Micro-organisms make up a large part of the planet’s living material and play a major role in maintaining the Earth’s ecosystem.

Learning Objectives

Define the differences between microbial organisms.

Key Points:-

Micro-organisms are divided into seven types: bacteria, archaea, protozoa, algae, fungi, viruses, and multicellular animal parasites ( helminths ).

Each type has a characteristic cellular composition, morphology, mean of locomotion, and reproduction. Micro-organisms are beneficial in producing oxygen, decomposing organic material, providing nutrients for plants, and maintaining human health, but some can be pathogenic and cause diseases in plants and humans.

Key Terms Gram stain:

A method of differentiating bacterial species into two large groups (Gram-positive and Gram-negative).

peptidoglycan: A polymer of glycan and peptides found in bacterial cell walls.

Micro-organisms or microbes are microscopic organisms that exist as unicellular, multicellular, or cell clusters. Micro-organims are widespread in nature and are beneficial to life, but some can cause serious harm. They can be divided into six major types: bacteria, archaea, fungi, protozoa, algae, and viruses.



Bacteria are unicellular organisms. The cells are described as prokaryotic because they lack a nucleus. They exist in four major shapes: bacillus (rod shape), coccus (spherical shape), spirilla (spiral shape), and vibrio (curved shape). Most bacteria have a peptidoglycan cell wall; they divide by binary fission; and they may possess flagella for motility. The difference in their cell wall structure is a major feature used in classifying these organisms.According to the way their cell wall structure stains, bacteria can be classified as either Gram-positive or Gram-negative when using the Gram staining. Bacteria can be further divided based on their response to gaseous oxygen into the following groups: aerobic (living in the presence of oxygen), anaerobic (living without oxygen), and facultative anaerobes (can live in both environments).According to the way they obtain energy, bacteria are classified as heterotrophs or autotrophs. Autotrophs make their own food by using the energy of sunlight or chemical reactions, in which case they are called chemoautotrophs. Heterotrophs obtain their energy by consuming other organisms. Bacteria that use decaying life forms as a source of energy are called saprophytes.



Archaea or Archaebacteria differ from true bacteria in their cell wall structure and lack peptidoglycans. They are prokaryotic cells with avidity to extreme environmental conditions. Based on their habitat, all Archaeans can be divided into the following groups: methanogens (methane-producing organisms), halophiles (archaeans that live in salty environments), thermophiles (archaeans that live at extremely hot temperatures), and psychrophiles (cold-temperature Archaeans). Archaeans use different energy sources like hydrogen gas, carbon dioxide, and sulphur. Some of them use sunlight to make energy, but not the same way plants do. They absorb sunlight using their membrane pigment, bacteriorhodopsin. This reacts with light, leading to the formation of the energy molecule adenosine triphosphate (ATP).



Fungi (mushroom, molds, and yeasts) are eukaryotic cells (with a true nucleus). Most fungi are multicellular and their cell wall is composed of chitin. They obtain nutrients by absorbing organic material from their environment (decomposers), through symbiotic relationships with plants (symbionts), or harmful relationships with a host (parasites). They form characteristic filamentous tubes called hyphae that help absorb material. The collection of hyphae is called mycelium. Fungi reproduce by releasing spores.



Protozoa are unicellular aerobic eukaryotes. They have a nucleus, complex organelles, and obtain nourishment by absorption or ingestion through specialized structures. They make up the largest group of organisms in the world in terms of numbers, biomass, and diversity. Their cell walls are made up of cellulose. Protozoa have been traditionally divided based on their mode of locomotion: flagellates produce their own food and use their whip-like structure to propel forward, ciliates have tiny hair that beat to produce movement, amoeboids have false feet or pseudopodia used for feeding and locomotion, and sporozoans are non-motile. They also have different means of nutrition, which groups them as autotrophs or heterotrophs.



Algae, also called cyanobacteria or blue-green algae, are unicellular or multicellular eukaryotes that obtain nourishment by photosynthesis. They live in water, damp soil, and rocks and produce oxygen and carbohydrates used by other organisms. It is believed that cyanobacteria are the origins of green land plants.



Viruses are noncellular entities that consist of a nucleic acid core (DNA or RNA) surrounded by a protein coat. Although viruses are classified as microorganisms, they are not considered living organisms. Viruses cannot reproduce outside a host cell and cannot metabolize on their own. Viruses often infest prokaryotic and eukaryotic cells causing diseases.

Multicellular Animal Parasites:-

Multicellular Animal Parasites

A group of eukaryotic organisms consisting of the flatworms and roundworms, which are collectively referred to as the helminths. Although they are not microorganisms by definition, since they are large enough to be easily seen with the naked eye, they live a part of their life cycle in microscopic form. Since the parasitic helminths are of clinical importance, they are often discussed along with the other groups of microbes.

This tree of life shows the different types of microorganisms.

