Micro-organisms are tiny living things that are too small to be seen with the naked eye and can only be observed clearly with the aid of a microscope. They are found almost everywhere — in the air, water, soil, on our bodies, and even inside our bodies. Micro-organisms are very important because some are useful while others are harmful. The useful ones help in food production, medicine, and nutrient recycling, while harmful ones cause diseases, spoil food, and spread infections. Understanding micro-organisms and their activities is necessary because of the great influence they have on human health, the environment, and industry.
Micro-organisms in Air and Water Air and water are natural habitats for many micro-organisms. In the air, dust particles and water droplets often carry bacteria, fungal spores, and viruses. These can enter the human body through breathing or contact, leading to infections such as influenza, tuberculosis, and common cold. In water, micro-organisms such as protozoa, bacteria, and algae thrive, especially when the water is stagnant or polluted. Some waterborne pathogens like Vibrio cholerae (which causes cholera) and Salmonella typhi (which causes typhoid fever) can spread diseases when contaminated water is consumed. Thus, clean air and safe water supply are essential for good health.
Groups of Micro-organisms Micro-organisms are broadly classified into five major groups: bacteria, viruses, fungi, protozoa, and algae. Each group has unique structures, modes of life, and effects on humans and the environment.
Viruses Viruses are the smallest micro-organisms, so small that they can only be seen with an electron microscope. They are not considered true living organisms because they cannot carry out life processes independently. Instead, they must invade a living host cell to reproduce. Structurally, a virus consists of genetic material (DNA or RNA) surrounded by a protein coat called a capsid. Some have an extra envelope around them. Viruses are responsible for several diseases such as HIV/AIDS, influenza, measles, and COVID-19. They spread easily through air, water, contact, and vectors. Despite their harmful roles, viruses are used in genetic engineering and vaccine production.Examples of viruses include: Adenovirus, Togavirus, Coronavirus,Picornavirus, etc.
Bacteria Bacteria are single-celled organisms that have a simple structure without a true nucleus (prokaryotes). Their genetic material is found in a region called the nucleoid, and they reproduce mainly by binary fission, a form of asexual reproduction. A bacterium has a slimy capsule, cell wall, a cell membrane, a dense cytoplasmic granules with no clear nucleus but it has a nuclear material called DNA (Deoxyribonucleic Acid) which spreads through the cell.
Types of Bacteria Bacteria are grouped based on shape, gram staining reaction, mode of nutrition and oxygen requirement
Grouping by Shape Bacteria occur in different shapes which can be observed under the microscope.
Coccus (spherical bacteria) These are round-shaped bacteria which may occur singly, in pairs (diplococci), chains (streptococci), or clusters (staphylococci). Examples are Streptococcus pneumoniae (causes pneumonia), Staphylococcus aureus (causes boils and food poisoning), and Neisseria gonorrhoeae (causes gonorrhea).
Bacillus (rod-shaped bacteria) These are straight, cylindrical bacteria which may be short or long. Some form resistant spores. Examples are Bacillus anthracis (causes anthrax), Clostridium tetani (causes tetanus), and Escherichia coli (found in the intestine; some strains cause food poisoning).
Spirillum (spiral-shaped bacteria) These bacteria have a spiral or helical shape and move with the help of flagella. Examples are Spirillum volutans (found in stagnant water), Treponema pallidum (causes syphilis), and Helicobacter pylori (causes stomach ulcers).
Vibrio (comma-shaped bacteria) These are curved rod-like bacteria shaped like a comma. The most important example is Vibrio cholerae, which causes cholera. Grouping by Gram Staining Reaction Bacteria can be divided into two groups based on how they react to Gram’s stain, which depends on the structure of their cell wall.
Gram-positive bacteria These retain the purple crystal violet stain because they have thick peptidoglycan walls. Examples include Streptococcus, Staphylococcus, and Clostridium.
Gram-negative bacteria These lose the purple stain and appear red or pink because they have thin peptidoglycan walls and an extra outer membrane. Examples include Escherichia coli, Salmonella, and Neisseria.
Grouping by Mode of Nutrition Bacteria may be autotrophic (make their own food) or heterotrophic (depend on other organisms).
Autotrophic bacteria These manufacture their own food. Photosynthetic bacteria, such as cyanobacteria (Anabaena, Nostoc), use sunlight, while chemosynthetic bacteria, such as Nitrosomonas and Nitrobacter, obtain energy from chemical reactions like nitrification.
Heterotrophic bacteria These cannot make their own food. Saprophytic bacteria feed on dead matter (e.g., Bacillus subtilis), while parasitic bacteria live in hosts and cause diseases (e.g., Mycobacterium tuberculosis causing TB, Salmonella typhi causing typhoid).
