Hey, there, in this article we will learn about the most important and interesting topic in microbiology “Size, Shape and Arrangement of Bacterial Cells“.
You have to know about the Shapes, Size, and arrangements of bacterial cells before going to dip in microbiology.
Now let’s jump into the main article……..
Bacterial cells come in an array of shapes, sizes and arrangement. They are tiny enough to not be seen by the naked eyes. The measurement unit used to measure microorganisms is micrometer (mm); 1 mm equals 0.001 millimeter. One nanometer (nm) equals 0.001 millimeter. The majority of bacteria are 0.2 to 2 um in size and range from 1 to 6 um in length. To demonstrate how tiny bacteria are, think 500 bacteria that are 1 millimeter across could easily be found in a row from end to end throughout the space at the end of this paragraph.
What are bacteria?
Bacteria is a single-celled, prokaryotic, and microscopic organism and sometimes carried plant characters, which live in both living and nonliving substances and whose reproduction takes place through binary fission process.
- The size, shapes, and arrangement of bacterial cells varies based on the types of species.
- We can not see bacteria in naked eyes. We need a microscope to see them.
- Bacterial cells are much smaller than eukaryotic cells, generally one-tenth of the size of the eukaryotic cell.
- They lack membrane-bound organelles like eukaryotes.
- Bacteria can thrive in extreme habitats, for example, hydrothermal vents, hot springs, and in your digestive tract.
- They mainly reproduced by binary fission.
- Bacterial cells can quickly increase their numbers of identical cells to form a colony.
- Bacterial cells lack chlorophyll pigments.
Size of Bacterial Cell
The primary unit we used to measure the size of a bacterial cell is micron (micrometer). Where I micrometer (um) is one-thousandth of a millimeter. The average size of a bacteria is varied between 0.2 and 2.0 um.
The resolution limit of our eyes (Naked eye) is between 100 and 200 um, which is about the diameter of our hair. Whereas a bacterial cell’s size varies between 0.2 and 2.0 micrometers (diameter).
Benefits of Bacterial Cell Being Small In Size
There are different benefits of being small in size;
- The variation in size of bacterial cells helps them to thrive in various microenvironments. For example, vertical gradients in intertidal marine sediments in which various types of bacteria have been found.
- Bacterial cells also get benefits for parasitism and oligotrophy because of small size.
- They can survive in such environments where nutrient concentration is low.
- Being small, bacterial cells have a high surface area to volume ratio, which helps them to take up as many nutrients as possible for survival.
Variations in Size
The bacterial cells are divided into this following groups based on the size range;
A. Average Size Bacteria
The average size of a bacterial cell varies between 0.2 to 2.0 micrometers (diameter). However, there are few bacteria that hold this to range between 1 and 10 micrometers.
- The size range of Escherichia coli bacteria is between 1.1 and 1.5 um in diameters.
- The size range of Bacillus anthrac is between 1.0 and 1.2um.
- Bacillus subtilis range between 0.25 and 1.0um in diameter.
B. Smallest Size Bacteria
- The size range of the smallest bacteria is between 0.2 and 0.5um (200 to 500 nanometers) there have been studies where some were found to filter through 0.1um filters.
- Smallest bacteria also known as nanobacterium.
- Most of them are Gram-negative in nature.
- In aerobic conditions, they grow slow.
- Mycoplasma genitalium size range is approximately 200 to 300 nm, which is considered as the smallest bacteria.
- The size of Haemophilus influenzae ranges between 0.2 and 0.3um, and
- The size of Mycoplasma gallicepticum ranges between 200 and 300 nanometers in diameter.
C. Large Bacteria
This type of bacteria is large enough to see in the naked eye. For example;
- Thiomargarita namibiensis size range between 0.1–0.3 mm (100–300 μm) in diameter, but sometimes attaining 0.75 mm (750 μm), which is considered as the largest bacteria.
Smallest Bacteria – Mycoplasma genitalium
- The size range of Mycoplasma genitalium is between 200 to 300 nm (approximately), which makes it smaller than other small bacteria, including rickettsia and chlamydia.
