Intro to antibacterial agents

Question 1

difference between bacteriostatic and bactericidal?

Answer 1

Bacteriostatic: Kill microbe
Bactericidal: Inhibits growth of microbe

Question 2

difference between an antiseptic and a disinfectant?

Answer 2

Antiseptic: kill microbes topically on surface
Disinfectant: Kill microbes on inanimate objects

Question 3

The basic target sites for the action of antibacterial agents?

Answer 3

1. Cell wall synthesis: effective w GPos
2. DNA synthesis
3. RNA synthesis
4. Protein synthesis (50S + 20S ribosomes)
5. Antimetabolites: biochem. pathways

Question 4

Difference between the MIC and the MBC for an antimicrobial agent?

Answer 4

MIC: Min. Inhibitory Conc. = no growth in tube
MBC: Min. Bactericidal Conc. = no growth in tube & culture plate

Question 5

List 5 ideal qualities for an antimicrobial agent and explain the benefit of each? (7)

Answer 5

1. Non-toxic to eukaryotic cells: selective toxicity
2. Broad spectrum: targets range of microbes
3. Bactericidal: Kills microbes
4. No acquired or intrinsic resistance: antimicrobial effective & not gain genetic material
5. Slow clearance time: not degraded quickly
6. Effective @ low dose: less harmful to person
7. Min. impact to host’s NF:

Question 6

Define drug Autonomy, antagonism, synergism

Answer 6

Autonomy: 2x anti-b have no effect on each other if tog.
Antoagonism: 2x anti-b that cancel each other if tog.
Synergism: 2x anti-b have a better effect when tog.

Question 7

What bacterial genus has been the source of many antimicrobial agents?

Answer 7

Streptomyces sp.

Question 8

Why would norfloxacin/nitrofuranstoin NOT be prescribed for septicaemia or meningitis?

Answer 8

bc the kidney rapidly excretes it (can’t get to the site of infection) = only suitable for treating UTI

Question 9

Why should nitrofurans not be prescribed with quinolones?

Answer 9

bc antagonistic to each other i.e. cancel the effect of each other

Question 10

Adv & disadv. of nitrofurans

Answer 10

Adv: bactericidal
Disadv: antogonistic w/ quinilones; Only oral use; Only use for UTI

Question 11

Why is tetracycline not used in children (unless it were a treatment of last resort)?

Answer 11

stain teeth of kids <8-9 yrs old during calcification

Question 12

Adv & disadv. of tetracycline

Answer 12

Adv: inhibit protein synthesis; variable resistance in GPos & GNeg (susceptible)
Disadv: bacteriostatic; oral use only; stain kid’s teeth;

Question 13

Why is chloramphenicol primarily only used as a topical agent?

Answer 13

bc broad spectrum - can cause toxixity problems

Question 14

Adv & disadv. of chloramphenicol

Answer 14

Adv: inhibit protein synthesis; suitable for oral & parenteral (IV), topical use
Disadv: bacteriostatic; broad spectrum => toxicity problems

Question 15

What is the similarities & difference b/w ampicillin and amoxicillin?

Answer 15

Both: have the same bacterial activity
Amoxillin: absorbed better by GIT

Question 16

How would aminoglycosides be administered?

Answer 16

IV only bc not absorbed by gut

Question 17

Adv & disadv. of aminoglycosides

Answer 17

Adv: bactericidal; active against staph (not MRSA), Enterobacteria, most aerobic GNB (e.g. P. aeruginosa) inhibit protein synth.
Disadv: IV only; not active against anaerobes or streptococci;

Question 18

Why are some bacterial agents suitable for oral ingestion and others are not?

Answer 18

bc it can be absorbed in GIT and remain a good concentration when reach target area.
Others are not suited as can be toxic in the body or easily excreted out by the kidneys

Question 19

Name and define the three methods of expression of enzyme induced antibacterial resistance? (Inducible, Constitutive)

Answer 19

• Inducible: enzyme only produced if organism is EXPOSED to corresponding B-lactam anti-b
• Constitutive: produced CONSTANTLY
• Constitutive-inducible: produced CONTANTLY @ LOW lvls & INC. as EXPOSED to anti-bact.

Question 20

Why don’t sulphonamides, trimethoprim or their combination affect humans but do affect bacteria?

Answer 20

Bc these antibacterials inhibit folate pathway = bact. can’t make folate ≠ make dNTP ≠ make DNA. Humans not affected bc get folate from diet

Question 21

Explain how the CDS (Calibrated dichotomous sensitivity) method of disc susceptibility are performed.

Answer 21

1. Fresh colony is picked & mixed in saline.
2. innoculum is distributed evenly on sensitised agar media with excess removed.
3. It is let to dry then anti-b disk are placed on the media & inoculated for 18 hrs.
4. The radius of the disks are measured and interpreted. if <6 = Resistant. ≥6= susceptible

Question 22

Explain how the CLSI/EUCAST method of disc susceptibility are performed.

Answer 22

1. Fresh colony of test organism is suspended to a density of ~0.5 McFarland std.
2. A lawn Innoculum is spread on MH agar plate in 3x directions w/ a swab dipped once in inoculum.
3. Incubated in air or CO2 for 18-24hrs.
4. Zone diameter measured in mm & use a table to determine result = susceptible, intermediate or resistant

Question 23

Explain how to set up and read an E-test strip so as to report an MIC for gentamicin?*

Answer 23

1. Make 0.5 Mcfarland standard w/ test organism
2. A lawn Innoculum is spread on MH agar plate in 3x directions w/ a swab dipped once in inoculum.
3. Incubated in air or CO2 for 18-24hrs.
4. Read MIC where zone intersect strip (always go to next highest value if in b/w values)

Question 24

What is _ & what is it caused by

a) Colitis
b) Thrush

Answer 24

a) Colitis: inflammation of LI bc of Clostridium deficile

b) Thrush: yeast infection caused by Candida albicans

Question 25

What is the therapeutic index?

Answer 25

Ratio of a reagent that causes toxicity to the dose that causes a therapeutic effect.
TD50 / ED50
TD50: toxic dose where 50% of pop respond
ED50: therapeutic dose where 50% of pop respond
=> Lo diff = dangerous bc easily overdose = need regular monitor in biochem

Question 26

Define these terms:

a) Antibiotic:
b) CHemotherapeutic agents
c) Semi-synthetic antibiotics

Answer 26

a) naturally occurring substances produced by micro-o to kill/inhibit other micro-o
b) Synthetically made - completely artificial
c) chemically modified from natural compoun

Question 27

Similarities & differences of Penicillin G, V (B-lactams)

Answer 27

Adv: bactericidal, high activity, low toxicity
Dis: Not oral (bc inactivated by gastric acid), narrow spectrum, inc bact. resistance
Diff: Penicillin V can be administered orally

Question 28

Similarities & differences of Amoxicillin & ampicillin

Answer 28

Sim: works against Enterobacteria
Diff: Amoxicillin better GIT absorption

Question 29

What does Clauvanic acid do?

Answer 29

inhibits PLASMID encoded B-lactamases = allows Antibiotic to work