Bacteria: Definition, Types, Classification, Facts

Have you heard of someone suffering from typhoid or cholera? Do you know these diseases are caused by minute organisms called bacteria? These bacteria are microscopic living cellular organisms that often remain associated with animals, plants, and even humans and many of them cause disease. In the \({\rm{1}}{{\rm{7}}^{{\rm{th}}}}\) century, a Dutch scientist, Antonie van Leeuwenhoek created a single-lens microscope with which he saw what he called animalcules, later known as bacteria. He is considered to be the first microbiologist.

Sometimes they live on as a helper by turning milk into curd or helping in our digestion. On the other hand, they are also destructive, causing diseases such as pneumonia and methicillin-resistant staphylococcus aureus (MRSA). Thus, some bacteria or bacteria are harmful, but most have some useful purpose. They support many forms of life, both plant, and animal, and are also used in industrial and medicinal processes. In this article, let’s learn everything about bacteria in detail.

What are Bacteria?

Bacteria are prokaryotes lacking well-defined nuclei and membrane-bound organelles and with chromosomes composed of a single closed DNA circle. They are classified in a separate kingdom called Monera. They are microscopic, unicellular organisms that flourish in diverse environments. These organisms can live in soil, the ocean, and inside the human gut.

Diagram and Structure of Bacterium

A typical bacterial cell contains the following structures:

1. Cell Wall: An outer covering of the cell that protects the bacterial cell and gives it shape. The cell wall of bacteria consists of peptidoglycan, an essential protective barrier for bacterial cells that encapsulates the cytoplasmic membrane of both Gram-positive and Gram-negative bacterial cells. 
2. Cytoplasm: A gel-like substance composed mainly of water that also contains enzymes, salts, cell structures, and various organic molecules. 
3. Cell Membrane or Plasma Membrane: Surrounds the cell’s cytoplasm and regulates the flow of substances in and out of the cell.  
4. Flagella: They are long hairy structures that help in locomotion found at either or both ends of a bacterium or all over its surface. 
5. Ribosomes: They have \({\rm{70}}\,{\rm{S}}\) (where \({\rm{S = }}\) Svedberg units) ribosomes which are responsible for protein production. 
6. Plasmids: Gene carrying, circular DNA structures that are not involved in reproduction. 
7. Nucleoid: Area of the cytoplasm that contains the single bacterial DNA molecule.

Classification or Types of Bacteria

Bacteria are classified on the basis of various features like shape, mode of nutrition, respiration, etc. In the year \(1872\) scientist Cohn classified bacteria to \(4\) major types depending on their shapes.

Classification on Bacteria Based on Shape

Based on Shape	Examples
Cocci: These types of bacteria are unicellular, spherical, or elliptical. Either they may remain as a single cell or may aggregate together for various configurations.	Diplococcus pneumonia
Bacilli: These are rod-shaped or cylindrical bacteria that either remain singly or in pairs.	Bacillus thuringiensis
Vibrio: These are curved, comma-shaped bacteria and represented by a single genus.
Spirilla: These types of bacteria are spiral or spring-like with multiple curvature and terminal flagella.	Spirillum volutans
Actinomycetes are branching filamentous bacteria, so-called because of a fancied resemblance to the radiating rays of the sun when seen in tissue lesions.
Mycoplasmas are bacteria that are cell wall deficient and hence do not possess a stable morphology. They occur as round or oval bodies and as interlacing filaments. Mycoplasma is known as Jockers of the Biological kingdom.

Classification on Bacteria Based on Respiration

Based on Respiration	Examples
Aerobic Bacteria: They require oxygen for survival. They are present in aerated moist soil containing organic carbon sources.	Bacillus anthracis
Anaerobic Bacteria: They are bacteria that do not live or grow when oxygen is present.	Prevotella

Classification on Bacteria Based on Nutrition

Based on Nutrition	Examples
Autotrophic Bacteria: They are capable of synthesizing their own food from simple inorganic nutrients.	Thiobacillus
Heterotrophic Bacteria: They are a type of bacteria that take the sugars they need to survive and reproduce from their environment.	Clostridium

Reproduction in Bacteria

Asexual Reproduction

1. Bacteria reproduce by binary fission.
   a. The cell of a bacterium just divides into two new daughter cells.
   b. The two daughter cells are identical.
   c. An Escherichia coli cell can divide in just \(20\) minutes under favourable conditions.

