In the image, drugs 1, 3, & 4 are streptomycin, polymyxin B, & imipenem, respectively. Drugs 2, 5, & 6 are penicillin G, bacitracin, & vancomycin, respectively. Knowing that, what can you reasonably conclude from this test?

• A. A broad-spectrum drug would be the most effective at treating this infection.
• B. Drugs that target the cell wall are the most effective against this bacterium.
• C. Drugs 3 and 4 work synergistically.
• D. The bacterium on the plate is likely a Gram-negative bacterium.

Answer: (D)

Verified step by step guidance

1. Identify the antibiotics and their targets: Drugs 1, 3, and 4 are streptomycin (an aminoglycoside targeting protein synthesis), polymyxin B (disrupts Gram-negative outer membranes), and imipenem (a carbapenem targeting cell wall synthesis). Drugs 2, 5, and 6 are penicillin G, bacitracin, and vancomycin, all of which target the bacterial cell wall, primarily effective against Gram-positive bacteria.
2. Observe the zones of inhibition around each antibiotic disc. Larger clear zones indicate greater bacterial susceptibility to that drug. In the image, drugs 1, 3, and 4 show clear zones of inhibition, while drugs 2, 5, and 6 show little to no inhibition.
3. Recall that penicillin G, bacitracin, and vancomycin are generally more effective against Gram-positive bacteria due to their mechanism of targeting the thick peptidoglycan layer, which is more accessible in Gram-positive bacteria.
4. Polymyxin B (drug 3) is known to be effective against Gram-negative bacteria by disrupting their outer membrane, which is absent in Gram-positive bacteria. The presence of a clear zone around polymyxin B suggests the bacterium is Gram-negative.
5. Conclude that since the bacterium is resistant to cell wall-targeting drugs (2, 5, 6) but susceptible to polymyxin B and other drugs effective against Gram-negative bacteria, the bacterium on the plate is likely Gram-negative.