Microbes in Human Welfare — Notes

• Curd: micro-organisms such as Lactobacillus and others commonly called lactic acid bacteria (LAB) grow in milk and convert it to curd. They produce acids that coagulate and partially digest milk proteins. A small amount of curd added to fresh milk as inoculum (starter) contains millions of LAB, which multiply at a suitable temperature. LAB also improve nutritional quality by increasing vitamin B₁₂, and in our stomach they help check disease-causing microbes.
• Dough for idli and dosa is fermented by bacteria; the puffed-up appearance is due to carbon dioxide.
• Bread: dough is fermented by baker’s yeast, Saccharomyces cerevisiae; carbon dioxide makes it rise.
• Toddy, a traditional drink of southern India, is made by fermenting sap from palms.
• Cheese: the large holes in Swiss cheese are due to a large amount of CO₂ produced by the bacterium Propionibacterium sharmanii; Roquefort cheese is ripened by growing specific fungi on it, which give it a particular flavour.

Microbes are grown on a large scale in very large vessels called fermentors.

Fermented beverages: Saccharomyces cerevisiae (brewer’s yeast) ferments malted cereals and fruit juices to produce ethanol. Wine and beer are produced without distillation; whisky, brandy and rum are produced by distillation of the fermented broth.

Antibiotics are chemical substances produced by some microbes that can kill or retard the growth of other (disease-causing) microbes. Penicillin was the first antibiotic discovered — by Alexander Fleming, who saw that a mould, Penicillium notatum, growing in an unwashed culture plate of Staphylococcus, prevented the bacteria from growing around it. Its full potential as an effective antibiotic was established by Ernst Chain and Howard Florey; it was used extensively to treat soldiers in World War II. Antibiotics have greatly reduced deaths from diseases such as plague, whooping cough, diphtheria and leprosy.

Chemicals, enzymes and other bioactive molecules:

• Lipases are used in detergent formulations to remove oily stains from laundry.
• Pectinases and proteases make bottled fruit juices clearer.
• Streptokinase, produced by Streptococcus and modified by genetic engineering, is a “clot buster” that removes clots from blood vessels of patients who have had a myocardial infarction (heart attack).
• Cyclosporin A, from the fungus Trichoderma polysporum, is an immunosuppressive agent used in organ-transplant patients.
• Statins, from the yeast Monascus purpureus, are blood-cholesterol-lowering agents; they competitively inhibit the enzyme responsible for cholesterol synthesis.

Sewage is municipal waste water containing large amounts of organic matter and microbes, many of them pathogenic. It is treated in sewage treatment plants (STPs) in two stages.

Primary treatment — physical removal of particles: floating debris is removed by sequential filtration; grit (soil and small pebbles) by sedimentation. All solids that settle form the primary sludge; the supernatant forms the effluent, which goes to secondary treatment.

Secondary (biological) treatment — the primary effluent is passed into large aeration tanks, where it is constantly agitated and air is pumped in. This allows vigorous growth of useful aerobic microbes into flocs — masses of bacteria associated with fungal filaments forming mesh-like structures. The microbes consume most of the organic matter, significantly reducing the BOD.

BOD (biochemical oxygen demand) is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. The greater the BOD, the greater the polluting potential (model plant: 360 → 240 → 24 mg/L, a fall of 93.3 percent).

When BOD has fallen enough, the effluent passes into a settling tank, where the flocs sediment as activated sludge. A small part is pumped back to the aeration tank as inoculum; the major part goes to anaerobic sludge digesters, where anaerobic bacteria digest the bacteria and fungi, producing gases such as methane, hydrogen sulphide and carbon dioxide — biogas. The treated effluent is released into rivers and streams. The Ministry of Environment and Forests began the Ganga Action Plan and Yamuna Action Plan to build STPs.

Biogas is a mixture of gases — predominantly methane, with carbon dioxide and hydrogen sulphide — produced by microbial activity and used as fuel.

Methanogens, such as Methanobacterium, grow anaerobically on cellulosic material and produce large amounts of methane along with CO₂ and H₂. They are found in anaerobic sludge during sewage treatment and in the rumen of cattle, where they help break down the cellulose of the cattle’s food. Cattle excreta (gobar) are therefore rich in methanogens.

Biogas plant: a concrete tank (10–15 feet deep) in which bio-wastes are collected and a slurry of dung is fed. A floating cover is placed over the slurry; it keeps rising as gas is produced by microbial activity. The plant has an outlet connected to a pipe that supplies biogas to nearby houses for cooking and lighting. The spent slurry is removed through another outlet and used as fertiliser. The technology was developed in India mainly by the Indian Agricultural Research Institute (IARI) and the Khadi and Village Industries Commission (KVIC).

Biocontrol is the use of biological methods for controlling plant diseases and pests. An organic farmer holds that eradicating pests is undesirable, since predatory and parasitic insects that depend on them would also die; the aim is to keep pests at manageable levels through a balanced system.

Examples:

• Ladybird beetles feed on aphids; dragonflies feed on mosquitoes.
• Bacillus thuringiensis (Bt): spores (available in sachets) are mixed with water and sprayed on plants such as brassicas and fruit trees; they are eaten by caterpillars, in whose gut the toxin is released, killing the larvae without harming other insects.
• Trichoderma: free-living fungi common in root ecosystems, effective biocontrol agents against several plant pathogens.
• Baculoviruses (genus Nucleopolyhedrovirus) attack insects and other arthropods; they are species-specific and narrow-spectrum, with no negative impact on plants, mammals, birds, fish or non-target insects — ideal for integrated pest management (IPM) in ecologically sensitive areas.

Harmful effects of chemical pesticides: they are toxic to humans and animals; they kill useful and harmful organisms indiscriminately; they pollute soil, water bodies and food (including by biomagnification in food chains); and pests develop resistance. IPM combines biocontrol, resistant varieties, crop rotation and minimal, targeted use of chemicals.

Biofertilisers are organisms that enrich the nutrient quality of the soil; the main sources are bacteria, fungi and cyanobacteria. They reduce the use of chemical fertilisers.

• Rhizobium forms nodules on the roots of leguminous plants and fixes atmospheric nitrogen into organic forms usable by the plant (symbiotic).
• Azospirillum and Azotobacter are free-living bacteria that fix atmospheric nitrogen in the soil.
• Mycorrhiza: symbiotic associations of fungi, many of the genus Glomus, with plant roots. The fungus absorbs phosphorus from the soil and passes it to the plant; mycorrhizal plants also show resistance to root-borne pathogens, tolerance to salinity and drought, and better growth.
• Cyanobacteria (blue-green algae) are autotrophic microbes, many of which fix atmospheric nitrogen — for example Anabaena, Nostoc, Oscillatoria. In paddy fields they are important biofertilisers and also add organic matter to the soil.