Bacteria and its Types

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Bacteria is a single-celled organism. Bacteria are not visible to our naked eyes and can be only seen through a microscope.  

Bacteria can be categorised as gram-positive and gram-negative bacteria. Gram-positive bacteria are Pediococcus pentosaceus, Staphylococcus aureus and Streptococcus pyogenes.  

Gram negative bacteria are Pseudomonas aeruginosa and Escherichia coli. Gram-negative bacteria are more resistant to antibiotics than gram-positive bacteria, comparatively.

small single-celled organisms

What are bacteria?

Bacteria are prokaryotic organisms, single-celled organisms. The nuclear membrane is absent in bacteria, and they divide by binary fusion.  

Bacteria are both parasitic and free-living forms. They are omnipresent and can easily adapt to changes by selective mutations.

Escherichia coli is a common bacterium that is used in the research field because of its easy availability, and it is relatively simple compared to other bacteria.  

They can also be propagated and studied easily in laboratories. The advancement of knowledge in protein expression studies also makes the organism suitable for research.

Bacteria can be categorised as good bacteria and bad bacteria. Good bacteria live in our gut and promote gut health.  

Examples of good bacteria are bifidobacteria and lactobacillus species. Bad bacteria will have adverse effects on our health. Examples of bad bacteria are Clostridium and Staphylococci species.

Types of bacteria  

Bacteria are divided into types depending on their size and shape. They are classified depending on the three basic shapes.  


Spherical shaped bacteria

Bacteria that are spherical or round in shape are called coccus or cocci in the plural. The spherical-shaped bacteria are the most studied.

Phylogenetic studies reveal that spherical bacteria could have evolved from rod-shaped bacteria.

The rod-shaped bacteria, by mutation, could have been converted to spherical-shaped bacteria. The cell wall or the peptidoglycan is responsible for retaining the shape after isolation.

The spherical bacteria are further divided into types depending on their cocci arrangement.

Cocci arranged in pairs are called diplococcus bacteria. Acinetobacter baumannii is a diplococci bacterium that is pathogenic.

Streptococci bacteria will have cocci arranged in a chain. Streptococcus pyogenes is a facultative, gram-positive bacteria.

Tetrad bacteria will have their cocci arranged in squares of 4. Lactic acid bacilli, Aerococcus and Pediococcus species will have a tetrad arrangement.

Sarcina cocci will be arranged in cubes of 8. Sarcina lutea will have cuboid arrangement.

Staphylococci have an irregular arrangement. Staphylococcus aureus has an irregular arrangement.


Rod shaped bacteria

The rod-shaped bacteria occur either in a single form or as multiple cells. Bacilliform bacteria usually occur in a rod shape. They are both aerobic and facultative anaerobic bacteria.  

They are resistant to heat and produce endospores to survive in unfavourable conditions.  

Bacillus anthracis and Bacillus cereus are rod-shaped bacteria and are pathogenic. Rod shaped-bacteria are classified into types depending on the arrangement.

Bacillus is a single, unattached rod-shaped bacteria. Salmonella enterica is an organism that will have such an arrangement.

Diplobacilli have an arrangement of two rods attached together. Klebsiella rhinoscleromatis can be seen in such an arrangement.

Streptobacilli are arranged in a chain, and Streptobacillus felis is an example of a streptobacilli arrangement.

Coccobacilli will be oval and have the appearance of cocci. Chlamydia trachomatis can be seen in in cocci.

Palisades have a fence-like structure. After the cell division, they bend and form a structure that reassembles a palisade. Corynebacterium diphtheria has a palisade structure.


Spiral shaped bacteria

As the name suggests, the bacteria are spiral or helical in structure. Spiral bacteria is a third major morphological category of the prokaryotic organism.

Some spiral-shaped bacteria may be pathogenic to humans. These bacteria are commonly found in the aquatic region except a few.

