Bacteria: History, Taxonomy, Structure, Morphology, Functions, Nutrition


Bacteria are single-celled microorganisms that belong to the domain Bacteria. They are one of the most abundant and diverse kinds of life on Earth, and can be found in a variety of habitats. These microscopic organisms have a distinct cellular structure and a remarkable ability for adaptation and survive in a variety of environments, including soil, water, air, and even within other organisms.


  • They are prokaryotic and unicellular organisms.
  • They are cosmopolitan in distribution.
  • Bacteria are generally smaller than other types of cells, ranging from 0.5 to 5 micrometres in length.
  • The cell wall is made up of murein or mucopeptide or peptidoglycan.
  • There is lack of membranes bound cell organelles
  • Mode of nutrition is autotrophic or heterotrophic
  • Nucleus is represented by single circular DNA called nucleoid
  • Reproduction tales place by vegetative, asexual and sexual methods.


Bacteria were first discovered by Antonie van Leeuwenhoek (1676), a Dutch scientist, in pond water and called them tiny “animalcules”.

Our understanding of infectious diseases was fundamentally altered in the 19th century by the introduction of the germ hypothesis of disease by scientists like Louis Pasteur and Robert Koch. As a result, there were considerable improvements made in sanitation procedures, sterilization methods, and the development of vaccinations and antibiotics, all of which had a significant positive effect on human health and longevity.

The present name “bacteria” was given by Ehrenberg (1829). Muller placed bacteria in phylum protozoa. But Nageli (1857) included them in Fungi of plant kingdom due to the presence of plant characters in them.

Classification and Taxonomy:

Understanding and organizing the diversity of bacteria depends on taxonomy and classification. The classification of bacteria involves arranging them according to similar traits and evolutionary relationships into hierarchical groups. The study of naming, characterizing, and categorizing species is known as taxonomy.

Bacterial taxonomy and categorization have changed over time as science and technology have advanced. Historically, the classification of bacteria was based on observable traits such cell shape, staining characteristics, metabolic capabilities, and growth requirements. To give a more precise understanding of bacterial interactions, new approaches are increasingly incorporating molecular methods, such as DNA sequencing and genomic analysis.

This is an overview of the hierarchical levels of bacterial classification:

Domain: Bacteria belong to the domain Bacteria, which is one of the three domains of life, along with Archaea and Eukarya. Bacteria are distinct from archaea and eukaryotes based on fundamental genetic and cellular differences.

Phylum: Bacteria are further divided into various phyla (singular: phylum). Phyla represent major evolutionary lineages and are characterized by unique genetic, physiological, and structural features. Some well-known bacterial phyla include Proteobacteria, Firmicutes, Actinobacteria, and Cyanobacteria.

Class, Order, Family, and Genus: These are subsequent levels of classification that provide more specific details about bacterial relationships. Classes are subdivisions within phyla, orders are subdivisions within classes, and families are subdivisions within orders. Genera (singular: genus) represent groups of closely related bacterial species.

Species: The species is the most specific level of classification, referring to a group of bacteria with shared characteristics and capable of interbreeding. Bacterial species names are usually written in italics and consist of two parts: the genus name (capitalized) and the species epithet (lowercase). For example, Escherichia coli is the species name for a common bacterium.


Bacteria are one of the smallest organisms capable of free existence. They are basically unicellular. Sometimes, they form loose irregular masses or incipient filaments. The average size of bacterial cell is about 2um. The smallest one is cold causing bacterium called Dialister pneumosintes (Size 0.15-0.3um). The largest bacteria belong to Spirochaete, some of which are 500um long. Bacteri possess the following forms.