- It’s a type of parasitic bacterium.
- Mycoplasma genitalium is mainly found in primate bladder, waste disposal organs, genital, and respiratory tracts.
- Mycoplasma genitalium also known as ultra microbacterium because they are smaller than 0.1 μm3 under all growth conditions.
- This bacteria is responsible for urethritis in both men and women, and also cervicitis and pelvic inflammation in women.
- Mycoplasma genitalium has 580,070 base pairs.
- It also has 525 genes.
Largest Bacteria -Thiomargarita namibiensis
- The size of Thiomargarita namibiensis is between 0.1–0.3 mm (100–300 μm) in diameter, but sometimes attaining 0.75 mm (750 μm). This bacteria is large enough to be visible to the naked eye.
- It is a Gram-negative coccoid Proteobacterium.
- The term “Thiomargarita” means “sulfur pearl” and “namibiensis” means “of Namibia”.
- They are mainly found in ocean sediments of the continental shelf of Namibia.
- This bacteria contain microscopic sulfur granules which scatter incident light, lending the cell a pearly luster.
Shapes of bacterial cell
There are present different varieties of bacterial cells in different shapes. This variation in shape of a bacterial cell is determined by the bacterial rigid cell wall and cytoskeleton.
Environmental factors are also responsible for the variations in shapes. Research studies have shown that the variation in bacterial cells helps them to survive in extreme environmental conditions.
Naturally there are present three types of bacterial cell, which are classified based on their shapes;
- Coccus: These types of bacterial cells appear as spherical or round in shape.
- Bacillus: These types of bacterial cells appear as rod shapes.
- Spiral: These types of bacterial cells appear as curved, spiral or curved in shape.
Reason For Variation In Shape of Bacterial Cell
The shape of a bacterial cell depends on a special protein called MreB. MreB. This protein forms a spiral band which is a simple cytoskeleton around the interior of the cell just under the cytoplasmic membrane.
It is believed to outline form by recruiting further proteins that then direct the particular sample of bacterial cell development.
If somehow the MreB gene in bacillus-shaped bacteria is inactivated, it will become a coccoid shaped. Coccid shaped bacteria naturally lacked the MreB gene.
Arrangement of Bacterial Cell
There are present mainly five types of bacterial cell arrangement such as;
- Diplo: In this type cells remain in pairs after division.
- Strepto: In this type cells remain in chains after division.
- Tetrad: In this type cells remain in groups of four and divide in two planes.
- Sarcinae: In this type cells remain in groups of eight and divide in three planes.
- Staphylo: In this type cells remain in clusters and divide in multiple planes.
Arrangement of Coccus (plural, cocci)
When bacterial cells appear spherical or oval in shape, this type of bacteria are called coccus. Coccus cells may appear as single cells or remain attached to each other.
They are divided into several classes based on their arrangements, such as;
- When a coccus cell stays singly in a plane, then this type of coccus cells are called monococci.
- Example: Micrococcus flavus
- When two Spherical Bacteria stay together in a plane then they are called diplococci.
- In this type, the Cells are divided into one plane and remain attached predominantly in pairs.
- Example: Diplococcus pneumonia, Streptococcus pneumoniae, Moraxella catarrhalis, Enterococcus spp, Neisseria gonorrhea
- When more than two coccus are arranged in a plane and form a chain then they are called Streptococci.
- In this type, the Cells divide into one plane and remain attached to form a chain staructure.
- Example: Streptococcus pyogenes, Streptococcus pyogenes, Streptococcus pneumoniae, S. mutans.
- When a number of coccus cells are divided into three planes (3D) in an irregular pattern and produce a branch of coccus then these are called staphylococci.
- Example: Staphylococcus aureus, Staphylococcus epidermidis, Staphylococcus haemolyticus, Staphylococcus aureus, Staphylococcus capitis.
- When cells are divided in two planes and characteristically form four cells then they are called it tetrads.
