Grade 8 Biology: Microorganisms Complete Notes

Fundamental Concepts of Microorganisms

A wide variety of living organisms inhabit both the external environment and the internal systems of higher life forms. While macroorganisms possess structural dimensions that are readily visible to the naked eye, microorganisms—commonly referred to as microbes—are extraordinarily small living organisms that can usually be observed only with the help of a microscope.

A significant proportion of microorganisms are single-celled, or unicellular, meaning that their entire biological structure and metabolic framework consist of only one cell. In terms of biological impact, microorganisms display diverse ecological roles: many species are beneficial and perform essential functions in ecosystems and industrial processes, whereas others are harmful pathogens that induce disease in humans, animals, and plants.

Primary Biological Classifications of Microorganisms

Microorganisms are categorized into five major taxonomic groups: Bacteria (singular: bacterium), Viruses, Protozoa (singular: protozoan), Fungi (singular: fungus), and Algae (singular: alga). This foundational group classification can be recalled using the structural sequence Bacteria, Viruses, Protozoa, Fungi, and Algae.

Bacteria were among the earliest living organisms to appear on Earth. They exhibit remarkable adaptability, occupying virtually all environmental niches on the planet as well as living inside other host organisms. Bacteria occur in several characteristic morphological shapes: Cocci (singular: coccus) are spherical; Spirilla (singular: spirillum) are spiral or helical; and Bacilli (singular: bacillus) are rod-shaped. Representative bacterial species include Streptococcus and Lactobacillus.

Viruses are acellular entities that infect all forms of cellular life. They exist in various shapes and sizes and are so minuscule that they can be visualized only using high-powered microscopes. Many viruses act as biological pathogens causing infectious diseases. For example, Papillomavirus is known to induce cellular mutations that can lead to cancer, while the Varicella-zoster virus causes chickenpox.

Protozoa are single-celled eukaryotic organisms that thrive predominantly in soil matrixes and aquatic environments, including freshwater and marine ecosystems. Despite being composed of only one cell, these microscopic organisms perform all primary life functions. Classic examples of protozoa include Amoeba and Euglena.

Fungi represent plant-like organisms that do not contain chlorophyll, rendering them incapable of photosynthesis and requiring heterotrophic or saprophytic absorption of nutrients. The fungal group includes single-celled species such as yeast, as well as macroscopic spore-bearing structures such as mushrooms.

Algae encompass both single-celled and multicellular plant-like organisms that contain photosynthetic pigments. They reside in diverse habitats including water bodies, moist rocks, and snowfields. Representative algal species include Spirogyra, a multicellular filamentous form, and Chlorella, which is a single-celled, spherical alga.

Habitats and Population Density of Microorganisms

Microorganisms are ubiquitous and inhabit almost every environment across the biosphere, including atmospheric layers, terrestrial soils, clothing fibers, surface skin, beneath fingernails, and within animal intestinal tracts.

The population density of microorganisms in natural and biological environments is extremely high. A single gram ( $1\,\text{g}$ ) of fertile soil can contain between $100\text{ million}$ and $1,000\text{ million}$ bacteria. Similarly, collections of dead epidermal cells and organic debris on the human scalp can harbor $500\text{ million}$ or more bacteria.

Environmental Factors Governing Microbial Growth

Different microorganisms exhibit specific environmental preferences, but their overall growth dynamics are governed by five primary environmental parameters: Oxygen, Water, Temperature, Darkness, and Food.

Regarding oxygen requirements, microorganisms that require molecular oxygen for cellular respiration are designated as aerobic organisms. Conversely, microorganisms that survive and respire without molecular oxygen are designated as anaerobic organisms. Examples of anaerobic organisms include yeast species and the pathogenic bacterium responsible for causing tetanus.

Moisture and water availability are essential for most microorganisms to maintain metabolic function and cellular growth. In terms of thermal tolerance, specialized microorganisms can tolerate extreme temperature environments ranging from $-10\,^\circ\text{C}$ to well above $100\,^\circ\text{C}$ . However, optimal conditions for rapid growth and multiplication across most microbial species occur between $25\,^\circ\text{C}$ and $38\,^\circ\text{C}$ .

Light conditions significantly impact microbial survival. Many microorganisms thrive in dark environments because direct sunlight contains ultraviolet radiation that destroys cellular structures. However, photosynthetic microorganisms require direct light exposure to synthesize cellular nutrients.

Nutritional requirements among microorganisms are met through diverse metabolic pathways. Saprophytes, which include most fungi and specific soil bacteria, feed on dead organic matter and drive natural decomposition. Parasites, including specific bacteria, fungi, and protozoa, derive nutrients directly from living host tissues. Additionally, autotrophic groups of bacteria, protozoa, and algae utilize photosynthetic mechanisms to manufacture their own food.

