Chapter 8: Microbes in Human Welfare

Introduction

Besides macroscopic plants and animals, microbes are the major components of biological systems on this earth. Microbes are present everywhere—in soil, water, air, inside our bodies, and in environments where no other life-form could possibly exist. While many microbes cause diseases, numerous others are useful to human beings in diverse ways.

Microbes in Household Products

• Curd (Dahi): Micro-organisms such as Lactobacillus and others commonly called Lactic Acid Bacteria (LAB) grow in milk and convert it to curd. During growth, the LAB produce acids that coagulate and partially digest the milk proteins, also increasing Vitamin \(B_{12}\).
• Dough: The dough used for making foods such as dosa and idli is fermented by bacteria. The puffed-up appearance of dough is due to the production of \(CO_2\) gas. Bread dough is fermented using baker’s yeast (Saccharomyces cerevisiae).
• Cheese: Different varieties of cheese are known by their characteristic texture, flavour, and taste. The large holes in ‘Swiss cheese’ are due to the production of a large amount of \(CO_2\) by a bacterium named Propionibacterium sharmanii. The ‘Roquefort cheese’ is ripened by growing a specific fungus on it.

Microbes in Industrial Products

Even in industry, microbes are used to synthesize a number of products valuable to human beings. Production on an industrial scale requires growing microbes in very large vessels called fermentors.

• Fermented Beverages: Saccharomyces cerevisiae (Brewer’s yeast) is used for fermenting malted cereals and fruit juices to produce ethanol. Depending on the type of raw material and processing (with or without distillation), different alcoholic drinks are obtained.
• Antibiotics: 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 from the mold Penicillium notatum.
• Chemicals, Enzymes, and Bioactive Molecules:
  • Aspergillus niger (a fungus) produces Citric acid.
  • Acetobacter aceti (a bacterium) produces Acetic acid.
  • Lactobacillus (a bacterium) produces Lactic acid.
  • Streptococcus produces Streptokinase, used as a ‘clot buster’ for removing clots from blood vessels.
  • Trichoderma polysporum produces Cyclosporin A, used as an immunosuppressive agent in organ-transplant patients.
  • Monascus purpureus produces Statins, used as blood-cholesterol lowering agents.

Microbes in Sewage Treatment

Large quantities of wastewater are generated every day in cities and towns. A major component of this wastewater is human excreta. This municipal wastewater is also called sewage. Before disposal, sewage is treated in sewage treatment plants (STPs) to make it less polluting.

• Primary Treatment: Physical removal of large and small particles from sewage through filtration and sedimentation.
• Secondary (Biological) Treatment: The primary effluent is passed into large aeration tanks where vigorous growth of useful aerobic microbes into flocs (masses of bacteria associated with fungal filaments to form mesh-like structures) takes place. While growing, these microbes consume the major part of the organic matter in the effluent, significantly reducing the Biochemical Oxygen Demand (BOD) of the effluent. The greater the BOD of wastewater, the more is its polluting potential. Once BOD is reduced, the effluent is passed into a settling tank where the bacterial ‘flocs’ are allowed to sediment. This sediment is called activated sludge. A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum. The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters, where anaerobic bacteria digest the bacteria and fungi in the sludge, producing a mixture of gases such as methane, hydrogen sulphide, and carbon dioxide (biogas).

Microbes in Production of Biogas

Biogas is a mixture of gases (containing predominantly methane) produced by the microbial activity and which may be used as fuel. Certain bacteria, which grow anaerobically on cellulosic material, produce large amounts of methane along with \(CO_2\) and \(H_2\). These bacteria are collectively called methanogens, e.g., Methanobacterium. They are commonly found in the anaerobic sludge during sewage treatment and in the rumen of cattle. The technology of biogas production was developed in India mainly due to the efforts of IARI and KVIC.

Microbes as Biocontrol Agents

Biocontrol refers to the use of biological methods for controlling plant diseases and pests.

• Bacillus thuringiensis (Bt): Used to control butterfly caterpillars. Available in sachets as dried spores which are mixed with water and sprayed onto vulnerable plants (such as brassicas and fruit trees), where these are eaten by the insect larvae. In the gut of the larvae, the toxin is released, and the larvae get killed.
• Trichoderma: Free-living fungi that are very common in the root ecosystems. They are effective biocontrol agents of several plant pathogens.
• Baculoviruses: Pathogens that attack insects and other arthropods. The majority of baculoviruses used as biological control agents belong to the genus Nucleopolyhedrovirus. They are specific and have no negative impacts on plants, mammals, birds, fish, or non-target insects.

Microbes as Biofertilisers

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

• Nitrogen-fixing Bacteria: Rhizobium (symbiotic in root nodules of leguminous plants), Azospirillum and Azotobacter (free-living in soil).
• Mycorrhiza: Fungi form symbiotic associations with plants. The fungal symbiont absorbs phosphorus from soil and passes it to the plant. The plant provides food to the fungus.
• Cyanobacteria: Autotrophic microbes widely distributed in aquatic and terrestrial environments, many of which can fix atmospheric nitrogen, e.g., Anabaena, Nostoc, Oscillatoria. In paddy fields, cyanobacteria serve as an important biofertiliser. Blue-green algae also add organic matter to the soil and increase its fertility.

Competency Based Questions

Q1. In a sewage treatment plant, a sample of incoming municipal wastewater is tested to have a Biochemical Oxygen Demand (BOD) of 400 mg/L. After successfully completing secondary biological treatment in the aeration tanks, the BOD of the effluent drops to 20 mg/L. What percentage of the origin biodegradable organic matter has been consumed by the microbial flocs during this treatment?

(A) 5%
(B) 20%
(C) 95%
(D) 100%

Q2. During the maturation of Swiss cheese, the bacterium Propionibacterium sharmanii ferments lactic acid to produce propionic acid, acetic acid, and \(CO_2\) gas. Assuming the basic stoichiometry of this fermentation produces 1 mole of \(CO_2\) for every 3 moles of lactic acid consumed, how many moles of \(CO_2\) gas are trapped in the large characteristic holes if the culture consumes 150 moles of lactic acid?

Q3. If an organ transplant patient is given a dose of 50 mg of Cyclosporin A daily to suppress T-cell activation, identify the microbial source of this crucial bioactive molecule.

Q4. Explain the dual purpose of returning a small fraction of the “activated sludge” back into the secondary aeration tanks during sewage treatment.

Q5. How do biofertilisers like Azospirillum and Azotobacter differ in their mode of nitrogen fixation compared to Rhizobium in agricultural systems?