Microbes in Human Welfare: One Mark Questions, Book Back Answers and Notes

About Microbes in Human Welfare

Medium ~120 min study

Most people think of microbes only as germs, but only a small fraction of them cause disease. Far more are harmless, and many are useful. This chapter is about the useful ones: the bacteria and fungi that make our food, medicines and fuels, and clean up our waste.

It starts in the kitchen, with Lactobacillus turning milk into curd and yeast making dough rise. It then moves to factories, where microbes grown in large fermenters produce antibiotics like penicillin, enzymes, citric and acetic acid, and alcohol. The last part deals with the environment: how microbes lower the organic load of sewage, produce methane in biogas plants, generate electricity in microbial fuel cells and break down oil and plastic.

Book back questions here are mostly direct. Expect to be asked which organism produces a given product, what happens in primary and secondary sewage treatment, what biogas contains, and which microbe is used for a given pollutant. A table of organism, product and use is the quickest way to revise.

What you'll learn

• Distinguish probiotics from disease-causing microbes by their effect on the body.
• Explain how lactic acid bacteria convert milk into curd.
• Describe how antibiotics and enzymes are produced in industrial fermenters.
• Explain the stages of sewage treatment and how microbes reduce BOD.
• Describe how biogas is produced under anaerobic conditions and what it contains.
• Assess how natural and genetically engineered microbes break down environmental pollutants.

Before you start

• Differences between prokaryotic and eukaryotic cells.
• Cellular respiration, especially anaerobic respiration and fermentation.
• Basics of genetic engineering, plasmids and recombinant DNA.

Microbes in Human Welfare explained

Where Microbes Help Us

Microbes in Household Products

Lactic acid bacteria (LAB) such as Lactobacillus convert milk into curd. They ferment lactose into lactic acid; the acid lowers the pH and makes the milk protein casein coagulate. LAB also partly digest milk proteins and raise the vitamin B₁₂ content. In bread making, the yeast Saccharomyces cerevisiae ferments sugars and releases carbon dioxide, which makes the dough rise and gives bread its soft, spongy texture. Probiotics are live beneficial microbes, such as LAB, that help keep the gut flora healthy and hold back harmful bacteria.

Microbes in Industrial Products

Industries grow selected microbes in large fermenters to make useful products. Antibiotics, such as penicillin from the fungus Penicillium, kill or stop the growth of disease-causing bacteria. Microbial enzymes include lipases, added to detergents to remove oily stains, and pectinases, used to clear fruit juices. Organic acids are also made this way: citric acid by Aspergillus niger and acetic acid by Acetobacter aceti. Yeast ferments sugars to produce alcoholic drinks; wine and beer are not distilled, while whisky, brandy and rum are distilled after fermentation.

Microbes in Sewage Treatment

Sewage treatment protects rivers and lakes from organic pollution. Primary treatment is physical: floating debris is filtered off and grit and solids settle out. Secondary treatment is biological: the effluent is aerated so that aerobic microbes form flocs and consume the organic matter, which lowers the BOD (biochemical oxygen demand). The settled sludge goes to anaerobic sludge digesters, where other bacteria break it down and release gases such as methane, hydrogen sulphide and carbon dioxide, which can be used as biogas. Tertiary treatment, where used, removes remaining nutrients and disinfects the water with chlorine or UV light.

Biogas Production

Biogas is made when organic waste such as cattle dung, crop residues and kitchen waste is broken down without oxygen inside a digester. Methanogens, such as Methanobacterium, produce the methane. Biogas contains about 63% methane, with carbon dioxide and some hydrogen and hydrogen sulphide. It is used for cooking and lighting in villages, and the leftover slurry makes a good manure.

Bioremediation

Bioremediation uses natural or genetically engineered microbes to remove pollutants from soil and water. Aerobic microbes break down pollutants where oxygen is present; anaerobic microbes such as Dechloromonas aromatica work where it is absent. Organic pollutants such as hydrocarbons and pesticides can be broken down into harmless products like carbon dioxide and water. Heavy metals cannot be broken down, but some microbes can take them up or convert them into less toxic, less mobile forms. The engineered strain of Pseudomonas putida degrades hydrocarbons in oil spills, and the bacterium Ideonella sakaiensis breaks down PET plastic using the enzymes PETase and MHETase. Bioremediation is usually cheaper and gentler on the environment than chemical clean-up.

Common mistakes to avoid

• Mixing up prebiotics and probiotics. Prebiotics are non-digestible food fibres that feed good gut bacteria; probiotics are the live beneficial microbes themselves.
• Assuming all microbes cause disease. Only a small fraction are pathogens; many are harmless and many are useful in food, industry and the environment.
• Thinking all alcoholic drinks are distilled. Wine and beer are made without distillation; whisky, brandy and rum are distilled.
• Placing biological treatment in the primary stage of sewage treatment. Primary treatment is physical (filtration and sedimentation); secondary treatment is the biological stage.
• Putting disinfection at the start of sewage treatment. Chlorination or UV treatment comes at the end, in the tertiary stage.

Test yourself on these with the practice test, then check the worked reasoning in the solved MCQs.

Frequently asked questions

How does milk turn into curd?

A small amount of curd added to warm milk introduces lactic acid bacteria, mainly Lactobacillus. These bacteria multiply and ferment lactose into lactic acid. The acid lowers the pH, and the milk protein casein coagulates, so the milk sets into curd. The bacteria also increase the vitamin B12 content, which makes curd more nutritious than milk.

How is the production of beer different from that of whisky?

Both start with fermentation of sugars by yeast. Beer and wine are not distilled, so their alcohol content stays relatively low. Whisky, brandy and rum are distilled after fermentation: the alcohol is separated from water and other substances, which gives a much higher alcohol concentration.

Why is BOD measured in sewage treatment?

BOD is the amount of oxygen that would be used up if all the organic matter in one litre of water were oxidised by bacteria. A high BOD means the water carries a lot of organic waste. During secondary treatment, aerobic microbes consume this organic matter and the BOD falls sharply. A low BOD shows that the water is clean enough to release into a river or lake.

How does a microbial fuel cell work?

A microbial fuel cell uses bacteria to turn the chemical energy in organic matter into electricity. At the anode, bacteria oxidise the organic matter under anaerobic conditions and release electrons and protons. The electrons travel through an external wire to the cathode, producing a current, while the protons pass through a membrane to the cathode, where they combine with oxygen and electrons to form water.

What gases are present in biogas?

Biogas is produced by methanogens from organic waste in the absence of oxygen. Methane is the main component, about 63%. The rest is mostly carbon dioxide, with small amounts of hydrogen and hydrogen sulphide.

What is bioremediation? Give an example.

Bioremediation is the use of microbes, natural or genetically engineered, to remove or neutralise pollutants. A standard example is the engineered strain of Pseudomonas putida, which can break down the hydrocarbons in oil spills into harmless substances such as carbon dioxide and water.

How does a fungus help clean up plastic waste?

The endophytic fungus Pestalotiopsis microspora can use polyurethane as its food. It produces enzymes that break the plastic down, even in the absence of oxygen, which makes it a possible tool for reducing plastic in landfills.