A 56-year-old female on clindamycin (Cleocin®) with a penicillin allergy presents with persistent diarrhea. Clindamycin is active against each of the following bacteria EXCEPT one. Which is the EXCEPTION?

Correct answer: D. Pseudomonas aeruginosa

• A. INCORRECT (clindamycin IS active against this organism). Staphylococcus aureus (MSSA) is susceptible to clindamycin. It is commonly used for skin and soft tissue infections caused by S. aureus, including some community-acquired MRSA strains with inducible resistance testing.
• B. INCORRECT (clindamycin IS active against this organism). Streptococcus pneumoniae is generally susceptible to clindamycin. Clindamycin is used as an alternative for respiratory infections caused by this organism in penicillin-allergic patients.
• C. INCORRECT (clindamycin IS active against this organism). Prevotella intermedius is an anaerobic gram-negative oral pathogen associated with periodontitis. Clindamycin has excellent anaerobic coverage and is effective against Prevotella species.
• D. CORRECT — this is the EXCEPTION. Pseudomonas aeruginosa is an aerobic gram-negative rod that is intrinsically resistant to clindamycin. Clindamycin's coverage does not extend to aerobic gram-negative bacilli. Pseudomonas requires antipseudomonal agents such as piperacillin-tazobactam, cefepime, carbapenems, or fluoroquinolones.