Antimicrobial drugs

Question 1

bactericidal

Answer 1

killing bacteria

Question 2

bacteriostatic

Answer 2

inhibiting/arresting the growth of bacteria

Question 3

anti bitotic characteristics based on target/mode of action

Answer 3

Antibiotics interfere with/inhibit essential cellular structures/processes
Antibiotics target bacteria-specific structures/processes
Such modes of action make them
Toxic to bacteria only
Innocuous to humans (little or no-side effects)

Question 4

The cell wall is a target for many antibiotics

Answer 4

The CW is responsible for cell integrity
The CW protects bacteria from toxic substances
Absence of functional CW = death (by autolysis)

Human cells do not have cell walls
The cell wall is an essential bacteria-specific organelle

Question 5

Gram positive cell walls

Answer 5

The CWs of Gram +ve bacteria have thick layers of peptidoglycan chains
Each peptidoglycan chain is build up of covalently linked sugars
In addition short peptide chains are attached to NAM = N-acetylmuramic acid
Peptidoglycan chains cross-link via peptidyl bridges

Question 6

Gram Negative cell walls

Answer 6

In contrast, the CW of Gram -ve bacteria have a thin peptidoglycan layer
This helps explain why antibiotics targeting peptidoglycan biosynthesis are not as effective against Gram -ve bacteria

Question 7

Anitbiotics that target peptidoglycan biosynthesis

Answer 7

beta lactams — Penicillins, Cephalosporins, Carbapenems

Vancomycin

Question 8

Beta-lactam antibiotics

Answer 8

are effective against growing and dividing cells

kill bacteria by autolysis (bactericidal)

resistant bacterial species produce beta-lactamase, a secreted enzyme which inactivates antibiotics by breaking down their beta-lactam ring

Question 9

How do Beta-lactam antibiotics work?

Answer 9

Inhibit the enzymes involved in the transpeptidase cross-linking reaction
Interfere with linking the individual chains together
Disrupt PG synthesis leading to autolysis

Question 10

Penicillins

Answer 10

Very effective against Gram +ve
Some allergy reactions
Aminopenicilins better tolerated
Natural 
(Penicillin G)
Aminopenicillin
(amoxicillin, ampicillin

Question 11

Cephalosporins

Answer 11

Different primary 
coverage:
1st – Gram +ve
2nd – Anaerobes
3rd – Gram –ve
4th - Pseudomonas

1st (Cefalexin; cefazolin)
2nd (Cefaclor, cefuroxime)
3rd (Cefixime, cefpodoxime)
4th (Cefepime)

Question 12

Carbapenems

Answer 12

Broad spectrum
Generally effective 
against all BUT:
MRSA and VRE
Only available IV
Meropenem
Ertapenem
Imipenem

Question 13

Vancomycin

Answer 13

An inhibitor of CW biosynthesis
different mode of action to beta-lactams
different chemical structure
Glycopeptide antibiotic
(more allergic reactions)
Effective against MRSA
Administered intravenously
However emerging cases of resistance
(VRSA; VRE)

Question 14

Antibiotics inhibiting protein synthesis

Answer 14

Protein synthesis requires rRNA-Protein complexes known as ribosomes
There is no synthesis of proteins in the absence of
functional ribosomes
Bacterial ribosomes differ from those in humans

Question 15

Examples of antibiotics which inhibit protein synthesis

Answer 15

Some bind to, and inhibit protein components of the 30S subunit
Tetracycline
Aminoglycosides (Gentamycin, Streptomycin)
Others bind to, and inhibit protein components of the 50S subunit
Macrolides (Erythromycin)
Chloramphenicol

Question 16

Antibiotics inhibiting DNA biosynthesis

Answer 16

Fluoroquinolones

Question 17

Fluoroquinolones

Answer 17

Broad-spectrum, synthetic
Inhibit bacterial enzymes (DNA gyrase) with essential roles in DNA replication
Effective against Gram –ve bacteria and intracellular pathogens (Legionella, Mycoplasma)
Higher levels of toxicity associated with them

Question 18

Antibiotics inhibiting RNA biosynthesis

Answer 18

RNA biosynthesis requires specialised enzymes known as DNA-dependent RNA polymerases
Rifampicin

Question 19

Rifampicin

Answer 19

Inhibits bacterial but not human RNA polymerases

Used predominantly for treating tuberculosis

Question 20

Nucleic Acids biosynthesis could be targeted indirectly

Answer 20

Biosynthesis of nucleotides requires folic acid
Folic acid synthesis is a target of man-made antibiotics- antimetabolites
Sulfonamides – analogues of PABA, act by substrate competition
Trimethoprim inhibits dihydrofolate reductase
Co-trimoxazole – a mixture of both (5:1), inhibits both enzymes

Question 21

Choice of antibiotic treatment depends on:

Answer 21

bacterial species
susceptibility to drug
site of infection
safety of drug
cost of therapy
patient factors

Question 22

Resistance to antibiotics comes from

Answer 22

Mutations in bacterial chromosomal genes encoding targets of common antibiotics (VGT)
Transfer between organisms of resistance genes carried by plasmids (HGT)
Biofilm formation
Biochemically resistance

Question 23

Biochemically resistance is manifested by

Answer 23

(i) decreased accumulation of the drug – increased efflux or reduced permeability of the drug
(ii) enzymatic inactivation of the drug - secretion of Beta- lactamase
(destroys beta-lactam) or chloramphenicol acetyl transferase (inactivates chloramphenicol)