They are gram-negative bacteria and are classified into further types depending on their physiology.  Spirillum is a gram-negative bacterium that is rigid and has external flagella. Helicobacter pylorus is an organism that has such an arrangement.

Another classification is a spirochete. They are spiral and thin. These bacteria are flexible and have internal periplasmic flagella.

Spirochaeta, Borrelia and Leptospira are some of the species that are spirochete in structure.

Structure of bacteria

Bacteria are unicellular organisms that lack definite nuclei and membrane-bound organelle. They come in different shapes and structures.

Bacterial cell includes  


A capsule is a protective layer that is made up of polysaccharides. The composition of the capsule is 98% water and 2% polysaccharide or polypeptide. The capsule is 0.2µm thick.

The significant role of the capsule is to protect the bacteria from dying and protecting from phagocytosis. The capsule is the virulent part of a bacteria.  

It is present in both gram-positive and gram-negative bacteria. The polysaccharide layer is present outside the cell wall, and it is hard to break the cell.  

The cell wall can be disrupted by vigorous washing or by organic laboratory methods. The lipoproteins are only found in gram-negative bacteria.  

The capsule is made up of amorphous viscid secretion and diffuses into the surrounding. The capsules are tightly packed and cannot be stained easily.  

The capsule contains water to keep the bacteria hydrated. Capsule helps the bacteria to adhere to surfaces.

The capsule also protects the bacteria from getting engulfed by WBCs.  

Some of the capsulated bacteria are Bacillus anthracis, Streptococcus pneumonia, Klebsiella pneumonia and Haemophilus influenza.

Cell wall

The cell wall is the most important structure of bacteria. It gives rigidity, support and shape to the bacteria.

Based on the composition of the cell wall, the bacteria are classified into major groups; the gram-positive bacteria and gram-negative bacteria.

The cell wall composition of gram-positive bacteria is a lipid, teichoic acid and lipid. The composition of gram-negative bacteria includes lipid, protein and lipopolysaccharide.

The cell wall gives the bacteria protection and structural support. The cell wall is a semi-permeable membrane acting as a filtering mechanism.

The cell wall protects the bacteria against osmotic lysis and acts as a molecular sieve. It also contains pigment for support.

The binding site for the enzymes is present in the outer layer of the cell wall. The outer layer of the cell wall acts as a medium between the cell and the environment.

The outermost layer of the cell wall employs a site for the attachment of the antigen and agglutin to the host, substrate and other cells.

Plasma membrane  

The bacterial cell wall contains a semi-permeable lipid bilayer. The cell membrane or plasma membrane separates the inner cell components from the outside environment.

The plasma membrane helps in the movement of water and other nutrients through the cell. It provides a fixed environment for the cell.

The plasma membrane contains proteins such as glycoprotein and lipid-protein in its outer environment that helps to interact with the other cells.  

The cell membrane also provides structural support for the cell.


The cytoplasm of the cell wall has a gel-like matrix that is composed of water, enzymes, nutrients, waste materials and gases.

The cytoplasm of the bacteria helps in cell growth, metabolism and replication. The prokaryotic cells do not have a membrane-enclosed nucleus.

The cellular components can be seen scattered throughout the cytoplasm. The cytoplasm promotes chemical reactions.

The cellular process such as protein synthesis, glycolysis, mitosis and meiosis take place in the cytoplasm.


The bacteria contain small circular molecules called bacterial chromosomes. In addition, to the bacterial chromosome, it also contains small circular DNA molecules called plasmids.

The bacterial DNA is in both circular and linear forms. The main function of DNA is to pass genetic information.  

The bacterial cell produces many copies of its own chromosomes. This is the reason bacteria is easy to isolate and preferred by many researchers in the laboratory.

Plasmids act as a vector or a delivery vehicle when a new DNA is introduced into bacteria. The plasmid also helps the bacteria to develop resistance to antibiotics.


