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212 SCIPAD - BIG IDEAS - BOOK 2
LIFE OF MICROBES
1. Define the term 'antibiotic'. 

2. Outline why some organisms produce antibiotics. 


3. Explain why a Doctor would not prescribe antibiotics for a patient suffering from a viral
infection such as a cold or the flu. 

4. (a) Referring to the 'Disk Diffusion Testing' image, state which antibiotic (A-D) is the
most effective. 
(b) Explain your answer to (a) above. 


Antibiotics
Antibiotics are chemicals that are produced naturally by bacteria
and fungi as a means of preventing the growth of competing
microbes around them.
Antibiotics are used to treat bacterial infections. Viruses are not
affected by antibiotics.
Antibiotics were discovered accidentally in 1928 by
Alexander Fleming. Fleming was investigating the properties of
staphylococci. His laboratory was often untidy. Before departing
for a family holiday he stacked all his cultures of staphylococci
in a corner of his laboratory. Upon returning from holiday, Fleming
noticed that one culture was contaminated with a mould (fungi), and
that the colonies of staphylococci that were surrounding the mould had
been destroyed.
Fleming grew the fungus in a pure culture and found that it produced a substance that killed a
number of disease-causing bacteria. The mould was from the Penicillium fungus, and after some
months of calling it “mould juice”, he named the substance penicillin - the world’s first antibiotic.
Numerous antibiotics have now been discovered and have prevented millions of deaths
worldwide.
Colonies of bacteria
Penicillium fungi
Area of no bacterial
growth created by the
antibiotic secreted by
the fungus. Paper disks soaked in different
antibiotics can be placed on an
agar plate inoculated with infectious
bacteria to determine which
antibiotic is the most effective.
Disk Diffusion Testing
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