Question

using the table below, determine which antibiotic would be a poor choice to treat an infection caused by staphylococcus aureus.

drug	zone size (mm) required for susceptibility	zone size (mm) required for resistance	example results (mm) for staphylococcus aureus
chloramphenicol	>19	<12	20
erythromycin	>18	<13	15
gentamycin	>13	<12	15
kanamycin	>18	<13	20
neomycin	>17	<12	12
penicillin g	>29	<20	19
polymyxin b	>12	<8	10
oxacillin	>15	<11	11
vancomycin	>12	<9	15
tetracycline	>19	<13	25

Explanation:

Step1: Analyze each antibiotic

For each antibiotic, compare the example result (zone size) with the resistance and susceptibility thresholds.
- **Bacitracin**: Susceptibility >13, Resistance <8. Example result =15. 15 >13 → Susceptible.
- **Chloramphenicol**: Susceptibility >19, Resistance <12. Example result =20. 20 >19 → Susceptible.
- **Erythromycin**: Susceptibility >18, Resistance <13. Example result =15. 15 <18 and 15 >13 → Intermediate (not resistant, but let's check others).
- **Gentamycin**: Susceptibility >13, Resistance <12. Example result =15. 15 >13 → Susceptible.
- **Kanamycin**: Susceptibility >18, Resistance <13. Example result =20. 20 >18 → Susceptible.
- **Neomycin**: Susceptibility >17, Resistance <12. Example result =12. 12 <17 and 12 ≥12 (not <12) → Intermediate? Wait, resistance is <12, so 12 is not resistant. Wait, maybe I misread. Wait, let's check **Penicillin G**: Susceptibility >29, Resistance <20. Example result =19. 19 <29 and 19 <20? No, 19 <20? 19 is less than 20? Yes, 19 <20. Wait, Penicillin G: Susceptibility zone >29, Resistance zone <20. Example result is 19. So 19 <29 (not susceptible) and 19 <20 (resistant? Wait, 19 <20, so it's resistant. Wait, no: Resistance is when zone size < resistance threshold. So for Penicillin G, resistance is <20. So if example result is 19, which is <20, so it's resistant. Wait, let's re - check all:

Wait, the table columns:

1. Drug: Bacitracin, Chloramphenicol, Erythromycin, Gentamycin, Kanamycin, Neomycin, Penicillin G, Polymyxin B, Oxytetracyclin, Vancomycin, Tetracycline

2. Zone size (mm) Required for susceptibility: >13, >19, >18, >13, >18, >17, >29, >12, >15, >12, >19

3. Zone size (mm) Required for resistance: <8, <12, <13, <12, <13, <12, <20, <8, <11, <9, <13

4. Example Results (mm) for Staphylococcus aureus: 15, 20, 15, 15, 20, 12, 19, 10, 11, 15, 25

Now let's go one by one:

- **Bacitracin**: Susceptibility >13, Resistance <8. Example =15. 15 >13 → Susceptible (not poor choice).
- **Chloramphenicol**: Susceptibility >19, Resistance <12. Example =20. 20 >19 → Susceptible (not poor choice).
- **Erythromycin**: Susceptibility >18, Resistance <13. Example =15. 15 <18 and 15 >13 → Intermediate (not resistant).
- **Gentamycin**: Susceptibility >13, Resistance <12. Example =15. 15 >13 → Susceptible (not poor choice).
- **Kanamycin**: Susceptibility >18, Resistance <13. Example =20. 20 >18 → Susceptible (not poor choice).
- **Neomycin**: Susceptibility >17, Resistance <12. Example =12. 12 <17 and 12 is not <12 (since resistance is <12), so it's intermediate (not resistant).
- **Penicillin G**: Susceptibility >29, Resistance <20. Example =19. 19 <29 (not susceptible) and 19 <20 (resistant, because resistance is when zone size < resistance threshold). So Penicillin G has a zone size of 19, which is less than the susceptibility threshold (29) and less than the resistance threshold (20), so it's resistant.
- **Polymyxin B**: Susceptibility >12, Resistance <8. Example =10. 10 >12? No, 10 <12. 10 <8? No, 10 >8. So intermediate? Wait, 10 <12 (susceptibility threshold) and 10 >8 (resistance threshold), so intermediate.
- **Oxytetracyclin**: Susceptibility >15, Resistance <11. Example =11. 11 <15 and 11 ≥11 (not <11), so intermediate.
- **Vancomycin**: Susceptibility >12, Resistance <9. Example =15. 15 >12 → Susceptible.
- **Tetracycline**: Susceptibility >19, Resistance <13. Example =25. 25 >19 → Susceptible.

So Penicillin G: example result 19. Susceptibility needs >29, so 19 is not susceptible. Resistance is <20, so 19 is <20, so it's resistant. So Penicillin G would be a poor choice because it's resistant (zone size indicates resistance). Wait, but let's check Polymyxin B: example result 10. Susceptibility >12, so 10 <12 (not susceptible). Resistance <8, 10 >8, so intermediate. But Penicillin G: 19 <29 (not susceptible) and 19 <20 (resistant), so it's resistant. So Penicillin G is resistant, so it's a poor choice.

Wait, maybe I made a mistake with Penicillin G. Let's confirm:

For an antibiotic to be effective, the zone size should be ≥ susceptibility threshold (susceptible) or at least not resistant (resistance is when zone size < resistance threshold). So if zone size is < susceptibility threshold and < resistance threshold → resistant. If zone size is < susceptibility threshold but ≥ resistance threshold → intermediate.

For Penicillin G:

Susceptibility threshold: >29 (so zone size ≥30 is susceptible)

Resistance threshold: <20 (so zone size ≤19 is resistant)

Example result: 19. So 19 <29 (not susceptible) and 19 ≤19 (which is <20? 19 <20, yes). So it's resistant. So Penicillin G is resistant, so it's a poor choice.

Another one: Polymyxin B:

Susceptibility: >12, Resistance: <8. Example result 10. 10 <12 (not susceptible), 10 >8 (not resistant), so intermediate. Not as poor as resistant.

Penicillin G is resistant, so it's a poor choice.

Step2: Confirm the poor - choice antibiotic

Among all the antibiotics, Penicillin G has an example result (19) that is less than the susceptibility threshold (>29) and less than the resistance threshold (<20), indicating resistance. So Penicillin G would be a poor choice to treat the infection caused by Staphylococcus aureus.

Answer:

Penicillin G