LIWA

Economic Importance of Bacteria

ICSE Class 9 · Biology · Chapter 9

What this chapter covers

Key definitions

Bacteria are microscopic, single-celled prokaryotic organisms that have a cell wall but no well-defined nucleus and no membrane-bound organelles such as mitochondria or chloroplasts.

An antibiotic is a chemical substance produced by a microorganism that, even in a very small amount, kills or stops the growth of other microorganisms, especially disease-causing bacteria.

A broad-spectrum antibiotic is an antibiotic that acts against many different kinds of bacteria.

A narrow-spectrum antibiotic is an antibiotic that acts against only a few kinds of bacteria.

Antibodies are proteins made by the body in response to a foreign substance, which combine with that substance and make it harmless.

An antitoxin is a serum whose antibodies neutralise the toxin produced by a particular bacterium.

A vaccine is a preparation of killed or weakened disease-causing microorganisms, or of their harmless products, which, when introduced into the body, makes the body produce its own antibodies and so gives protection against that disease.

Vaccination is the introduction of a vaccine into the body, by injection or by mouth, to produce immunity against a disease.

The nitrogen cycle is the continuous circulation of nitrogen between the atmosphere, the soil and living organisms, brought about mainly by the activities of different groups of bacteria.

Ammonification is the conversion of the nitrogen of proteins and other organic compounds in dead bodies and animal wastes into ammonia by decay bacteria and fungi.

Nitrogen fixation is the conversion of free atmospheric nitrogen into nitrogenous compounds, such as ammonia, that can be used by plants.

Biological nitrogen fixation is nitrogen fixation carried out by living organisms, mainly certain bacteria and cyanobacteria.

Key points

Questions answered

Define an antibiotic. Name one antibiotic obtained from a bacterium and the bacterium that produces it.
An antibiotic is a chemical substance produced by a microorganism that, in a very small amount, kills or stops the growth of other microorganisms, especially disease-causing bacteria. Streptomycin is produced by the bacterium Streptomyces griseus.
Why should a patient complete the full course of an antibiotic even after the fever has gone?
Stopping early leaves the toughest bacteria alive. They multiply and can give rise to bacteria that are resistant to the antibiotic, so the medicine may no longer cure the infection.
Why is an antibiotic of no use in treating the common cold?
The common cold is caused by a virus, and antibiotics act only on bacteria.
Distinguish between a serum and a vaccine with respect to (a) who makes the antibodies and (b) how long protection lasts.
(a) A serum contains ready-made antibodies made by another animal; a vaccine makes the person's own body produce antibodies. (b) Protection from a serum lasts only a few weeks; protection from a vaccine lasts months to many years.
What is a toxoid? Name one vaccine that contains toxoids.
A toxoid is a bacterial toxin treated so that it is no longer poisonous but can still make the body produce antibodies. The DPT vaccine contains diphtheria and tetanus toxoids.
Why can plants not use the nitrogen of the air directly?
The two atoms of a nitrogen molecule are joined by a very strong triple bond that plant cells cannot break. Plants can absorb nitrogen only in combined forms such as nitrates.
Name the bacterium responsible for each of the following: (a) fixing nitrogen in the root nodules of legumes; (b) converting ammonia to nitrites; (c) converting nitrites to nitrates; (d) converting nitrates to free nitrogen.
(a) Rhizobium; (b) Nitrosomonas; (c) Nitrobacter; (d) Pseudomonas denitrificans (or Thiobacillus denitrificans).
Define nitrification and denitrification. Which of the two lowers soil fertility?
Nitrification is the conversion of ammonia in the soil into nitrites and then nitrates by nitrifying bacteria. Denitrification is the conversion of soil nitrates back into free nitrogen gas by denitrifying bacteria. Denitrification lowers soil fertility.