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What are Antibiotics?
Antibiotics are antimicrobial agents produced naturally by other
microbes (usually fungi or bacteria).
The first antibiotic penicillin was discovered by Alexander
Flemming in from the filamentous fungus Penicilium notatum.…read more

Slide 3

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Action of Antibiotics
Many chemicals kill microbes. But a therapeutically useful anti
microbial agent must be selectively toxic i.e. it must kill
pathogenic microbes already growing in human tissue, without
also killing the host human cells.
Antibiotics do this by inhibiting enzymes that are unique to
prokaryotic cells, such those involved in synthesizing the bacterial
cell wall or 70S ribosomes.
Penicillin (and related antibiotics ampicillin, amoxicillin and
methicillin) inhibits an enzyme involved in the synthesis of
peptidoglycan for bacterial cell wall. This weakens the cell wall,
killing bacterial cells by osmotic lysis.
Streptomycin, tetracycline and erythromycin inhibit enzymes in
ribosomes. This stops protein synthesis so prevents cell division.…read more

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Development of Antibiotic Resistance
How do bacteria become resistant to antibiotics?
Resistance first develops due to a mutation often during
A mutation could slightly alter an enzyme, changing its substrate
specificity so that its active site will now bind penicillin. Some
bacteria now have a penicillinase enzyme that breaks down
penicillin, rendering the antibiotic useless.
Such mutations are very rare, but bacteria reproduce so rapidly,
and there are so many bacterial cells, that new resistance
mutations do crop up at a significant rate.
Remember that development of antibiotic resistance is a random
event, and is not caused by the presence of the antibiotic.
It is certainly not an adaptation that bacteria acquire.…read more

Slide 5

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Impact of Resistance on the use of Antibiotics
Antibiotic-resistant bacteria can spread to other people by any of
the normal methods of spreading an infection: through faeces,
water, food, sneezing, infected instruments, etc.
Some bacteria can form spores to aid their dispersal, and so a
mutated strain can survive long journeys and long periods of time.
In most new environments the mutated strain will die out through
competition, but whenever it encounters penicillin it will thrive,
out-competing all other bacteria.
The most common sources for antibiotic-resistant bacteria (and
especially multiple-resistant bacteria) are hospitals.
This is partly because they have a high concentration of people
with bacterial infections, but also because the environment is
awash with antibiotics.…read more

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Case 1 ­ Tuberculosis
Bacterial disease of the lungs ­ caused by Mycobacterium sp.
Problem with antibiotic treatment of TB ­ antibiotics should be
taken often for 6-9 months
Initially, when individuals are ill they take antibiotics ­ these
destroy only the least resistant strains of Mycobacterium
After few months, the patients feel better as most of the bacteria
are killed ­ use of antibiotics is stopped
Few remaining strains of Mycobacterium become resistant,
survive, multiply and spread to others ­ does not respond to
So what can be done? The simple solution is to use a wider range
of antibiotics (usually a cocktail of three or four antibiotics is used
to ensure that at least one will be effective.…read more

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