0.001 millimeters. The size of bacterias are constantly depends on the environment and growth conditions. Clinically important bacterias and pathogens typically range from 0.2 – 1.5 μm in diameter and 3-5μm in length.
This small size plays a major advantage in their survival and ability to inhabit several harsh environmental conditions. Since their small size bacterias can easily inhabit small spaces where it is not suitable for larger organisms.
In these harsh conditions competitions for the nutrition resource is relatively low allowing bacterias utilize the nutrients, which allows the steady growth and reproduction.
Bacterias smaller size gives them a higher surface area to volume ratio, meaning they have a relatively large cell surface through which nutrients can enter compared with the amount of cell material that needs those nutrients. This allows them to take up nutrients efficiently.
Morphology of a Bacteria
Bacterial cell wall is made up of peptidoglycan (murein), which is a polymer of sugars, alternating N-acetylglucosamine (NAG) linked to N-acetylmuramic acid (NAM) and amino acids peptide chain. Changes in the configuration of the polymer and its thickness is responsible for different morphology and shapes of a bacterial cell.
Although bacterias have many different shapes, each genus usually has only a few characteristic shapes.This strongly suggests bacterias shapes are not very random, they have evolved because they provide advantages in a particular environment.
Bacteria’s shapes constantly influence several properties like cell-surface area, nutrient absorption, attachment to the host or surface, motility, interaction with the surrounding environment.
Different Shapes of Bacteria
As we read above bacterias are in different shapes which are influenced by adaptability and functionality. These shapes are primarily categorized into three main groups: cocci, bacilli, and spirilla.
1. Cocci (Spherical): Cocci are bacterias that are in a spherical or round shape. Cocci may occur in singles, pairs, in chains, in clusters or even in groups.
2. Bacilli (Rod-shaped): Bacilli are bacterias that are in rod-like shapes. These rod shapes can be short, little rounder or long. Bacilli bacterias can exist in singles, in pairs, or in chains.
3. Spirilla (Spiral or Helical): Spirilla are bacteria with a spiral or helical shape. These bacteria have a single, rigid spiral or a flexible corkscrew shape.
Cocci (Spherical Bacteria)
Cocci bacteria can be single or multiple in pairs, in groups of four, in chains, in clusters. Cocci bacteria can be round, oval or bean-shaped. The cells do not separate after cell division. According to the arrangement of cells, cocci bacterias are subdivided into various subtypes:
Monococcus: A monococcus is a bacteria that exists as a single spherical cell. The monococcus term comes from the Greek mono (single) and kokkos (berry).
Diplococcus: Diplococcus bacteria are arranged in pairs after cell division. After the cell division some of the cells might remain spherical while some might appear flattened, elongated, or bean-shaped. Examples of diplococcus bacteria are: Neisseria spp, Moraxella catarrhalis, Streptococcus pneumoniae.
Tetrads: Tetrads bacterias are arranged in a group of 4 cells. In tetrads cell division occurs in two different planes. Examples of tetrad bacteria are: Micrococcus spp, Pediococcus, Tetragenococcus.
Sarcinae: Sarcinae bacterias occur as a group of 8 cells. Sarcine shape happens because the cells divide in three planes and form a cuboidal cell arrangement. These bacterias are commonly found in the large intestine and skin and these are anaerobic gram-positive bacteria. Examples of sarcinae are: Sarcina aurantiaca, Sarcina lutea, Sarcina ventriculi.
Streptococcus: Cocci divide in one plane and remain attached after division from a chain-like arrangement. These cocci’s are non-motile, aerobic and gram-positive bacteria which causes many diseases. Examples of streptococcus are Streptococcus pyogenes, Streptococcus pneumonia.
Staphylococcus: In this type bacteria cells are arranged in an irregular cluster, which looks like a grapes. This is due to the cell division in three planes. These types of bacterias are immotile and Gram-positive. Examples of staphylococcus are staphylococcus aureus, Staphylococcus haemolyticus.
Bacilli (Rod-shaped)
Bacilli are bacteria with rod-like shape, these bacterias can be short or long. Based on the arrangement of rod shaped cells, bacilli are classified into various types:
Bacillus: Bacillus are rod shaped bacteria and present as single cells. These are endospore forming bacteria. These bacterias aerobic or facultatively anaerobic. Examples of bacillus are; Bacillus cereus, Salmonella enterica.
Diplobacilli: Diplobacilli are two rod cells attached to each other and appear in pairs after cell division. Examples of Diplobacilli are: Moraxella bovis, Klebsiella rhinoscleromatis, Coxiella burnetii.
Streptobacilli: This type of bacilli are arranged in chains, as the cells divide in one plane. Streptobacilli bacteria are gram-negative, aerobic or facultative anaerobic bacteria. Examples are Streptobacillus moniliformis, Streptobacillus felis, Streptobacillus moniliformis, Streptobacillus Levaditi.
Coccobacilli: Compared to other bacilli, these are shorter and oval in shape, they appear more like a coccus. Examples are Chlamydia trachomatis, Haemophilus influenzae, Chlamydia trachomatis, Haemophilus influenza.
Palisades: Palisades are types of bacilli bacteria that fence like structure. This is a result of bent at the point of division during cell division. An example of palisades is Corynebacterium diphtheria.
Spirilla (Spiral or Helical)
Spirilla types of bacterias are spiral or helical in shape. Based on the thickness of the cell wall, flexibility and motility of cell they are further divided into three types;
Vibrio: These are slightly curved bacteria and appear more like a comma. Vibrio are most commonly gram-negative bacterias. These bacterias are commonly known for spreading food and water borne diseases. Examples: Vibrio mytili, Vibrio cholera.
Spirillum: Spirillum bacterias are rigid spiral shaped bacteria. These types of bacterias have an external flagella, which helps in their motility. Examples are Spirillum, Campylobacter jejuni, Helicobacter pylori, Spirillum winogradskyi.
Spirochet: Spirochetes are spiral, thin and flexible. They have an axial filament which runs throughout the length of the bacteria, helping in twisting motion of bacteria. These are one of the most pathogenic species that causes a lot of serious health issues. Examples are Leptospira, Treponema pallidum, Borrelia recurrentis.
Other Shapes of Bacteria’s
Filamentous: Filamentous bacterias have a very thin long filamentous structure. Some of the bacteria's branching elements result in a multiple or network of filaments called mycelium.
Appendaged: This type has a very unique structure like pillus or fimbriae. Due to their structure they are also known as budding bacteria. These bacterias are more virulent than other bacterial types. These types can be motile with flagella or non-motile.
Rectangular: These are box or rectangular shaped bacterias. Example: Haloarcula marismortui, Haloarcula vallismortis.
Pleomorphic: The bacteria that do not have a specified shape. These have the ability to change their shape and size due to various external factors like pressure, environmental stress, etc. Examples: Mycoplasma pneumoniae, M. genitalium.
Stalked: These are the bacteria that possess a stalk on one end of the cell. A stalk develops at one end of bacteria due to asymmetrical cell division.
Star-shaped: The bacteria that resemble a star. or are star-shaped are included in this group. Example: Stella humosa, Stella vacuolata.