Different morphology of bacteria

Fig: Different morphology of bacteria

  • Coccus- Bacteria having a spherical or oval shape are called Coccus bacteria. The cocci occurring singly are monococcus, in two diplococci, in tetrads tetracocci, in chain streptococci and in group staphylococci.
  • Bacillus- Bacteria of this form are straight and cylindrical like rod. The bacilli may occur singly pr in group. Depending upon the grouping, the bacteria are diplobacillus (in two), palisade bacillus (in stacks) and streptococcus (in chain).
  • Spirillum- These types of bacteria possess spirally coiled form like that of a cork screw They don’t aggregate to form colonies.
  • Vibrio- The body of bacterium is like comma
  • Mycelia Bacteria- They are aseptate filamentous bacteria which multiply asexually like fungi.
  • Stalked Bacteria- These bacteria possess stalks.

Based on flagellation bacteria are following types:

  • Atrichous: Absence of flagella
  • Monotrichous: Having single flagellum at their one end
  • Amphitrichous: One flagellum on each end
  • Cephalatrichous: A tuft of flagella at one end
  • Lophotrichous: Tuft of flagella present in each end
  • Petrichous: Flagella present throughput the surface.

Structure and Functions:

The parts of a bacterial cell can be classified into different categories based on their functions and locations within the cell.

External Structures

Cell Wall

It is the outmost layer of the cell of bacterium. But when a bacterium is viewed through an electron microscopy, they show three kinds of surface layers surrounding the cytoplasm- capsule or slime layers, cell wall and cell membrane. These layers differ appreciable in their structure, composition and function.

Capsule or slime layer: Capsule is frequent in bacteria but not a universal structure. When present, the capsule is always of a simpler composition. The most important component is polysaccharides or polypeptides containing a single amnio acids, mixed capsules composed of two polymeric components are found.

Structural Overview of a Bacterial Cell

Fig: Structural Overview of a Bacterial Cell

Cell wall: Beneath the capsules lies the cell wall. Cell wall is a rigid structure and imparts the characteristics shape to them. The cell wall is made up of two types of polymers. One consists of saccharides units and the other of amino acids sub units. Collectively, therefore bacterial cell wall is a glycopeptide, which is known as murein. The saccharide components of mureins comprise of two sugars- one is glucosamine (NH2– group attached to glucose). The other saccharise is a lactic acid derivates of glucosamine, the muramic acid. Additionally, the wall consists of polysaccharides, lipids, amino acids and protein.

Cell Membrane: It is the innermost layer which surrounds the protoplast. It is a dynamic structure. The removals of cell wall either mechanically or more accurately by lysosome or penicillin treatment, produces spherical protoplast bound by cell membrane. Under electron microscope, the cell or cytoplasmic membrane can be viewed as a typical triple layered unit membrane about 80 A in thickness. The unit membrane acts as a permeability barrier. It also plays importance role in biosynthetic and energy mediated metabolism.

Flagella: They are long, fine protoplasmic threads which occur in a number of bacteria that help the bacteria in swimming.

Pili/Fimbriae: They ate short, narrow protoplasmic outgrowth hair-like projections which occurs all around the bacterial cell. They help the bacteria in adherence to water surface for aeration, conjugation, and biofilm formation.

Outer Membrane (in Gram-negative bacteria): An additional layer outside the cell wall that contains lipopolysaccharides and serves as a protective barrier.

Bacterial cell wall structure of gram negative bacteria

Fig: Bacterial cell wall structure of gram negative bacteria

Cytoplasmic Structures

Cytoplasm: It is surrounded by thin, delicate, plasma membrane. As, in other prokaryotes, only two distinct regions can be distinguished in protoplast, cytoplasm, nucleus. These two are not separated by a membrane (nuclear membrane) and are relatively uniform in fine structure. Cytoplasm does not contain an endoplasmic reticulum, Golgi bodies, mitochondria and asp vacuoles. Osmotic pressure is high and cyclosis is absent. The important cytoplasmic inclusions are as follows.

Nucleoid:  As in other prokaryotes, an organised nucleus is absent. In bacterial cells hereditary material consists of a naked double strand of DNA which is folded variously with the help of RNA to form an oval, spherical, cylindrical or dumbbell shaped structure called nuclear body.

