SPI Biology · Microbiology and Disease

Microorganisms and Their Uses

Microorganisms are living things too small to see without a microscope: bacteria, fungi, protozoa, algae and viruses, found almost everywhere.

All Microbiology and Disease notes

Why this matters

Five questions in the bank, four EASY and one MODERATE from the 2022 paper: where microbes live, which organism digests its food outside its body, the microbe that enriches the soil, the scientist who made oil-eating bacteria, and who gave us pasteurisation. Each one pairs a microbe or a scientist with one fact.

Concept 1 of 2: The groups of microbes and how they feed

Microbes come in five main groups, and they live in every place you can think of: hot springs, ice, deserts, marshes, soil, water and inside other living things. Many cannot make their own food. Some feed on dead matter by digesting it outside their bodies; others live on a living host.

Definition

Bacteria: Rhizobium, Lactobacillus. Fungi: yeast, bread mould (Rhizopus), Penicillium. Protozoa: Amoeba, Plasmodium. Algae: Chlamydomonas, Spirogyra. Viruses: reproduce only inside a host cell.
Saprophytes (most fungi, many bacteria) pour digestive juices onto dead food, break it down outside the body and then absorb it. Bread mould is the usual example.
Parasites take food from a living host: Cuscuta (a plant), the tapeworm (an animal).
Symbiosis: two organisms live together and both gain, as Rhizobium does in the roots of legumes.
Microbes live in all habitats, from hot springs to deserts and marshy land.

How it feeds	What it does	Example
Saprophytic	Digests dead food outside its body, then absorbs it	Rhizopus (bread mould), yeast Outside the body, then absorbed: that is the mark of a saprophyte.
Parasitic (plant)	Draws food from a living host plant	Cuscuta
Parasitic (animal)	Absorbs food already digested by its host	Tapeworm
Symbiotic	Lives with a partner, both gain	Rhizobium in pea roots

Practice this conceptself-check · 4 quick reps

The same idea in a practice question:

Practice questionEasy

Example 1 · Microbiology and Disease · Microorganisms and Their Uses

An organism which breaks down the food material outside the body and then absorbs it is

A parasite as a saprophyte

A tapeworm absorbs food that its host has already digested, and Cuscuta draws food from a living plant. Only a saprophyte such as Rhizopus digests food outside its own body.

Picking one habitat when the options allow all

Microbes live in hot springs, deserts, ice and marshes alike. When the options list several habitats and 'all of these', every one of them is right.

Drill 1 more on the groups of microbes and how they feed

Concept 2 of 2: Useful microbes and the scientists behind them

Many microbes help us: they set curd, raise bread, make medicines and feed the soil. A few scientists are remembered for putting them to work or for keeping them out of food.

Definition

Lactobacillus: turns milk into curd.
Yeast: fermentation (alcohol) and making bread rise (it gives out carbon dioxide).
Rhizobium: fixes nitrogen in legume roots and so increases soil fertility.
Penicillium: gives the antibiotic penicillin, found by Alexander Fleming (1928).
Louis Pasteur: pasteurisation (heating milk to kill microbes) and early vaccines.
Edward Jenner: the first vaccine, against smallpox.
Ananda Mohan Chakrabarty: made an oil-eating bacterium (a Pseudomonas, the 'superbug') to clean up oil spills.

Scientist	Known for
Louis Pasteur	Pasteurisation
Alexander Fleming	Penicillin
Edward Jenner	Smallpox vaccine
Ananda Mohan Chakrabarty	Oil-eating bacteria for oil spills M S Swaminathan and Norman Borlaug are Green Revolution names, not microbiologists.
Robert Hooke	First saw cells

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

SPI · 2022 · SPI Entrance Examination 2022 · Q101Moderate

Example 2 · Microbiology and Disease · Microorganisms and Their Uses

The credit for discovering an environment friendly cost effective method to deal with oil spill by using oil digesting, fast multiplying bacteriagoes to____.

Green Revolution names for a microbe question

M S Swaminathan and Norman Borlaug raised crop yields; they did not engineer bacteria. The oil-eating bacterium was the work of Ananda Mohan Chakrabarty.

Lactobacillus for soil fertility

Lactobacillus sets curd and yeast ferments sugar. The microbe that adds nitrogen to the soil is Rhizobium.

Drill 2 more on useful microbes and the scientists behind them

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (2)

The groups of microbes and how they feed4 rows

How it feeds	What it does	Example
Saprophytic	Digests dead food outside its body, then absorbs it	Rhizopus (bread mould), yeast Outside the body, then absorbed: that is the mark of a saprophyte.
Parasitic (plant)	Draws food from a living host plant	Cuscuta
Parasitic (animal)	Absorbs food already digested by its host	Tapeworm
Symbiotic	Lives with a partner, both gain	Rhizobium in pea roots

Useful microbes and the scientists behind them5 rows

Scientist	Known for
Louis Pasteur	Pasteurisation
Alexander Fleming	Penicillin
Edward Jenner	Smallpox vaccine
Ananda Mohan Chakrabarty	Oil-eating bacteria for oil spills M S Swaminathan and Norman Borlaug are Green Revolution names, not microbiologists.
Robert Hooke	First saw cells

Watch out for (4)

A parasite as a saprophyte
→ The groups of microbes and how they feed
Picking one habitat when the options allow all
→ The groups of microbes and how they feed
Green Revolution names for a microbe question
→ Useful microbes and the scientists behind them
Lactobacillus for soil fertility
→ Useful microbes and the scientists behind them

Test yourself on a real paper

Sit a past SPI paper, timed and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

Sit an SPI past paper Drill all 5 questions