The gram-positive and gram-negative bacteria have a motility organelle called a flagellum. They help the bacteria in locomotion and chemotaxis.  

The flagella are hair-like structures that help the bacteria in locomotion. Bacteria can have the flagella all over their surface, as a cluster on one end or at one end.  

The flagella also perform other functions apart from locomotion. The flagella help in other functions such as protein synthesis, biofilm formation and adhesion.  

It also plays a major role in interaction with the host targets. The flagellated bacteria are more virulent than the non-flagellated bacteria.  

In species like E. coli, Clostridium difficile and Pseudomonas aeruginosa, the flagella play a multifunctional role. They help in locomotion and also adhesion.


Bacteria have small hair-like projections emerging from the cell surface called pili. The pili help the bacteria to attach to other cells like the intestines.

The pili can cause infection to the host they get attached to. Some specialised pili help in conjugation, where two bacteria exchange fragments of a plasmid.

What are bacteria feeding?  

Bacteria feed on most organic and inorganic compounds as food. They mostly can survive extreme conditions.

Heterotrophic bacteria feed on organic compounds. Most bacteria feed on dead and decomposing flesh.

Autotrophic bacteria make their food by a mechanism of photosynthesis. They also make their food by chemosynthesis. The bacteria that prepare their food by chemosynthesis are called chemoautotrophs.

Photosynthesis requires sunlight, water and carbon dioxide, whereas chemosynthesis requires carbon dioxide, chemicals and water.

Bacteria that prefer photosynthesis mechanisms are called photoautotrophs. Another type of bacteria called cyanobacteria produces oxygen.

Heliobacteria do not produce oxygen. Chemoautotrophs are commonly seen in oceans and roots of leguminous plants.  

Aerobic bacteria can only grow in an environment where oxygen is present. Some bacteria cause corrosion, water clarity problems and bad smells.

Anaerobic bacteria will be able to survive without oxygen. They are commonly found in the human intestinal tract.

Facultative bacteria will be able to survive with or without oxygen. Facultative bacteria grow favourably in an environment with oxygen.

Mesophilic bacteria can grow at moderate temperature and causes many infections in humans.  

Listeria monocytogenes, Pseudomonas maltophili and Thiobacillus novellus are some examples of mesophilic bacteria.

What are reproduction and transformation?

What are reproduction and transformation

Bacteria can reproduce, and they use different methods such as binary fission, conjugation and spores.

The bacteria take up foreign genetic material from the environment through a process called bacterial transformation.  

The gene transfer takes place horizontally, and it only requires the resolute DNA in the environment, not the living donor. The bacterial cell to undergo the transformation process, it must become a competent cell.  

Once the bacterial cell becomes competent, it can take up extracellular genetic material through the process of transformation.

The bacteria adapt the transformation to stabilise the fluctuation in population dynamics. The transformation has also helped to overcome challenges in harsh and extreme environmental conditions.

During transformation, bacteria release the bacterial DNA into the environment and take up the competent cells. The competent cells respond to environmental and genetic changes.

The competent cells also control the level of gene acquisition through the transformation process. Only some bacteria will be able to take up exogenous DNA from the environment.

To make the bacterial cell competent requires chemical treatment or electrical pulses. Electroporation is the technique to make bacterial cells permeable by subjecting them to electrical pulses.

The transformation technique is widely used in the field of molecular biology. Transformation helps to make multiple copies of DNA in a cloning procedure.

To express a protein or an enzyme, to create a cDNA library and in DNA linkage studies.

Uses of bacteria

As classified earlier, there are good and bad bacteria. Good bacteria benefit humans and help in their survival.

Human survival

The bacteria in our digestive system aid digestion and breaks down complex nutrients such as sugars, so the body can use them easily. These bacteria are classified as probiotics.

Probiotics are live bacteria in our intestines and balance the good and bad bacteria.  

The bifidobacteria and the lactic acid bacteria are the two important gut bacteria. Lactobacillus and the Leuconostoc species are the predominant species present in our intestines.

