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Bacteria: Structure, Types, Classification, and More

‘Bacteria’ which is the plural form of bacterium are small microorganism that are single celled. Bacteria can live in any possible environment which includes the soil and even the human body.

They are both useful (they can be used in medicinal processes) and harmful (they can cause disease like pneumonia and even tooth decay).


Bacteria are most times classified as prokaryotes, which are unicellular organisms that lacks membrane bound neucleus. Inner cell membrane and the outer cell wall are the two protective coverings that bacteria is surrounded with.

Some bacteria do not have a cell wall at all. Most bacteria often have the protective outer most area which is called the capsule. Extensions most times cover the surface of bacteria which help them move around to attach to a host.


Bacteria can be classified by the basis of their shape, basis of cell wall, basis of nutrition.

Based on Shape

There are four major types of bacteria under this group.

  • Cocci – These type of bacteria are spherical in shape. They may remain as a single cell or they could be together. Examples are diplococcus,streptococcus, monococcus, tetracoccus and sarcina.
  • Spirilla – These are bacteria that are spiral with terminal flagella.
  • Bacilli – These bacteria are either in pairs of can be single and they are cylindrical in shape.
  • Vibrates bacteria are known by a single genus and they are curved in shape.

Classification on the Basis of Cell Wall

It can be classified into two types. The Gram-negative and the gram-positive. While the gram-positive retains its crystal fire lit (gram stain), the gram-negative bacteria do not retain the stain.

Classification on the basis of temperature response

They are classified into;

  • Mesophilic bacteria
  • Thermophilic bacteria
  • psychrophilic bacteria

Classification on the Basis of Nutrition

Under this category, bacteria are classified into two groups.

  • Heterotrophic bacteria – This group of bacteria depend on external organic carbon for nourishment. They can be classified on the basis of media in which the bacteria are growing.
  • Autotrophic bacteria – These bacteria prepare their own food with the use of inorganic components and solar energy.

The bacteria also live freely. They have two types; photoautotrophs and the chemoautotrophs.


This occurs in three processes; Asexual, sexual an vegetative

Asexual Reproduction

This occurs by the formation of endospore (the thick oval structure that encloses the protoplast during conditions that are not favourable).

In the formation process, the bacteria protoplast shrink to the middle area of the cell enclosed within a Cladosporium while the cell remains intact.

When the extreme middle, it is followed by a two-layered wall.

Exosporium is when the area between the original cell wall and the endosporium become empty. The bacterial cell grows out when the wall is breached in favourable conditions.


This is divided into two, the budding and binary fission

  • Budding – This is a process in which vegetative cell creates a lateral protuberance in the form of an outer bulge containing a fragment of genetic material in it.
  • The bud goes along with the parent cell or it can also be separated from the parent cell.
  • Binary Fission – This is the most common way with which bacteria reproduce itself. In this process, the bacterial cell divides into two in half an hour.

Events of binary fission are;

  • The nucleoid forms a dumble shape structure and is longer in size.
  • With the help of the mesosome, they remain attached to the plasma membrane.
  • They separate from each other when the duplication of DNA and mesosome takes place.
  • The nucleoids and daughter mesosomes move toward the opposite pole.
  • The parent cell is divided into two identical cells when the plasma membrane introverts.

Sexual Reproduction of Bacteria

This takes place when enough water and substrate are not available in the environment. The types are;


This is the process where DNA fragmentations are transferred with the help of a bacteriophage from one bacteria into other bacteria.


During transformation, competent cells are ready to take up naked DNA and get transformed. A new genotype is developed as a result of the incorporation of DNA from one bacterium to another bacterium.


This process requires physical between the cell walls of bacteria. The DNA is transferred to the recipient from the donor cell. The events during the process of conjugation include coming together of two separate bacterial cells which belong to opposite strains

  • A conjugation bridge is formed between both cells using the sex pili
  • replication of the sex factor in the male
  • with the use of the conjugation tube, the sex factor slowly moves into the female and an incomplete zygote is formed.
  • The presence of sex factor in female transforms the recipient strain to the donor strain.
  • The genetic recombination product of the two strains of cells is the product of conjugation.

Bacteria in Human Health and Disease


Bacteria can be both good and bad to human health. The human gut is a very relaxing place for bacteria because there is plenty of nutrients for their sustenance.

Gut bacteria are beneficial to the host because it regulates gut mobility, it also controls the functions and maturation of microglia in the CNS(central nervous system) and it also helps in producing vitamins.

The absence of gut bacteria is linked with different central nervous system developmental problems and even specific disease conditions.

Other bacteria cause infections too. Necrotizing fasciitis is a serious soft tissue infection caused by Staphylococcus aureus. This infection can be treated if it is noticed early. The infection also affects tissues that are surrounding blood vessels and nerves.

Antibiotic Resistance

Antibiotics are drugs that help fight against infections. Antibiotics have been used for long and are most times always prescribed.

Because of the frequent use of antibiotics, the infectious bacteria have now adapted to the drugs which make it less effective. The antibiotics resistant bacteria can be contacted from family members and people close to you making it a new strain of the disease that is difficult to cure.

If an individual is always taking antibiotics, sensitive bacteria are killed but the resistant germs will still be there and they will end up growing and multiplying.

So individuals should make wise use of antibiotics so as to control the resistance spread.


  • Bacteria: Definition, types and infection; Live Science
  • Classification of bacteria; Boilogy For Everyone
Emmanuel Ekokotu
Ekokotu Emmanuel Eguono is a mass communicator and fashion designer. When he is not behind a laptop screen or on a sewing machine, he spends his time netflixing and swimming.

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