Comprehensive Study Notes on Microbes in Human Welfare
Microbes in Household Products
Microbes are microscopic organisms present everywhere, including in the air, water, soil, food, inside human bodies, and within extreme environments. Diverse groups of microbes such as bacteria, viruses, and fungi play fundamental roles in human welfare, particularly in household food production and processing.
Lactobacillus, commonly referred to as Lactic Acid Bacteria (LAB), converts milk into curd. During its growth, LAB produces lactic acids that cause the coagulation and partial digestion of milk proteins. Fresh milk can be converted into curd by introducing a small amount of existing curd containing LAB as an inoculum or starter culture. In addition to thickening the liquid, LAB increases the nutritional value of curd by significantly increasing its Vitamin B12 content. Furthermore, in the human stomach, LAB plays a crucial protective role by checking and preventing the growth of disease-causing pathogens, making curd an effective probiotic.
Bacterial fermentation, which occurs via anaerobic respiration, is widely utilized in preparing traditional foods such as idli and dosa. The puffed-up appearance of the fermented dough is directly caused by the production and entrapment of carbon dioxide gas () generated during microbial metabolic activity.
In bread production, Baker's Yeast, known scientifically as Saccharomyces cerevisiae, is utilized to ferment dough. The metabolic activity of the yeast ferments sugars present in the dough, producing gases that expand and soften the texture of the bread.
Microbes are also integral to traditional regional foods and beverages. Toddy, a traditional beverage in parts of southern India, is produced by fermenting the nutrient-rich liquid sap collected from palm trees. Additionally, microbial fermentation is routinely employed to process and preserve fish, soya beans, and bamboo shoots. Examples of fermented soya products include soya sauce, tofu, and tempeh.
Cheese production relies heavily on specific microbial species to achieve distinct textures, holes, and flavors. Swiss cheese features characteristic large holes formed by the production of large volumes of carbon dioxide gas () by the bacterium Propionibacterium sharmanii. Roquefort cheese is ripened by growing a specific fungal mold, Penicillium roqueforti, directly on the cheese, which imparts a unique texture and flavor profile.
Microbes in Industrial Products
The industrial-scale production of beverages, antibiotics, chemicals, and enzymes requires growing microorganisms in specialized, exceptionally large vessels termed fermentors.
Fermented beverages rely primarily on Brewer's Yeast, Saccharomyces cerevisiae, which ferments malted cereals and fruit juices to produce ethanol (). Alcoholic beverages are categorized based on whether the fermented broth undergoes distillation. Wine and beer are produced without distillation. In contrast, spirits such as whisky, brandy, rum, gin, and arrack are produced through the distillation of the fermented broth to concentrate the alcohol content.
Antibiotics are chemical substances produced by certain microorganisms that can kill or retard the growth of pathogenic microbes. The term "anti" originates from a Greek word meaning "against," and "bio" means "life," together signifying "against life" in the context of disease-causing pathogens, while being "pro-life" relative to human health. Antibiotics are critical medical tools used to treat severe infections such as plague, whooping cough, diphtheria, and leprosy.
The first antibiotic discovered was penicillin, identified by Alexander Fleming in 1928. Fleming observed that the bacterium Staphylococci failed to grow around a fungal mold, identified as Penicillium notatum, which was accidentally growing on unwashed culture plates. He isolated the active antimicrobial compound and named it penicillin. Subsequently, Ernest Chain and Howard Florey established the full medical potential and efficacy of penicillin as an antibiotic. Penicillin was extensively used to treat wounded American soldiers during World War II. For this landmark discovery and development, Alexander Fleming, Ernest Chain, and Howard Florey were jointly awarded the Nobel Prize in Physiology or Medicine in 1945.
In addition to fungal sources, several important antibiotics are derived from specific bacteria. Polymyxin-B is isolated from Bacillus polymyxa, Gramicidin is produced by Bacillus brevis, Terramycin is extracted from Streptomyces rimosus, and Erythromycin is synthesized by Streptomyces erythraeus. Other fungal antibiotics include Cephalosporin, which is derived from Cephalosporium acremonium.
Microbes are also primary producers of organic acids, ethanol, enzymes, and bioactive compounds. Fungal and bacterial species produce organic acids through fermentation: Aspergillus niger (a fungus) produces citric acid; Acetobacter aceti (a bacterium) produces acetic acid; Clostridium butylicum (a bacterium) produces butyric acid; and Lactobacillus (a bacterium) produces lactic acid. Ethanol is commercially manufactured using the yeast Saccharomyces cerevisiae.
Specialized microbial enzymes serve industrial and clinical functions. Lipases break down lipids and fats, making them vital additions to detergent formulations for removing oily stains in laundry. Pectinases and proteases are used industrially to clarify bottled fruit juices, removing cloudiness for a clear final liquid. Streptokinase, an enzyme produced by the bacterium Streptococcus, functions clinically as a "clot buster" to dissolve blood clots inside the blood vessels of patients suffering from myocardial infarction.
