Bacteria: Definition, Structure, Types and More

Microorganisms are tiny organisms that are microscopic in size. They can be divided into five major groups- bacteria, algae, fungi, archaea, protozoan, and viruses. 

Bacteria are unicellular organisms that belong to the prokaryotic category. Their size ranges between 1 and 10 micrometres. They reproduce by a process called binary fission. 

Bacteria are known to be the earliest forms of life to ever exist on Earth. They obtain necessary nutrients by decomposition of waste, photosynthesis, or chemical breakdown of compounds. Gram-positive and gram-negative bacteria are subdivisions of bacteria. A separate discipline called bacteriology deals with detailed research of bacteria. 

The fundamental bacterial cell

The bacterial cell is a simple structure. Since it is a prokaryote, the cell does not contain any organelles bound by any membrane. Following are the various parts of a bacterial cell:

• Cell wall

1. It is the external layer of the bacterial cell. 
2. It is a very firm layer responsible for maintaining the shape of the cell and keeping the cell contents intact.
3. Its function is to protect the cell from external objects, prevent bursting of the cell and give a shape to the cell.
4. It is made of peptidoglycan or murein and has teichoic acids embedded. 

• Cytoplasm

It is a colloidal complex that contains 80% water and 20% salts and proteins. All the organelles are scattered across the cytoplasm, including DNA and plasmids. 

• Nucleus

The nucleus is located in the cytoplasm. It has no distinct cytoplasmic membrane. The nucleus contains genetic substances. The DNA is haploid and round. There is also the presence of extracellular DNA known as a plasmid.

• Ribosomes

Ribosomes are where proteins are synthesised. Eukaryotic ribosomes are bigger compared to bacterial ribosomes. The difference is that prokaryotes have 70s ribosomes and Eukaryotes have 80s ribosomes. 

• Flagella

Flagella are identical to the shape of the hair. They emerge and elongate from the cytoplasmic membrane. The only function of flagella is the movement of the cell. They are divided into three parts: filament, basal, and hook. 

• Pili 

Their structure is hair-like and made up of proteins. They are extensions from the cell wall. They are shorter, thinner, and present in large numbers compared to flagella. Their function is to adhere the cell to various surfaces. They are also called Fimbriae.

Shapes of bacteria

Bacterial cells can be divided into three fundamental groups based on their shape. They are:

• Cocci 

The term ‘coccus’ came from the Greek word ‘Kokko’, which means seed. These bacterial organisms are shaped like spheres or ellipses (oval). They are present as independent cells or may be present in clusters. They can be further divided into five subclasses: 

1. Monococcus: They are also known as micrococcus, and they exist in mono and circular form. Ex: Micrococcus flavus.
2. Diplococcus: In this type of coccus bacteria, the division of cells takes place in a particular plane. Ex: Diplococcus pneumonia 
3. Streptococcus: Their cell division takes place repeatedly on one plane. Due to this, it forms chains. Ex: Streptococcus pyogenes
4. Tetracoccus: This type of coccus bacteria embodies four circular cells. Ex: Gaffkya tetragena.
5. Sarcina: In this type of bacteria, the division of cells takes place in 3 planes. They have a cubic structure with a definite shape and embody 8 to 16 cells. Ex: Sarcina lutea.

• Bacilli

The shapes of these bacteria can be cylindrical or rod-like. Their width ranges between 0.5 and 1 micrometre, and length varies between 1 and 4 micrometres. They can be subcategorised into: 

1. Bacillus: they appear in mono or single forms. Ex: Escherichia coli 
2. Streptobacillus: the division of cells forms chains in this type of bacilli. 
3. Coccobacillus: they are identical to the coccus form.

• Spirilla: They are helical in shape. They have a hard texture with motile flagella.  Ex: Spirillum winogradskyi
• Vibrio: They are shaped like a comma. However, they are not completely curved. Ex: Vibrio mytili
• Spirochete: Like the spirilla, the spirochete also has a coil shape. Unlike spirilla, they are flexible. It is observed they produce mycelium. They have axial filaments which are responsible for movement. Ex: Spirochaete.

Nomenclature 

The International Committee on Systematic Bacteriology is the governing body that takes care of naming bacterial cells. According to the rules given by this body, the name of a bacteria should contain two parts. The first part is the genus of the bacteria, and the second is the species name. This type of naming system is known as binomial nomenclature. 

Example: Escherichia coli –  Escherichia is the name of the genus, and coli is the name of the species

Applications

1. Anaerobic bacteria play a role in sugar fermentation.
2. It is a common misunderstanding that bacteria are harmful to humans. Some bacteria are good for the body too. For example, the bacteria inside our body are vital for maintaining good gut health. They help to break down sugars and nutrients into smaller fragments.
3. In the food industry, Lactobacillus is used in the production of cheese, fermented beans, yoghurt, vinegar, etc.
4. Bacteria help in breaking down large organic compounds, which helps in waste disposal.
5. The plasmid present in a bacterial cell is used as recombinant DNA.
6. Bacterial cells help in making antibacterial vaccines and tablets.

Conclusion  

According to the definition of bacteria, they are unicellular organisms grouped under the prokaryotic category. They are omnipresent. Bacteriology is the study and research of bacterial organisms. The structure of a typical bacterial cell comprises a cell wall, cytoplasm, nucleus, flagella, ribosomes, and pili. Bacteria are classified into five types according to the shape of their cells- spirilla, bacilli, cocci, vibrio, and spirochete. The International Committee on Systematic Bacteriology governs the naming of bacteria and uses the binomial nomenclature. Bacteria have various applications in antibiotics, fermentation, digestion, waste disposal, and DNA recombination.