Useful Micro-organisms :-

The environment is incomplete without microorganisms. With every breath you take, there are millions of microscopic organisms that you breathe in. Apart from that, the human body hosts a plethora of microbes both inside and outside. Besides this, they are a crucial part of the ecosystem and take part in activities like production of minerals like nitrogen, gases like oxygen, carbon dioxide, taking care of dead and decaying materials etc. Uses of Micro-organisms. As discussed, microorganisms are beneficial for humans in various ways. They play an important role in human welfare and for the environment. These include processing and preservation of food, production of biomolecules, manufacture of pharmaceutical products, cosmetics industries, recycling the nutrients in the soil and so on. Listed below are some of the applications of microorganisms in human welfare.

Food Industry and Beverages:-

The role of microorganisms in food preparation and beverage manufacturing is known for ages. They are used in the manufacture of bread, curd, wine, and alcohol etc. Lactobacillus bacteria are responsible for the curd formation. They multiply and convert milk into curd. Another example is Yeast which is used commercially for alcohol and wine production. The process is called fermentation. Yeast is also used in baking industries for the preparations of bread and cakes. Micro-flora Bacteria are not only present outside the human body, but they also live inside the body too. This aggregate collection of microorganisms that is present in the human body is termed as Microflora.


The human population is growing and we need to produce enough food for everyone. Some microbes negatively impact agriculture by causing diseases in crop plants and livestock but we can use other, beneficial microbes to increase food availability. Several insect pathogens are used as biological pesticides in the biocontrol of insect pests that reduce crop yields and limit food production.Farmers can buy endospores of the bacterium Bacillus thuringiensis (Bt) to use as an organic pesticide. Some strains produce crystals in the endospores that are toxic to insects if they eat them.While controversial, crop plants genetically modified with the Bt toxin genes are resistant to insect pests. Baculo-viruses infect insects and turn them into zombies. If a caterpillar eats leaves contaminated with baculo-virus occlusion bodies occlusion derived virus (ODV) infect cells in the insect gut. The virus replicates by budding from cell to cell, and it makes the infected insect move to the top of the plant towards the light. The baculo-viruses produce an enzyme called chitinase which dissolves the insect tissues and turns the caterpillar into a pool of mush. When it rains, occlusion bodies are washed on to the leaves below ready to infect a new host.

Crop yield:-

Microbes can also be used as bio-fertilisers to improve crop yield. The large-scale cultivation of crop plants rapidly depletes nutrients in the soil and limits plant growth. Nitrogen can be replenished by using legume plants in crop rotation programmes or by the application of free-living nitrogen-fixing bacteria such as Azotobacter to the soil. Phosphate-solubilizing bacteria such as Pseudomonas putida can be introduced to increase soil phosphates, and plant growth promoting bacteria (eg Pseudomonas fluorescens) can be applied to improve the health of crop plants. These bio-fertilizers help reduce the need for synthetic fertilizers and pesticides, which are produced using fossil fuels that contribute to climate change.


Bacteria are also present in the gut, and they aid the process of digestion by releasing certain enzymes. They live in a symbiotic relationship with a human. Other roles of microflora are vitamin K production, which is crucial in enabling blood clotting. They also prevent the invasion of the foreign bodies, by acting against other fatal microbes.Bacteria are used in brewing and baking and other fermentation processes. They play a key role in nitrogen fixation. Bacteria are also used as pesticides and in composting processes in agriculture.Pharmaceutical IndustryAntibiotics go hand in hand with microbes in the medical field. Typically, antibiotics are obtained from a weakened form of an otherwise harmful microorganism. This is then injected into the body and the body learns to fight off the diseases caused by these organisms. Examples of this include mumps and the measles.


In the environment, microorganisms have two vital roles- one is an enhancement of soil fertility another is cleaning. Azotobacter, Rhizobium, Clostridium are few examples of Nitrogen-fixing bacteria which play a primary role in transforming atmospheric nitrogen into inorganic compounds which are then used by the plants. Without this process, the vast majority of the nitrogen present in the atmosphere becomes unusable.Microorganisms also act as cleaners. Plants and animals eventually die and their bodies are turned into nutrients which the environment can use.


Fungi play a major role in the decomposition of dead and decaying matter. Fungi such as mushrooms are edible. They are also used in fermentation during the production of bread, cheese, beverages, and various other food products. Antibiotics such as penicillin are obtained from fungi.


Protozoa are beneficial in mineralizing nutrients and making them available to the plants and other soil organisms. They also feed on bacteria and regulate the bacteria.

Harmful Micro-organism:-

Harmful effects of microorganisms:

Bacteria: Causes various diseases such as typhoid, diarrhea, and cholera.

Fungi: Causes a large number of diseases in plants and in animals such as rust diseases in plants, fruit rot in apple, red rot in sugar cane and ring worm disease in human beings.

Algae: Algal boom in water (rapid growth of algae) causes poisonous effect after they die, which in turn results in the death of aquatic organisms.

Protozoa: Causes Amoebic dysentery, pyorrhoea and sleeping sickness etc.

Virus: Cause small fox, common cold, influenza, herpes, hepatitis, polio and rabies

Micro-organisms related experiment:

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