Bioactive molecules with significant medical applications are synthesized by specific fungi and yeasts. Cyclosporin A, produced by the fungus Trichoderma polysporum, acts as an essential immunosuppressive agent given to organ transplant patients to prevent organ rejection. Statins, produced by the yeast Monascus purpureus, serve as blood-cholesterol lowering agents by competitively inhibiting the specific liver enzymes responsible for synthesizing cholesterol.
Microbes in Sewage Treatment
Sewage, or municipal wastewater, consists of large volumes of human excreted waste, washroom waste, organic matter, and pathogenic organisms. Discharging untreated sewage directly into natural water bodies causes severe pollution. Wastewater treatment is carried out in Sewage Treatment Plants (STPs) using naturally occurring heterotrophic microbes present in the sewage itself to reduce its polluting potential.
Sewage treatment is conducted in two primary stages: Primary Treatment and Secondary Treatment. Primary treatment involves the physical removal of coarse and fine suspended particles through physical processes. Floating debris is systematically removed using sequential filtration. Subsequently, grit consisting of soil, sand, and small pebbles is removed by sedimentation in grit chambers. The solids that settle at the bottom form the primary sludge, while the clear supernatant liquid floating above constitutes the primary effluent.
Secondary treatment, also known as biological treatment, processes the primary effluent to break down dissolved organic matter. The primary effluent is transferred into large aeration tanks where it is constantly agitated mechanically and supplied with air. This environment promotes the vigorous growth of aerobic microbes into functional structures termed flocs, which consist of bacterial masses held together by mesh-like networks of fungal filaments. As these aerobic organisms grow, they consume a major portion of the organic matter present in the effluent.
This microbial consumption leads to a significant reduction in the Biochemical Oxygen Demand (BOD) of the wastewater. BOD is defined as the amount of oxygen () consumed by bacteria to oxidize all organic matter in one litre () of water. BOD serves as a direct quantitative measure of the amount of organic pollution present in water; a higher BOD value indicates a greater polluting potential.
Once the BOD of the effluent is substantially lowered, the liquid is transferred into a settling tank where the bacterial flocs are allowed to settle down by gravity. This settled biological sediment is called activated sludge. A small fraction of this activated sludge is pumped back into the primary aeration tank to act as an inoculum or starter culture, supplying active microorganisms to digest incoming wastewater.
The remaining major portion of the activated sludge is pumped into large, sealed tanks known as anaerobic sludge digesters. In these digesters, specialized anaerobic bacteria digest the aerobic bacteria and fungi present in the sludge. During this anaerobic digestion process, gases such as methane (), hydrogen sulphide (), and carbon dioxide () are generated. These gases collectively form biogas, which can be harvested as a renewable energy source. The clear treated effluent exiting the secondary treatment is safe to be discharged into natural water bodies like rivers and streams.
To combat river pollution caused by urban waste, the Ministry of Environment and Forests initiated major national conservation efforts including the Ganga Action Plan and the Yamuna Action Plan. Under these national programs, the government proposed building numerous Sewage Treatment Plants along these major river basins to ensure that only treated sewage is discharged into these vital natural rivers.
Microbes in the Production of Biogas
Biogas is a combustible mixture of gases produced by the microbial breakdown of organic substrates under anaerobic conditions. It serves as a clean, sustainable energy source used primarily for rural cooking, lighting, and electricity generation. Biogas consists predominantly of methane (), alongside carbon dioxide (), hydrogen (), and hydrogen sulphide ().
The primary organisms responsible for biogas generation are obligate anaerobic bacteria known as methanogens, with Methanobacterium being a prime example. Methanogens naturally thrive on cellulosic material under strictly anaerobic environments. They are abundant in anaerobic sewage sludge as well as inside the rumen—a specialized portion of the stomach—of cattle, where they assist in digesting dietary cellulose. Because cattle dung (gobar) is exceptionally rich in Methanobacterium, dung serves as the ideal primary substrate for biogas generation, often referred to as gobar gas.
A standard biogas plant consists of a concrete tank constructed to deep into the ground. This pit collects bio-wastes and a thoroughly mixed slurry of cattle dung and water. A floating cover or gas holder is fitted over the slurry pit; as gases are produced by microbial digestion, the cover rises upward. The plant features an outlet pipe connected to a distribution system to deliver biogas directly to households, as well as a separate sludge removal outlet to collect spent slurry, which is rich in nitrogen and organic nutrients and is utilized as high-quality biofertilizer.
The biochemical process of biogas production occurs through a sequence of microbial breakdown stages inside the anaerobic digester. Initially, complex organic waste—such as crop residues, food waste, livestock dung, and sewage—is mixed with water to form a uniform slurry. Inside the oxygen-free digester, hydrolytic enzymes break down large, complex organic polymers into simpler soluble units.