- Example: Micrococcus sp. , Pediococcus, Tetragenococcus
- When coccus divided in three planes in a regular pattern and produced a cuboidal arrangement they are called Sarcina.
- Example: Sarcina maxima, Sarcina aurantiaca, Sarcina lutea, Sarcina ventriculi
Arrangement of Bacilli (plural, bacilli)
Bacillus is a rod-shaped, endospores forming, and facultative anaerobic bacteria. Like as cocci, bacillus also has different types of arrangements such as;
- When Straight rod shaped bacteria stay single then they are called mono bacillus.
- Example: Lactobacillus sp.
- When Straight rod shaped bacteria stay in pairs then they are called Diplobacillus.
- Example: Bacillus Licheniformis, Coxiella burnetii, Klebsiella rhinoscleromatis, Moraxella bovis.
- When bacillus form chain-like structures, then they are called streptobacillus.
- Example: Bacillus Subtilis, Streptobacillus moniliformis, Streptobacillus Levaditi, Streptobacillus felis, Streptobacillus hongkongensis.
- When bacillus are arranged side by side like a match stick is called palisad.
- Example: Corynebacterium diphtheriae
- Sometimes bacillus form chains which have a much larger area of contact between the adjacent cells then streptobacillus, then they are called trichomes.
- Example: Saprospira grandis
Arrangement of Spiral
- This type of bacteria are elongated, spiral-shaped, rigid cells. They also contain flagella, which help them in movements.
- Example: Spirillum minus, which causes rat-bite fever.
- This type of bacteria appears as a comma shape or slightly curved in shape.
- Vibrio bacteria are Gram-negative facultative anaerobes and can survive without oxygen.
- They also contain flagellum, which helps the theme in movement. Some of Vibrio bacteria are responsible for food poisoning.
- Example: Vibrio mytili, Vibrio anguillarum, Vibrio parahaemolyticus, Vibrio cholerae.
2. Spirilla bacteria
- When a bacterial cell completes one or more complete turns and produces helical shape and stays rigid, then these bacteria are called spirilla.
- Like as spirochetes they lack endoflagella
- Example: Aquaspirillum, Campylobacter jejuni, Spirillum winogradskyi, Oceanospirillum.
- When a bacterial cell completes one or more complete turns and has the ability to control their shape are called spirochetes.
- They are spiral bacteria and contain helical shape.
- Spirochete can be distinguished with their characteristic feature called axial filament, which helps them in motility.
- Axial filament is located on both outer and inner membranes of Spirochete.
- Axial filaments run along the length of the bacteria during motion.
- Axial filaments are also responsible for the twisting motion of the organisms.
- This bacteria is flexible and produces mycelium.
- Example: Leptospira, Spirochaeta, Treponema
These are various shaped bacteria, which have no particular structure.
- Corynebacterium spp: Pleomorphic gram-positive rods
- Haemophilus influenzae: Pleomorphic gram-negative coccobacilli
- Francisella tularensis: Small, pleomorphic, gram-negative rod
- Bartonella henselae: Small, pleomorphic gram-negative rod
- Cardiobacterium hominis: Gram-negative pleomorphic rod
- Propionibacterium acnes: Pleomorphic gram-positive rods
Other shapes of Bacteria
1. Rectangular bacteria
- This type of bacteria appears as Rectangular shape.
- Example: Haloarcula marismortui
2. Star-shaped bacteria
- This type of bacteria appears in the star shape.
- Example: Stella humosa
3. Stalked bacteria
- These types of bacteria are includes C. crescentus that possess a stalk on one end of the cell
- They are Triangular in shape.
5. Club-shaped Rod Bacteria
- They have thinner on one side than the other in shape
- Example: Corynebacterium.
6. Filamentous Bacteria
- This type of bacteria are thin, long and filament-shaped. Sometimes, they divide to form branches resembling strands of hair or spaghetti called mycelium.
- Example: Actinomycetes.
7. Appendaged Bacteria
- They produce pillus or fimbriae like structure, which is called appendaged bacteria. This bacteria are more poisonous than others.