Sexual Reproduction

There is an exchange of genetic material in bacteria during sexual reproduction:

1. Conjugation is the process by which one bacterium transfers genetic material to another through direct contact.
2. Transduction is the process by which DNA is transferred from one bacterium to another by a virus. This property is used by molecular biologists.
3. Transformation is the process by which DNA is transferred from one bacterium to another by the bacterium. This property was discovered and used in understanding the nature of genetic material.

Survival Mechanism

1. Interestingly, bacteria can withstand various harsh climatic conditions.
2. An endospore is a dormant, tough, and non-reproductive structure produced by certain Gram-positive bacteria.
   a. These structures are extremely resistant to pH, chemicals and can allow the bacterium to survive in a harsh environment.
   b. Many endospore-producing bacteria are nasty pathogens, for example, Bacillus anthracis, the cause of anthrax.

Useful Bacteria

Probiotics which are usually beneficial bacteria, provide all sorts of powerful benefits for a person’s body and brain. They may improve digestive health, reduce depression and promote heart health.

Harmful Bacteria

1. Many known diseases are caused by bacteria, both in humans and plants.

Spoilage Bacteria are microbes too small to be seen without a microscope that causes food to deteriorate and develop unpleasant odours, tastes, and textures. These one-celled microorganisms can cause fruits and vegetables to get mushy or slimy or meat to develop a bad odour.

Interesting Facts About Bacteria

1. Researchers at Chalmers University of Technology, Sweden, have developed a new material that prevents infections in wounds – a specially designed hydrogel that works against all types of bacteria, including antibiotic-resistant ones.
2. Superbugs are bacteria or fungi that have developed the ability to withstand commonly prescribed drugs.
3. Archaebacteria are a group of microbes considered to be an ancient form of life that evolved separately from the bacteria and blue-green algae (BGA), and they are sometimes classified as a kingdom.
4. Drying your hands with a paper towel will reduce the bacterial count by \({\rm{45 – 60\% }}\) on your hands.

Bacterial Diseases

When bacteria break down inside the body, as in typhoid, or induce sensitivity to their antigenic properties, as in tuberculosis, they cause disease by secreting or excreting toxins. Other serious bacterial diseases include cholera, diphtheria, bacterial meningitis, tetanus, Lyme disease, gonorrhea, and syphilis. By the middle of the 20th century, bacterial pneumonia was probably the leading cause of death in the elderly. Improved hygiene, vaccines, and antibiotics have reduced mortality from bacterial infections, although some diseases have increased again as antibiotic resistance develops.

The overuse of antibiotics is making the treatment of bacterial infections difficult. As the bacterium mutates, it becomes more resistant to existing antibiotics, making it difficult to treat. Bacteria naturally mutate as well, but overuse of antibiotics is speeding up this process. In the early 21st century, several strains of Mycobacterium tuberculosis developed resistance to one or more of the drugs widely used to treat the infection, making TB one of the deadliest infectious diseases worldwide.

Summary

Bacteria belong to the only prokaryotic kingdom, Monera. They are microscopic living organisms and often cause various diseases to humans, other animals, and plants. The cells do not have membrane-bound nuclear material and cellular organelles. Despite this fact, they are complex in the life machinery and are of tremendous importance and interest in science.

FAQs

Q.1. What are the negative effects of bacteria?
Ans: Harmful bacteria are called pathogenic bacteria because they cause disease and illnesses like strep throat, staph infections, cholera, tuberculosis, and food poisoning. Some of the bacteria cause food spoilage, reduction of soil fertility, biowar, etc.

Q.2. What is the simple definition of bacteria?
Ans: Bacteria are single-celled prokaryotes, lacking well-defined nuclei and membrane-bound organelles, and with chromosomes composed of a single closed DNA circle. They are invisible to the naked eye.

Q.3. What are the four types of bacteria on the basis of shape?
Ans: Bacteria are classified into four groups according to their basic shapes: spherical (cocci), rod (bacilli), spiral (spirilla), and comma (vibrios).

Q.4. What are the seven types of microbes?
Ans: Microorganisms are divided into seven types: bacteria, archaea, protozoa, algae, fungi, viruses, and multicellular animal parasites (helminths).

Q.5. Is a virus a bacteria?
Ans: Viruses are another type of tiny microorganism, although they’re even smaller than bacteria. A virus is a small collection of genetic code, either DNA or RNA, surrounded by a protein coat.

Q.6. What can kill bacteria?
Ans: Antibiotics are medicines that fight bacterial infections in people and animals. When diluted in water, alcohols are effective against a wide range of bacteria, though higher concentrations are often needed to disinfect wet surfaces.