Next, acidogenic bacteria ferment these simpler organic compounds into intermediate products, including carbon dioxide (), hydrogen (), ammonia (), and volatile organic acids. Acetogenic bacteria then process these organic acids further into acetic acid (), additional hydrogen (), and carbon dioxide ().
In the final stage, methanogenic bacteria (methanogens) consume the acetic acid, hydrogen, and carbon dioxide, converting them into methane () and carbon dioxide (), which constitute the primary components of biogas. The technology for rural biogas production was systematically developed in India through pioneering work by the Indian Agricultural Research Institute (IARI) and the Khadi and Village Industries Commission (KVIC).
Microbes as Biocontrol Agents
Biocontrol refers to the application of biological methods and living organisms to control plant pests and disease-causing agents. Unlike synthetic chemical insecticides and pesticides—which non-selectively kill both harmful and beneficial soil organisms, pollute ecosystems, and accumulate in food chains—biological control systems rely on natural ecological interactions such as predation and parasitism without harming non-target species. Examples of natural biological predators include ladybird beetles, which control populations of destructive aphids, and dragonflies, which hunt and suppress mosquito populations.
Microbial biocontrol agents provide targeted, environmentally safe alternatives for crop protection. The soil bacterium Bacillus thuringiensis (Bt) is widely used to control pests such as butterfly caterpillars. Commercial formulations of Bt consist of dried bacterial spores sold in sachets. These spores are mixed with water and sprayed onto vulnerable crops, including brassicas and fruit trees.
When caterpillars feed on the treated foliage, they ingest the bacterial spores. In the alkaline environment of the insect gut, the dormant Bt endotoxin protein is solubilized and activated, causing pore formation in the gut wall, cell lysis, and ultimate death of the caterpillar larvae. Genetic engineering has enabled scientists to transfer the toxic gene from Bacillus thuringiensis directly into plant genomes, creating pest-resistant transgenic crop cultivars such as Bt cotton.
Fungi also serve as effective biocontrol organisms. Species belonging to the fungal genus Trichoderma are free-living fungi abundantly present in plant root ecosystems. Trichoderma species actively suppress and control a wide spectrum of plant root pathogens.
Viruses belonging to the family Baculoviridae, specifically those within the genus Nucleopolyhedrovirus, serve as targeted biological control agents against insects and other arthropods. These viruses are highly species-specific and possess a narrow spectrum of insecticidal activity, meaning they infect only target pest species without causing harm to non-target organisms, plants, mammals, birds, or beneficial insects. Consequently, baculoviruses are ideal components in Integrated Pest Management (IPM) programs aimed at conserving beneficial pollinators and natural predators.
Microbes as Biofertilisers
Biofertilisers are living organisms that enrich the nutrient quality and physical fertility of soil by making essential plant nutrients available through natural biological processes. Primary groups of biofertilisers include free-living and symbiotic bacteria, mycorrhizal fungi, and cyanobacteria.
Bacteria play a dominant role in converting atmospheric nitrogen () into bioavailable organic forms that plants can assimilate. The bacterium Rhizobium forms a symbiotic association within specialized root nodules of leguminous plants, fixing atmospheric nitrogen directly for host plant growth. Additionally, free-living nitrogen-fixing bacteria present in the soil, such as Azospirillum and Azotobacter, absorb gaseous nitrogen and enrich the nitrogen content of surrounding soil.
Fungi form vital symbiotic associations with plant root systems known as mycorrhizae. Fungi belonging to the genus Glomus are prominent mycorrhizal symbionts. In this mutualistic partnership, the plant provides photosynthetic nutrients to the fungus, while the fungal hyphae absorb essential minerals—particularly phosphorus—from deep soil layers and transfer them efficiently to the host plant. Beyond mineral absorption, mycorrhizal fungi impart secondary benefits to the plant, including enhanced resistance to root-borne soil pathogens, increased tolerance to environmental stress like drought and salinity, and an overall boost in plant growth and development.
Cyanobacteria, commonly referred to as blue-green algae, are autotrophic photosynthetic microbes capable of fixing atmospheric nitrogen in aquatic and terrestrial habitats. Genera such as Anabaena, Nostoc, and Oscillatoria are key nitrogen fixers. In paddy fields (rice cultivation), cyanobacteria serve as vital biofertilisers that significantly reduce the need for synthetic nitrogenous fertilizers while continually adding organic matter to the soil to enhance overall fertility.
Furthermore, the aquatic fern Azolla forms a specialized symbiotic relationship with the nitrogen-fixing cyanobacterium Anabaena azollae. Cultivated in flooded paddy fields, Azolla serves as a natural, highly effective biological nitrogen source that substantially enriches paddy soils.