Course 26.1Lesson 3 of 3

Antibiotics, enzymes and bioactive molecules

An antibiotic is a chemical made by a microbe that kills or slows the growth of other microbes. Penicillin was the first one, found by Alexander Fleming in 1928. Microbes also make enzymes such as lipases, pectinases, proteases and streptokinase, and the bioactive molecules cyclosporin A and the statins.

about 30 min1 simulation1 practice question

Lesson 3 of 3Show all lessons

Try the code and the simulations beside the lesson.

A dish with a clear patch

In 1928 Alexander Fleming was working with Staphylococcus bacteria in dishes of agar. On one dish, he saw that a mould was growing, and that no bacteria grew near it. There was a clear zone around the mould.

He understood that the mould made a chemical that stopped the bacteria. The mould was Penicillium notatum, and the chemical was penicillin. It was the first antibiotic. The word means a chemical against life, and in this use it means against the life of disease causing microbes.

A round dish of agar seen from above, covered with a lawn of tiny golden dots, the bacterium Staphylococcus. Near the left side is a green colony of the mould Penicillium with a ring of paler spots at its edge. Around the colony is a round clear zone, drawn with a dashed blue line, where no bacteria grow. Labels point to the lawn, the mould colony and the clear zone. At the right, a short timeline says: 1928 Fleming sees the clear zone on a dish; Chain and Florey show it works as a medicine; 1945 Nobel Prize to the three.

Fleming saw the effect, but it was Ernest Chain and Howard Florey who showed that penicillin works as a medicine. It was used on a large scale to treat wounded soldiers in the Second World War. Fleming, Chain and Florey received the Nobel Prize in 1945.

Antibiotics have greatly improved the treatment of deadly diseases such as plague, whooping cough, diphtheria and leprosy.

A note on the name. NCERT writes Penicillium notatum. Modern books say that the mould of Fleming was a close relative, Penicillium rubens. The idea is the same.

Enzymes from microbes

Microbes make enzymes that are used in industry and in medicine.

Lipases break fats. They are added to detergents to remove oily stains from clothes.
Pectinases and proteases are used to clear bottled juices. They break the substances that make the juice cloudy.
Streptokinase is made by the bacterium Streptococcus. It is a clot buster. It is used to remove clots from the blood vessels of patients who have had a heart attack.

Bioactive molecules

Some microbes make bioactive molecules, which are substances that act on the body.

Cyclosporin A is made by the fungus Trichoderma polysporum. It is an immunosuppressive agent. It is given to patients after an organ transplant, so that the body does not reject the new organ.
Statins are made by Monascus purpureus. They lower the cholesterol of the blood. They are competitive inhibitors of the enzyme that makes cholesterol. A statin has a shape that fits the active site of the enzyme, so the real substrate cannot enter.

Two boxes side by side. Each shows a purple enzyme, the enzyme that makes cholesterol, with a notch in its top, which is the active site. On the left, without a statin, a yellow substrate sits in the notch and the words say that cholesterol is made. On the right, with a statin, a red statin molecule of the same shape sits in the notch, and a yellow substrate above it is crossed out with the words substrate cannot enter.

Notes on the names. NCERT calls Monascus purpureus a yeast. It is a mould, a fungus. NCERT names Trichoderma polysporum as the source of cyclosporin A. Today the fungus is classified as Tolypocladium inflatum. For a test that follows the textbook, use the NCERT name.

How much antibiotic is enough?

This part is not in the NEET syllabus, but A-Level and IB microbiology ask it.

A laboratory finds the least dose of an antibiotic that stops a bacterium. It sets up a row of tubes of broth with bacteria. Tube 1 has a high dose, and each next tube has half the dose of the one before. After a day, some tubes are cloudy, because the bacteria grew, and some are clear. The lowest dose that gives a clear tube is read as the least dose that stops the bacterium.

You should find that a bacterium that needs a higher dose leaves fewer tubes clear. The reading is always one of the doses in the row, so it can be a little higher than the true least dose.

Try it in Python

The program goes along a row of eight tubes and finds the lowest clear one.

Practise with new numbers

Common mistakes

Saying that penicillin was found in a bacterium. It was found in a mould.
Giving all the credit for the medicine to Fleming. Chain and Florey showed that penicillin works as a medicine.
Saying that cyclosporin A lowers cholesterol. It suppresses the immune response. The statins lower cholesterol.
Saying that statins are made by a bacterium. Streptokinase is made by the bacterium Streptococcus. The statins come from the fungus Monascus purpureus.
Thinking that an antibiotic can kill a virus. An antibiotic acts against bacteria.

Remember these

Penicillin: Penicillium mould, seen by Fleming in 1928 on a dish with Staphylococcus. Chain and Florey made it a medicine. Nobel Prize in 1945.
Lipases: detergents. Pectinases and proteases: clear juices. Streptokinase from Streptococcus: clot buster.
Cyclosporin A: Trichoderma polysporum, immunosuppressant after a transplant. Statins: Monascus purpureus, lower blood cholesterol, competitive inhibitors.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

What did Fleming see on the dish, and what did he conclude?

Who showed that penicillin could be used as a medicine, and when did the three scientists receive the Nobel Prize?

Give the use of lipases, of pectinases and of streptokinase.

Name the source and the use of cyclosporin A.

How do statins lower the cholesterol of the blood?

References

NCERT (2023). Biology, Textbook for Class XII, chapter 8, Microbes in Human Welfare.
Fleming A. (1929). On the antibacterial action of cultures of a Penicillium. British Journal of Experimental Pathology, 10, 226 to 236.
Endo A. (2010). A historical perspective on the discovery of statins. Proceedings of the Japan Academy, Series B, 86, 484 to 493. doi:10.2183/pjab.86.484.
Andrews J. M. (2001). Determination of minimum inhibitory concentrations. Journal of Antimicrobial Chemotherapy, 48, Supplement 1, 5 to 16.