Probiotics also enhance the human immune system. Pediococcus pentosaecus, a gram-positive bacterium isolated from the betel leaf, is said to boost the immune system.

Lactic acid bacteria present in our digestive system help in digestion and also prevent getting any digestive problems such as diarrhoea and stomach pain.

Nitrogen fixation

Nitrogen is important for plants to grow. Plants require nitrogen to perform photosynthesis, but they cannot use the nitrogen present in the atmosphere right away.

The nitrogen-fixing bacteria help the plants to perform photosynthesis by converting the atmospheric nitrogen into nitrates, nitrites or ammonia.  

The plants, in turn, provide a space for the bacteria to live. The bacteria and the plant help each by the process of symbiosis.

Bradyrhizobium japonicum is a nitrogen-fixing bacteria that helps the soybean plant fix nitrogen in its nodules.

Rhizobium leguminosarum is a bacterium that helps beans and plants to fix nitrogen in their roots.

Food technology

Lactic acid bacteria are a predominant group of bacteria among the microorganism that is used in food industries.

Kimichi, a famous Korean food, is rich in probiotics. The bacteria that ferment Kimichi are Lactobacillus, Weisella and Leuconostoc species.

Lactic acid bacteria produce bacteriocins that have anti-microbial properties. They are reported to have protection against enteric pathogens such as E.coli and Shigella species.

Lactobacillus reuteri is one of the baceriocin producing bacteria. They are used to stabilise the food and also as food additives.

Bacteria in industry and research  

Bacteria play an important role in industry and research. Bacteria helps to break down complex compounds into simplified products.

Lactobacillus species play an important role in winemaking. They are also used to process the waste and clean up oil spills in oceans.

Ideonella sakaiensis bacteria are capable of breaking down and consuming plastic.  

Bacteria in molecular biology play a vital role. They are used to express certain protein and gene expressions.

Bacillus thuringiensis a bacterium used in the agriculture industry to prevent the pests from consuming the pests. These bacteria prevent the undesirable effects of chemical pesticides in the environment.


Though the bacteria are beneficial to humans, there are bacterial species that cause infections and undesirable side effects.

Some bacteria cause diseases such as Cholera, Diptheria, Tuberculosis and Typhoid.

Clostridium species, a gram-positive bacterium, is known to cause fatal food poising in humans. They are capable of producing toxins that cause infection in humans.

Neisseria gonorrhoeae is a sexually transmitted bacteria that affects the mucous of the reproductive tract.  


Though bacteria help humans to survive, some bacteria infect humans. When left untreated, these bacterial infections may be fatal.

Always be thoughtful in the foods you consume. The bacterial infection mostly spreads through water and food. Heat the food before consuming it, and only drink boiled water to keep yourself away from such infections.


What are the four types of bacteria?

The four types of bacteria are spherical, rod, spiral and comma-shaped bacteria.

What are the growth characteristics of bacteria?

The bacterial growth curve includes the lag phase, exponential phase, stationary phase and death or decline phase.

What are the characteristics of bacteria?

The bacteria are unicellular, microscopic organisms that lack membrane-bound organelle.

What are the morphological characteristics of bacteria?

Bacteria are complex organisms and present in different shapes.

What is the identification of bacteria?

 Bacteria are identified through serotyping and antibiotic inhibition tests.

What is the structure and shape of bacteria?

The bacteria are divided into four types depending on their shape. The four types of bacteria are spherical, rod, spiral and comma-shaped bacteria.


The Information including but not limited to text, graphics, images and other material contained on this blog are intended for education and awareness only. No material on this blog is intended to be a substitute for professional medical help including diagnosis or treatment. It is always advisable to consult medical professional before relying on the content. Neither the Author nor Star Health and Allied Insurance Co. Ltd accepts any responsibility for any potential risk to any visitor/reader.

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