Antibiotics

Question 1

Define chemotherapy

Answer 1

Chemotherapy can be defined as the use of chemicals

to destroy bacteria, fungi, viruses or cancer cells

Question 2

What does chemotherapy depend on?

Answer 2

Chemotherapy depends on drugs being selectively toxic
to the unwanted cells, but not too toxic to the host’s cells.

Therefore, one of the principles of chemotherapy is to
look for differences between the disease state and the
host’s normal cells.

Since bacteria, fungi and viruses have many biochemical
differences to mammalian cells, many drugs used to
treat such infections are selectively toxic can show low
levels of human toxicity.

Question 3

What is one of the key problems with antimicrobial therapy?

Answer 3

it is important for patients to comply with their dosing instructions (regular administration and completing courses of treatment) as inadequate antimicrobial dosing will encourage the development of ‘resistance’.

Question 4

How long should antimicrobial therapy continue?

Answer 4

Therapy should be continued until cure, which for most acute infections treatments last for 5-10 days (there are many exceptions to this)

Question 5

What reasons can microbes be resistant to antimicrobial therapy?

Answer 5

• Innate - a microbial species has never been susceptible to a particular drug.
• Acquired - a microbial species that was once susceptible becomes resistant

Question 6

What ways can a microbe develop acquired resistance?

Answer 6

• Selection - within a population some microbes may be resistant. Once a drug has eliminated the sensitive ones,
the resistant microbes can proliferate and become dominant. Spontaneous mutations may result in such resistant microbes being present in a population.

• Transmission - genes coding for resistance can be carried by plasmids (small circular strands of extrachromosomal DNA), which can be transferred between microbes.

Question 7

How can plasmids be transferred?

Answer 7

• Conjugation - where bacteria join up to each other using pili (hollow tubes), through which plasmids can now pass.
• Transduction - where plasmids are encapsulated in bacteriophages (bacterial viruses), which then go and infect other bacteria, which now become resistant

Question 8

Give examples of the different mechanisms of resistance

Answer 8

• Producing inactivating enzymes that destroys drugs.
• Developing ways of reducing drug accumulation.
• Altering drug binding sites, so that there is no affinity for
the drug.
• Developing alternative metabolic pathways that are not
sensitive to drugs.

Question 9

By what mechanisms can antibacterial drugs kill bacteria?

Answer 9

Affecting bacterial genetic material
Affecting the bacterial cell wall
Affecting bacterial peptide synthesis

Question 10

What types of antibacterial drugs affect the bacterial genetic material?

Answer 10

Sulphonamides
Quinolones
5-Nitroimidazoles
Rifamycins

Question 11

What types of antibacterial drugs affect the bacterial cell wall?

Answer 11

b-Lactams

Glycopeptides

Question 12

What types of antimicrobial drug affect bacterial peptide synthesis?

Answer 12

Tetracyclins
Aminoglycosides
Macrolides
Chloramphenicol

Question 13

What is the mechanism of action for sulphonamides?

Answer 13

Bacteria normally produce folate from p-aminobenzoic acid (PABA):

The sulphonamides (sulfamethoxazole, sulfadiazine) have a similar structure to PABA and competitively inhibit dihydropteroate synthetase.

Therefore folate synthesis declines, which adversely affects DNA synthesis and cell division.

The host’s defence mechanisms now have a chance to
cope and kill off the bacteria. So, these drugs will not be
very effective in immunocompromised patients (long term
glucocorticoid therapy, immunosuppressants, AIDS).