Microbes in Human Welfare Comprehensive Study Notes

Microbes in Household Products

Microbes play a vital role in our daily dietary needs, particularly in the fermentation of dairy and grain-based products. Milk is converted into curd through the action of Lactic Acid Bacteria (LAB), such as Lactobacillus. This transformation not only changes the consistency and taste of the milk but also improves its nutritional quality by increasing the content of specific vitamins. In household food preparation, the puffed-up appearance seen in the dough used for making bread is primarily due to the production of $CO_2$ gas. Similarly, the dough used for traditional dishes like idli and dosa is fermented by various bacteria.

Another significant application is in the cheese industry. Different varieties of cheese are distinguished by their unique texture, flavor, and taste. For example, Swiss cheese is characterized by large holes, which are formed by the production of large amounts of $CO_2$ by the bacterium Propionibacterium sharmanii. Roquefort cheese is ripened by growing a specific fungus on it to provide a particular flavor. Beyond dairy, microbes are used to ferment other food sources such as fish, soyabean, and bamboo-shoots to create traditional edible products.

Microbes in Industrial Production

On an industrial scale, microbes are grown in very large vessels called fermentors to produce a variety of products, including beverages, antibiotics, and bioactive molecules. Saccharomyces cerevisiae, commonly known as Baker’s yeast or Brewer’s yeast, is central to the production of ethanol and bread. The production of alcoholic beverages involves the fermentation of broth; some products like wine are produced without distillation, while others like whisky, brandy, and rum are products of the distillation of the fermented broth.

Antibiotics are chemical substances produced by microbes that can kill or retard the growth of other disease-causing microbes. The word "anti" is a Greek term meaning 'against', while "bio" means 'life'. Penicillin was the first antibiotic to be discovered. Alexander Fleming discovered it serendipitously while working on Staphylococci bacteria; he observed a mould (later identified as Penicillium notatum) growing in an unwashed culture plate that inhibited the growth of the bacteria.

Microbes are also utilized for the production of organic acids and bioactive molecules. Examples include citric acid from the fungus Aspergillus niger, acetic acid from the bacterium Acetobacter aceti, and butyric acid from Clostridium butylicum. In the pharmaceutical sector, various bioactive molecules are produced:

Lipases: Used in detergent formulations to remove oil stains.
Pectinases and Proteases: Used to clarify bottled fruit juices, making them clearer than home-brewed juices.
Streptokinase: Produced by Streptococcus and modified by genetic engineering, it is used as a 'clot buster' for removing clots from the blood vessels of patients who have undergone myocardial infarction.
Cyclosporin A: An immunosuppressive agent used in organ-transplant patients, produced by the fungus Trichoderma polysporum.
Statins: Produced by the yeast Monascus purpureus, these act as blood-cholesterol lowering agents.

Microbes in Sewage Treatment (STP)

Sewage, which is municipal waste-water containing human excreta, must be treated in Sewage Treatment Plants (STPs) before being discharged into water bodies to reduce its polluting potential. This process occurs in two main stages: Primary and Secondary treatment. Primary treatment is a physical process that involves the removal of large and small particles through filtration and sedimentation. All solids that settle form the primary sludge, while the supernatant forms the effluent.

Secondary treatment, also known as biological treatment, involves the use of heterotrophic microbes naturally present in the sewage. The effluent from primary treatment is passed into large aeration tanks where it is constantly agitated. This allows for the growth of flocs, which are masses of aerobic bacteria associated with fungal filaments. These microbes consume the organic matter in the effluent, significantly reducing the Biochemical Oxygen Demand (BOD). BOD is a measure of the organic matter in water; a greater BOD indicates higher polluting potential. Once the BOD is reduced, the effluent is passed into a settling tank where the bacterial flocs settle; this sediment is called activated sludge. A small part of this sludge is pumped back into the aeration tank to serve as an inoculum, while the rest is pumped into anaerobic sludge digesters. In these digesters, anaerobic bacteria digest the bacteria and fungi in the sludge, producing a mixture of gases known as biogas.

Microbes in Biogas Production

Biogas is a mixture of gases produced by microbial activity and is used as a source of energy because it is inflammable. It consists primarily of methane ( $CH_4$ ), carbon dioxide ( $CO_2$ ), and hydrogen ( $H_2$ ). The bacteria responsible for this production are known as methanogens, with Methanobacterium being a common example. These bacteria are often found in the anaerobic sludge during sewage treatment and are also present in the rumen of cattle. In the rumen, they help in the breakdown of cellulose. The excreta of cattle (dung) is rich in these bacteria and is used in gobar gas plants to produce biogas for rural lighting and cooking.

Microbes as Biocontrol Agents

Biocontrol refers to the use of biological methods for controlling plant diseases and pests, moving away from the use of toxic chemicals and pesticides. This approach relies on natural predation. Examples of biocontrol agents include:

Ladybird beetles: Used to get rid of aphids.
Dragonflies: Used to control mosquitoes.
Bacillus thuringiensis (Bt): Used to control butterfly caterpillars. It is now incorporated into plants like Bt-cotton via genetic engineering.
Trichoderma: A free-living fungus common in root ecosystems that acts as a biocontrol agent against several plant pathogens.
Baculoviruses: Specifically the genus Nucleopolyhedrovirus, these are excellent candidates for species-specific, narrow-spectrum insecticidal applications. They attack insects and other arthropods but have no negative impacts on plants, mammals, birds, or non-target insects, making them ideal for Integrated Pest Management (IPM) programs.

Microbes as Biofertilisers

Biofertilisers are organisms that enrich the nutrient quality of the soil. The main sources of biofertilisers are bacteria, fungi, and cyanobacteria.

Bacteria: Rhizobium forms symbiotic associations in the nodules of leguminous plants, fixing atmospheric nitrogen. Free-living nitrogen-fixing bacteria include Azotobacter and Azospirillum.
Fungi: Fungi form symbiotic associations with plants known as mycorrhiza. Many members of the genus Glomus form mycorrhiza, which help the plant absorb phosphorus from the soil and provides resistance to root-borne pathogens and salinity/drought tolerance.
Cyanobacteria: Autotrophic microbes like Anabaena, Nostoc, and Oscillatoria fix atmospheric nitrogen. In paddy fields, cyanobacteria serve as an important biofertiliser and add organic matter to the soil to increase fertility.

General Information and Classification

Microbes are diverse and found everywhere on Earth. They include protozoa, bacteria, fungi, and microscopic animal and plant viruses, as well as viroids and prions. Note that higher plants and animals are excluded from the category of microbes. In the context of human welfare, some microbes are associated with diseases that have common vernacular names in India, such as whooping cough (Kali khansi), diphtheria (Gal ghotu), and leprosy (Khusht rog).