In adults, which organisms most often cause cellulitis?

• A. Group A beta-hemolytic streptococci
• B. Staphylococcus aureus
• C. Pseudomonas aeruginosa
• D. Both A and B

Answer: (D)

In adults, cellulitis is most often caused by bacteria that are common on the skin and in the upper respiratory tract—specifically group A beta-hemolytic streptococci and Staphylococcus aureus. These two organisms are the usual culprits because they readily invade through minor skin breaks and spread in the soft tissues, leading to the classic tender, red, warm, swollen infection.

Pseudomonas aeruginosa isn’t a common cause of uncomplicated cellulitis in healthy adults. It tends to appear in particular situations, such as puncture wounds (especially through footwear), burns, chronic ulcers, or certain hospital exposures, where the environment or patient factors favor its growth. That’s why the combination of the two more frequent pathogens is the best answer for the typical adult case.