Grouping by Oxygen Requirement Bacteria differ in their need for oxygen in respiration.
Aerobic bacteria These require oxygen to survive. Examples are Mycobacterium tuberculosis and Bacillus subtilis.
Anaerobic bacteria These do not require oxygen, and some are even killed by it. Examples are Clostridium tetani and Clostridium botulinum.
Facultative anaerobes These can survive with or without oxygen. Examples are Escherichia coli and Salmonella.
Microaerophilic bacteria These need oxygen but in smaller amounts than what is present in the atmosphere. An example is Helicobacter pylori, which causes stomach ulcers.
Fungi
Fungi are simple organisms that may be unicellular (like yeast) or multicellular (like moulds and mushrooms). They do not contain chlorophyll and therefore cannot produce their own food. Instead, they feed on dead or decaying matter (saprophytic) or live on other organisms as parasites. The body of multicellular fungi consists of thread-like structures called hyphae, which form a network known as mycelium. Useful fungi include yeast, used in bread and alcohol production, and Penicillium, from which the antibiotic penicillin is made. Harmful fungi cause diseases such as ringworm and athlete’s foot, and they also spoil food.
Protozoa
Protozoa are single-celled, animal-like micro-organisms. They are eukaryotic, meaning they have a nucleus enclosed in a membrane. Protozoa are mostly found in water, soil, or inside living hosts. They move using special structures: flagella (as in Trypanosoma), cilia (as in Paramecium), or pseudopodia (as in Amoeba). Some protozoa are harmless, but others cause diseases such as malaria (Plasmodium), sleeping sickness (Trypanosoma), and dysentery (Entamoeba histolytica). Diagram to sketch: Draw an Amoeba showing pseudopodia, nucleus, and contractile vacuole; also draw a Paramecium with cilia around the body, nucleus, and food vacuole.
Algae
Algae are simple plant-like organisms that contain chlorophyll and carry out photosynthesis. They may be unicellular, such as Chlorella, or multicellular, such as seaweeds. Algae are mainly aquatic, found in fresh water and marine environments. They play important roles in producing oxygen and serving as food for aquatic animals. Some algae, however, cause problems such as algal blooms, which reduce oxygen in water and kill fish. Algae are also used in industry for making agar, cosmetics, and as food supplements. Diagram to sketch: Draw a simple filament of green algae (like Spirogyra) with rectangular cells joined in a chain. Label the cell wall, chloroplast (spiral shape), and nucleus.
Concept of Culturing
Culturing refers to the process of growing micro-organisms in a prepared medium under controlled laboratory conditions. The medium may be solid (like agar plates) or liquid (broth) containing nutrients needed for growth. Culturing allows scientists to study the characteristics, growth patterns, and effects of micro-organisms. Since most micro-organisms cannot grow on plain water alone, they need specific food substances and conditions to multiply.
For example, bacteria can be cultured to identify disease-causing strains or to produce antibiotics. Proper culturing requires sterile conditions to prevent contamination and ensure accurate results.
Steps in Preparing a Culture Solution
Selection of Nutrients
Nutrients such as sugars, proteins, minerals, and sometimes vitamins are chosen depending on the type of micro-organism to be cultured.
Example: Glucose, peptone (protein source), and salts.
Preparation of the Medium
The selected nutrients are dissolved in water to make a solution.
If a solid medium is required, a substance like agar-agar (a jelly-like material obtained from seaweed) is added.
For liquid cultures, the solution is left without agar.
Sterilization
The prepared culture medium is sterilized by heating (using an autoclave or boiling) to destroy all unwanted micro-organisms that may contaminate the medium. 4. Pouring and Cooling
The sterile medium is poured into containers such as Petri dishes, test tubes, or flasks.
It is allowed to cool and solidify (in the case of agar medium). 5. Inoculation
The micro-organism to be studied is carefully introduced (inoculated) into the medium under sterile conditions. 6. Incubation
The inoculated culture medium is kept in an incubator at a suitable temperature, pH, and oxygen condition (depending on the type of organism) until colonies of the micro-organisms grow.
Types of Culture Medium
Liquid Medium (Broth) – used for growing large numbers of micro-organisms.
Solid Medium (Agar Plates) – used for isolating and observing colonies of micro-organisms.
Selective Medium – contains substances that allow only specific microbes to grow.
Instruments required for the preparation of Culture Solution
Microscope
Petri-dishes
Test tubes
Test tube holders
Hand lens
Innoculating needles and loops
Glass rod
Spatula
Pipettes and micro-pipettes
Slides
Cover slips
Microscope
Precautions to be taken during preparation of culture solution
Ensure sterility – all instruments and containers must be properly sterilized before use. Hands must also be washed before and after preparation of agar solution.