Ecological Contributions: Soil Fertility and Environmental Sanitation

Saprophytic bacteria and fungi serve as primary decomposers in terrestrial and aquatic ecosystems. They break down dead plant and animal matter along with organic agricultural wastes such as livestock dung, straw, and fallen grass. This biological decay process returns essential mineral nutrients to the soil, prevents the physical accumulation of dead organic matter, and enhances overall soil fertility.

Microorganisms play a critical role in environmental sanitation by processing raw sewage containing urine and faeces. Sewage-decomposing bacteria break down hazardous organic waste into non-toxic, harmless substances. These residual materials can be utilized safely in land reclamation and landfills while replenishing soil concentration levels of nitrates ( $NO_3^-$ ) and phosphates ( $PO_4^{3-}$ ) and improving the soil's physical water-retention capacity.

In rural areas across nations such as India and China, microbial metabolism is harnessed in specialized biogas fermenters. Human faeces, animal manure, and leafy crop residue undergo microbial decomposition in the absolute absence of oxygen. This anaerobic digestion yields methane ( $CH_4$ ) gas, commonly called biogas, which provides a clean, renewable fuel source for rural cooking and household lighting.

Biogeochemical Dynamics of the Nitrogen Cycle

Nitrogen gas ( $N_2$ ) constitutes approximately $78\%$ of atmospheric air by volume, making it the most abundant gaseous element in the atmosphere. Nitrogen is a vital component of essential biological molecules, including structural and functional proteins and nucleic acids. However, the vast majority of living organisms cannot assimilate gaseous atmospheric nitrogen directly. Plants absorb nitrogen primarily in fixed inorganic forms, specifically as nitrate ions ( $NO_3^-$ ) or ammonium compounds ( $NH_4^+$ ). Because crop plants consume substantial quantities of soil nitrogen compounds during growth, these reserves are rapidly depleted unless naturally replenished.

Biological nitrogen fixation is the process whereby specific specialized microorganisms convert atmospheric gaseous nitrogen into usable nitrogenous compounds such as ammonia ( $NH_3$ ). Leguminous plants, including beans, peas, and soya beans, feature specialized root structures termed root nodules. Symbiotic bacteria belonging to the genus Rhizobium reside inside these root nodules and reduce atmospheric nitrogen gas directly into ammonia ( $NH_3$ ), enriching the soil with soluble nitrogen compounds necessary for plant protein synthesis.

Nitrification is the biological conversion of nitrogen compounds carried out by nitrifying bacteria (nitrifiers). In this multi-step pathway, specific bacteria convert ammonia into nitrites ( $NO_2^-$ ), after which secondary nitrifying bacteria convert nitrites into nitrates ( $NO_3^-$ ). Non-biological fixation also occurs during atmospheric lightning events or high-temperature combustion in factory furnaces and vehicle engines; these processes cause atmospheric nitrogen and oxygen to combine into nitrogen oxides, which dissolve in rainwater to form dilute nitric acid ( $HNO_3$ ). Upon reaching the soil, this acid reacts with soil minerals to yield nitrates.

Plants absorb soil nitrates through root systems to synthesize plant proteins, which are subsequently consumed by animals. When plants and animals die, decomposers perform ammonification, breaking down nitrogenous cellular waste and organic tissue to release ammonia ( $NH_3$ ) back into the soil matrix. Soil nitrates may be stored within organic humus, reabsorbed by growing flora, leached into aquatic ecosystems, or converted back into gaseous nitrogen ( $N_2$ ) by denitrifying bacteria in a process known as denitrification, which returns free nitrogen gas to the atmosphere.

Commercial, Industrial, and Food Biotechnology

Fermentation is defined as the metabolic breakdown of organic substrates by the biochemical activity of fungi and bacteria, typically occurring under anaerobic conditions and resulting in the evolution of carbon dioxide ( $CO_2$ ) gas alongside alcohols or organic acids. Yeast cells ferment simple sugars to produce ethyl alcohol. In commercial beverage industries, yeast ferments fruit sugars present in grapes to make wine, and converts grain sugars derived from barley and rice to produce beer.

In commercial baking, yeast is incorporated into dough where it ferments available sugars, producing bubbles of carbon dioxide ( $CO_2$ ) gas. These expanding gas pockets cause dough to rise, producing soft, porous bread and culinary items like dhokla. Similarly, traditional batter preparations for idli and dosa rely on naturally occurring environmental bacteria that ferment ground rice and pulse pastes over several hours, imparting light texture and characteristic flavor.

In textile manufacturing, linen fibers derived from flax plants and jute fibers obtained from jute plants are harvested through a process termed retting. Plant stems tied in bundles are submerged in water where ambient bacteria penetrate stem tissues and digest soft cellular structures. This enzymatic degradation loosens strong structural plant fibers so they can be separated and spun into commercial threads.