Extra Nuclear DNA: A bacterial cell may contain one to several additional segment of circular DNA which is Capable of replication. Moreover, bacteria can also have extra-nuclear DNA in the form of plasmids in addition to their chromosomal DNA, which is found in the nucleoid portion of the bacterial cell. Small, circular DNA molecules known as plasmids exist independently of chromosomal DNA. They are frequently present in bacteria and offer extra genetic data that can give the bacterial cell a number of advantages.

Gas vacuoles: It is made up of a variable number of sub-microscopic hexagonal gas vesicles. Each gas vesicle is surrounded by a single protein membrane having transverse thickenings.

Volutin granules: They contain reserve phosphate.

Food reserve: The food is stored in the form of polyhydrobutyric acid bodies, lipid globules, protein crystal and glycogen particles.

Ribosomes: The bacterial ribosomes are 70Stype.

Mesosomes: They are small, circular or semi-circular bodies having coiled membrane which are attached to the plasmalemma and are formed by the infoldings, Mesosomes help in separating the replicated DNA during division. They take part in respiration.

 Endospores (in some bacteria): Highly resistant structures formed under unfavourable conditions for survival.


Bacteria have a diverse set of nutritional strategies that allow them to acquire energy and nutrients from a variety of sources.

Heterotrophic Nutrition

The bacteria having heterotrophic types of nutrition are called heterotrophs. Heterotrophs are of two types-


Saprophytes, also known as saprophytic organisms, are organisms that receive nutrition from the decomposition of decaying organic waste such as excreta, leaves, fruits. Meat and other products of plants and animals. The bacteria secret digestive enzymes which break down the complex food into simpler and soluble form, The solubilized substances are than absorbed and converted into complex protoplasmic constituents. They are essential in ecosystems because they break down complex organic substances and recycle nutrients back into the environment.


The parasitic bacteria get food from a living organism called host. They may be completely harmless to the host which are called non pathogenic or causes serious diseases. The disease-causing form is called pathogenic bacteria. The pathogenic bacteria cause diseases either due to eating directly or the hosts cells or due to liberation of poisonous substances called toxins. The toxins may be excreted by bacteria during their metabolism or formed after their death and disintegration. Understanding parasites is critical in areas such as medicine, veterinary science, ecology, and parasitology in order to prevent and treat parasitic diseases and to understand the interactions of hosts and parasites in ecosystems.

Autotrophic Nutrition

The autotrophic bacteria have the capacity to produce their own food from inorganic substances just like higher plants. They, however, do not utilize water as a raw material. Instead, another compound hydrogen is used. Therefore, no oxygen is evolved.

Autotrophs, also known as primary producers, play an important role in ecosystems by converting solar or inorganic compound energy into usable organic energy for themselves and other species in the food chain.

Chemoautotrophic bacteria

The process of obtaining energy from chemical reaction and its utilisation in the formation of food material form inorganic raw materials is called chemosynthesis. The bacteria obtain chemical energy by oxidising the medium in which they live. Chemoautotrophic bacteria are fascinating organisms that have developed to survive in extreme conditions by using alternate energy sources. They contribute to the elemental cycling and energy flow in their ecosystems, demonstrating the diversity and adaptability of life on Earth.

Photo autotrophic bacteria

Photoautotrophic bacteria are a type of bacteria that use sunlight as their primary source of energy for photosynthesis, allowing them to synthesize organic chemicals from inorganic substances. They have pigments that collect light energy, such as chlorophyll or bacteriochlorophyll. They transform carbon dioxide and water into glucose or other organic compounds via photosynthesis. Photoautotrophic bacteria are classified as oxygenic or anoxygenic based on whether they produce oxygen as a by-product. They are important primary producers in ecosystems, contributing to the production of organic matter and providing as a food supply for other creatures.


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  • Prescott, L.M., 2002. Microbiology.
  • Murray, P.R., Rosenthal, K.S. and Pfaller, M.A., 2020. Medical microbiology E-book. Elsevier Health Sciences.

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