Use only distilled water – tap water contains impurities that can affect results.
Measure accurately – chemicals should be weighed and measured correctly to get the right concentration.
Avoid contamination – do not touch the inside of test tubes, Petri dishes, or pipette tips. If you must open the petri-dish, do so slightly and cover at once.
Sterilize inoculating loops – by flaming before and after use.
Work in a clean area – preferably in a sterile environment to reduce contamination from the air. Sick persons should not be allowed to take part in culturing experiments.
Cover containers properly – keep test tubes, flasks, and Petri dishes closed when not in use.
Label solutions clearly – with name, date, and type of medium prepared.
Wear protective gear – lab coat, hand gloves, and goggles for safety.
Dispose of waste properly – contaminated materials should be sterilized before disposal.
WAYS AND PLACES THROUGH WHICH MICRO-ORGANISMS ENTER THE BODY
Micro-organisms such as bacteria, viruses, fungi, and protozoa have different entry points into the human body. These are usually natural body openings, breaks in the skin, or through contact with infected materials.
1. Through the Mouth (Oral Route): Germs can enter the body when contaminated food, drinks, or unwashed hands are taken in. Diseases like cholera, typhoid, and polio are spread this way.
2. Through the Nose (Respiratory Route): Micro-organisms can be inhaled through infected droplets in the air when a sick person sneezes or coughs. Examples include influenza, tuberculosis, and COVID-19.
3. Through the Skin (Cuts and Wounds): The skin protects the body, but when it is cut or injured, micro-organisms can pass through. Tetanus bacteria and staphylococcus infections often enter this way.
4. Through Insect and Animal Bites: Some insects and animals carry disease-causing organisms in their saliva. When they bite, they transfer these germs into the body. For example, mosquitoes spread malaria, and dogs can transmit rabies through bites.
5. Through Blood Contact: Pathogens can be introduced directly into the bloodstream through contaminated needles, sharp objects, or transfusion of infected blood. HIV and hepatitis viruses are spread in this way.
6. Through the Eyes: When dirty hands or contaminated water touches the eyes, germs may enter. A common example is conjunctivitis, also called Apollo.
7. Through the Ears: The ears can be infected when dirty water enters them, or when sharp, unclean objects are used to scratch inside. This often results in ear infections.
8. Through the Genitals: During unprotected sexual contact, disease-causing organisms can pass from one person to another. Examples are syphilis, gonorrhea, and HIV.
9. Through the Anus: Some parasites and worms can enter the body through the anus, especially in unhygienic environments. An example is threadworm infection.
10. Through the Placenta (Mother to Child): Certain diseases can be passed from an infected mother to her unborn baby through the placenta. Examples include HIV and syphilis.
Carriers of Micro-organisms
Carriers are living or non-living agents that harbour and spread disease-causing micro-organisms from one person, animal, or place to another. They may not always show symptoms of the disease, but they help in transmitting it.
Living Carriers
Humans: Humans can serve as carriers of micro-organisms. Some people, called healthy or passive carriers, may not show signs of illness but still spread germs through coughing, sneezing, touching, or handling food. A common example is the spread of typhoid by carriers of Salmonella typhi.
Insects: Insects are also important carriers of disease and are often referred to as vectors. They may transfer germs through bites or by contaminating food and surfaces. For instance, mosquitoes transmit malaria, houseflies spread cholera and dysentery, while tsetse flies spread sleeping sickness.
Animals: Animals act as carriers of diseases that affect humans. Domestic and wild animals can harbour harmful germs and pass them on. Dogs, for example, can transmit rabies, rats can spread plague, while monkeys have been linked to the spread of Ebola virus.
Birds: Birds may also carry micro-organisms on their feathers, beaks, or droppings. Pigeons, for example, can spread fungal infections that affect humans.
Non-living Carriers
Water: Water is a major non-living carrier of micro-organisms. When water is contaminated with germs, it can spread diseases such as cholera or guinea worm infection.
Air can also carry micro-organisms. When an infected person sneezes or coughs, tiny droplets containing germs are released into the air. People who inhale this contaminated air can become infected with diseases such as tuberculosis or influenza.
Food may serve as a carrier when it is spoiled, improperly cooked, or left exposed to flies. Harmful germs such as those causing food poisoning or tapeworm infection can be transmitted this way.
Objects, also called fomites, are another example of non-living carriers. Everyday items like towels, door handles, or needles can harbour micro-organisms and transfer them to people who touch or use them. Contaminated needles, for example, are a common way HIV and hepatitis viruses are spread.
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