In the leather manufacturing industry, specialized bacteria act on raw animal hides during processing. Microbial action degrades unwanted epidermal tissues and non-collagenous proteins, converting stiff animal skins into soft, pliable, and workable leather.

Dairy biotechnology utilizes lactic acid bacteria to convert lactose (milk sugar) into lactic acid ( $C_3H_6O_3$ ). Accumulation of lactic acid increases milk acidity, souring the liquid and causing casein, the primary milk protein, to coagulate into curd. In industrial cheese production, lactic acid bacteria convert lactose to lactic acid, followed by the addition of setting enzymes. The resulting semi-solid curd is isolated from liquid whey and processed into cheese. Specific fungi and bacterial strains are then introduced during aging to generate distinct structural textures, aromas, and flavors.

Vinegar is produced via a two-stage microbial pathway: yeast initial fermentation breaks fruit sugars down into ethyl alcohol, after which specialized acetic acid bacteria act on the alcohol, converting it into acetic acid (vinegar).

Algae yield valuable thickeners and nutritional products. Extraction of marine algae yields agar, a vegetarian gelatine substitute used in food formulation and biological culture media. Algal sodium compounds act as emulsifiers in ice cream production to maintain smooth textures and prevent ice crystal formation. Additional algal derivatives function as food stabilisers and thickeners in commercial salad dressings. Furthermore, single-celled spherical algae such as Chlorella represent concentrated nutritional sources rich in protein and vitamins, studied extensively as prospective human food supplies for long-duration space flight missions.

Pharmacology: Antibiotics, Immunity, and Vaccination

An antibiotic is a specific chemical substance secreted by microorganisms that selectively kills or inhibits the biological growth of other susceptible microorganisms. Medical antibiotics selectively eliminate disease-causing bacteria without causing severe toxic injury to human body tissues or disrupting essential host physiological functions. Modern clinical medicine utilizes over $100$ distinct antibiotic formulations, each operating through specialized modes of action against target bacterial species. Key fungal secretions yield antibiotics such as penicillin and cephalosporin, while specific bacterial cultures produce antibiotics including streptomycin, chloramphenicol, and tetracycline. Antibiotics are also applied in agriculture to manage crop diseases and mixed into livestock feed to treat microbial infections in poultry and cattle.

The immune system protects the body against disease through antigen-antibody interactions. Any foreign substance or disease-causing microorganism entering the host organism is recognized as an antigen. In response, host white blood cells synthesize specific blood proteins called antibodies. Antibodies exhibit precise structural affinity, binding exclusively to the specific antigen that triggered their production to neutralize or destroy it. Immunity denotes the body's physiological ability to resist infectious disease. Natural immunity can be inherited genetically from parents, whereas acquired immunity develops after recovering from clinical infections such as measles or chickenpox. Following recovery, protective antibodies persist in circulation, and the immune system maintains an enhanced capacity to rapidly produce these antibodies upon re-exposure.

Vaccination or immunisation induces acquired immunity against specific pathogens by introducing a vaccine containing dead, weakened, or altered microbial material into the body. The host immune system identifies the introduced antigen and synthesizes protective antibodies and cellular memory without inducing clinical disease. Vaccination confers long-term protection against severe infectious diseases including tetanus, typhoid, poliomyelitis, measles, and hepatitis B. Modern vaccines utilize three major categories of antigenic materials: dead pathogens (used in cholera and typhoid vaccines), live but attenuated/weakened pathogens (used in oral polio and measles vaccines), and toxoids, which are chemically neutralized, harmless forms of toxins produced by germs (used in the tetanus vaccine).

Pathogenic Mechanisms and Disease Transmission

Pathogens are defined as disease-causing microorganisms capable of infecting humans, animals, or plants. Diseases that can be transmitted from an infected host or environmental reservoir to another individual are classified as communicable or infectious diseases.

Communicable pathogens are transmitted through several primary environmental routes:

Direct Contact: Spread via physical contact with infected individuals, animals, or contaminated surfaces (e.g., ringworm and chickenpox).
Air and Droplet Infection: Pathogens released in respiratory droplets when an infected host coughs or sneezes are inhaled by nearby individuals (e.g., common cold and influenza).
Contaminated Food and Water: Ingestion of food or water contaminated with human or animal faeces introduces enteric pathogens (e.g., cholera, typhoid, and amoebic dysentery).
Insect Vectors: Biological vectors transmit pathogens during feeding; mosquitoes vector malaria, dengue, and yellow fever; fleas transfer bubonic plague bacteria; and houseflies mechanically carry pathogens from waste directly onto exposed food.
Animal Bites: Salivary transmission of viral pathogens through bites from infected animals (e.g., rabies transmission via infected dog bites).
Cuts and Wounds: Direct entry of environmental pathogens or bacterial endospores into compromised skin tissues or deep puncture wounds (e.g., tetanus).

