Which organism is NOT susceptible to clindamycin?

Correct answer: D. Pseudomonas aeruginosa

• A. Viridans streptococci are gram-positive cocci and are well covered — this is why clindamycin was historically used for penicillin-allergic endocarditis prophylaxis.
• B. Bacteroides fragilis is an anaerobic gram-negative rod and falls squarely within clindamycin's spectrum, though resistance rates have been climbing.
• C. Peptostreptococcus species are anaerobic gram-positive cocci and are reliably covered.
• D. CORRECT. PSEUDOMONAS AERUGINOSA is an aerobic gram-negative rod with intrinsic resistance to clindamycin — its outer membrane is impermeable to the drug and it possesses efflux pumps. Clindamycin has essentially no gram-negative AEROBIC coverage at all.

Note: Clindamycin's spectrum in one line: gram-positive aerobes plus anaerobes, both gram-positive and gram-negative. It does NOT cover aerobic gram-negatives (Pseudomonas, Enterobacteriaceae) or enterococci. Note that the 2021 AHA update REMOVED clindamycin from endocarditis prophylaxis because of its C. difficile risk.