M15- Antibiotics

Question 1

what is an antibiotic?

Answer 1

a drug used to treat or prevent infection caused by micro-organisms

Question 2

what is bacteriostatic?

Answer 2

inhibit growth of bacteria

Question 3

what is bactericidal?

Answer 3

kill bacteria

Question 4

How can antibiotics be narrow spectrum or broad spectrum?

Answer 4

-e.g. Penicillin narrow spectrum active against Gram-positive bacteria
• Tetracycline active against many Gram-positive & Gram-negatives

Question 5

what are antibiotics not a substitute for?

Answer 5

functional immune system

Question 6

what are features of an ideal antibiotic?

Answer 6

•  Selective toxicity/minimal toxicity to host 
•  Cidal
–  (i.e. kills bacteria) 
•  Long half life
–  (e.g. low binding to plasma proteins)
•  Appropriate tissue distribution
•  No adverse drug interactions/side effects
 •  Oral & parenteral preparations

Question 7

Name 3 antibiotic targets.

Answer 7

• cell wall (peptidoglycan synthesis)
• ribosomes (protein synthesis)
• DNA replication (nucleic acid synthesis)

Question 8

what is cell wall synthesis targeted by?

Answer 8

ß-lactam (penicillins) & Glycopeptide antibiotics

Question 9

How does penicillin work?

Answer 9

prevents cross linking of bacterial cell wall

Question 10

Name 3 beta-lactams.

Answer 10

• Penicillin V (narrow spectrum)
• Amoxicillin (entended spectrum)
• Co-amoxiclav (inhibits b-lactamase)

Question 11

What is the main B-lactams prescribed by dentists?

Answer 11

amoxycillin

Question 12

what are features of amoxycillin?

Answer 12

• spectrum extended
• absorbed through intestine
• stable in presence of stomach acid
• peak plasma concentration 60-120 minutes after taking
• wide tissue distribution
• 20% binds to plasma proteins
• plasma half life is 60 minutes

Question 13

what is not suitable for empiric prescribing?

Answer 13

co-amoxiclav

Question 14

what are the features of penicillin V?

Answer 14

• narrower spectrum
• absorbed through intestine
• sensitive to stomach acid
• peak plasma concentration 30-60 minutes aft6er taking
• 75%-90% binds to plasma proteins
• serum levels maintained for 30-60mins

Question 15

what are the advantages of B-lactam antibiotics?

Answer 15

– Well characterised
– Safe
– Spectrum (narrow to wide)
– Variety/choice

Question 16

what are the disadvantages of B-lactam antibiotics?

Answer 16

– Resistance
– Rapidly excreted
– Hypersensitivity (1-10%) – CDI (C. diff infections)

Question 17

what is the caution when using B-lactam antibiotics?

Answer 17

• Antibiotic antagonism

- Penicillin & bacteriostatic antibiotics

Question 18

what are not B-lactam antibiotics but similar mode of action ?

Answer 18

Glycopeptides

Question 19

How does glycopeptides work?

Answer 19

• Cell-wall active antibiotics,
– Binds to end of peptide side chains (D-alanine D-
alanine) & prevent its incorporation into the cell wall.
– Blocks access to substrate by PBP (transpeptidase)
– Inhibits peptidoglycan synthesis,
– Reduced cross-linking & activity of lytic enzymes
weaken the bacterial cell wall
• Bactericidal

Question 20

Name some antibiotics that target protein synthesis?

Answer 20

• tetracyclines - deoxycycline

- macrolides - erythromycin

Question 21

Name the features of gentamicin (aminoglycosides).

Answer 21

• not absorbed from gut -intravenously
• binds to ribosomes inhibiting protein synthesis: bactericidial
• active against gram negative
• excreted in urine
• toxicity -so monitored blood levels

Question 22

what is the mode of action of tetracyclines (bacteriostatic ) ?

Answer 22

Actively transported into cell. Binds to 30S subunit. Prevents attachment of tRNA to acceptor sites. Stops chain elongation

Question 23

what are the adverse effects of tetracyclines?

Answer 23

Destruction of normal intestinal flora resulting in increased secondary infections

Question 24

what is the mode of action of lincosamides (bacteriostatic)?

Answer 24

– Bind to the 50S ribosome. Blocks Peptidyl-transferase.

– Prevents peptide Bond Synthesis.

Question 25

what does the macrolides (erythromycin ) group do/

Answer 25

• excreted via the liver,
  biliary tract and into the gut. – not excreted in urine
• Lipophilic antibiotics pass through cell membranes easily.
  – They are useful for treating certain infections where intracellular bacteria “hide” from the host’s immune system
  – Erythromycin is safe in pregnancy

Question 26

Name an antibiotic that inhibits nucleic acid synthesis?

Answer 26

metronidazole

-anaerobes and protozoa

Question 27

How does metronidazole work ?

Answer 27

anaerobic conditions important beacuse its taken up into cell into inactive form - in reducing conditions becomes activated (charged molecule) -interfere and causes damage to DNA and leads to death

Question 28

what is the mode of action of Quinolones (bactericidal)?

Answer 28

Bind to the A subunit of DNA gyrase (topoisomerase) and prevent supercoiling of DNA. Indirectly inhibit DNA synthesis

Question 29

Name some side effects of antibiotics.

Answer 29

• Nausea, vomiting, diarrhoea – common – all antibiotics disrupt the gut bacterial flora
– may affect absorption of oral contraceptives
• Gentamicin
– renal & VIII nerve damage
• Metronidazole
– interacts with alcohol
• Broad spectrum antibiotics increase the risk of C. difficile infection in the elderly

Question 30

what are the 4 “C” antibiotics that we try avoid using and why?

Answer 30

These broad-spectrum antibiotics are particularly associated with an increased risk of Clostridium difficile (“C diff”) infection:
–  Cephalosporins 
–  Co-amoxiclav (MAIN)
–  Ciprofloxacin
–  Clindamycin