Pathogens cause pathology within host organisms by secreting destructive chemical toxins, invading individual host cells, or destroying tissue structures. Following initial infection, pathogens multiply rapidly within host tissues before symptoms become clinically apparent. The time interval between initial host infection and the first appearance of clinical disease symptoms is defined as the incubation period. Common clinical signs of infectious disease include fever, headache, nausea, vomiting, and skin rashes.

Systemic Survey of Human, Animal, and Plant Diseases

Multiple bacterial pathogens induce severe clinical diseases in human populations:

Cholera is caused by the bacterium Vibrio cholerae, transmitted via contaminated food or water, causing severe watery diarrhoea, acute dehydration, vomiting, and abdominal pain; prevented via cholera vaccination.
Tuberculosis (TB) is caused by Mycobacterium tuberculosis, transmitted through airborne respiratory droplets, presenting with persistent cough, low-grade fever, fatigue, weight loss, and hemoptysis (blood in sputum); prevented via BCG vaccination.
Typhoid is caused by Salmonella typhi, transmitted via contaminated food or water, producing sustained high fever, severe acute headache, constipation, and dry cough; prevented via anti-typhoid inoculations.

Viral, protozoan, and fungal agents cause various communicable health conditions:

Gastroenteritis is caused by diverse viruses and bacteria transmitted through contaminated food, resulting in acute diarrhoea and vomiting; managed through oral rehydration and targeted antibacterial therapy for bacterial cases.
Common cold is caused by multiple respiratory viruses spread via airborne droplets, presenting with fever, headache, runny nose, sneezing, and coughing; managed by avoiding crowded spaces and covering the mouth and nose during coughing or sneezing.
Poliomyelitis (polio) is caused by the polio virus transmitted via the faecal-oral route, causing nerve-cell damage, fever, headache, muscular stiffness, and paralysis; prevented via polio vaccines.
Malaria is caused by protozoan parasites of the genus Plasmodium, transmitted via infected mosquito bites, producing paroxysmal recurring chills and high fever; controlled by eliminating mosquito breeding sites.
Ringworm is a fungal infection transmitted via contact with infected animals, livestock, or contaminated bedding, forming itchy ring-shaped skin patches; treated with antifungal therapy and hygiene.

Microorganisms also act as major agricultural and veterinary pathogens:

Leaf mosaic disease in agricultural crops is caused by viral infections.
Potato blight is caused by fungal pathogens.
Powdery and downy mildew affecting grapes, roses, gooseberries, and ornamental plants are caused by fungi.
Rust diseases in wheat and cereal crops are caused by fungal pathogens.
Foot-and-mouth disease in livestock and cattle is caused by viral pathogens.

Food Preservation Principles and Methodologies

Microbial food spoilage occurs through the enzymatic breakdown of organic matter by saprophytic fungi and bacteria. As decomposers grow on stored food, they alter its physical properties and release toxic chemical by-products, rendering the food unfit for human consumption and capable of causing food poisoning. Beyond microbial decay, food products are subject to mechanical damage by insect pests such as flies and weevils, contamination by rodents such as mice and rats, and internal chemical oxidation.

Food preservation is defined as the processing and handling of food to prevent microbial spoilage, suppress chemical decay, and maintain nutritional value over extended periods. Effective preservation reduces domestic food wastage, extends storage life, enables long-distance transport of perishable items, and ensures year-round availability of seasonal produce.

Food preservation employs seven primary physical and chemical techniques:

Canning: Food products and beverages are thermally processed and hermetically sealed inside airtight containers to prevent microbial contamination.
Pasteurisation: Liquid foods such as milk are heated to high temperatures to destroy pathogenic vegetative bacteria and then quickly cooled, effectively sterilizing the liquid while preserving natural flavor.
Sun Drying: Removing unbound water from foods such as cauliflower and mangoes via solar drying deprives microbes of essential moisture, preventing microbial growth.
Salting: Application of sodium chloride ( $NaCl$ ), often combined with drying, draws cellular water out of microbial cells via osmotic action, inhibiting bacterial growth.
Pickling: Submerging raw mangoes, gooseberries, lemons, cauliflower, onions, and cucumbers in vinegar (acetic acid) or concentrated brine creates high acidity or salinity that prevents microbial growth while imparting a sharp, spicy flavor.
Adding Sugar: Preserving fruits in high-sugar preparations such as jams and jellies exerts high osmotic pressure, drawing water out of microbial cells and preventing microbial proliferation.
Refrigeration: Freezing reduces temperatures to levels that halt microbial enzymatic pathways and multiplication without necessarily killing the microbes; if thawed food warms up, surviving microbes resume active multiplication.