Which organism is most commonly associated with surgical site infections (SSIs) at about 20%?

• A. Staphylococcus aureus
• B. Escherichia coli
• C. Pseudomonas aeruginosa
• D. Streptococcus pyogenes

Answer: (A)

The main idea here is that SSIs are most often caused by organisms that normally inhabit the skin and upper respiratory tract. Staphylococcus aureus is the leading culprit because it readily colonizes the skin and nares, and during surgery it can contaminate the wound. Its traits—such as strong adherence to tissue and enzymes and toxins that help it invade and resist immune defenses—make it a frequent cause of infection after incision. In many settings, Staph aureus accounts for roughly 20% of SSIs, making it the most common single pathogen involved.

Escherichia coli is more commonly linked to infections related to the gut or urinary tract, especially after abdominal or urologic procedures. Pseudomonas aeruginosa tends to appear in specific contexts like burns, moist environments, or implanted devices and is not the top cause of SSIs overall. Streptococcus pyogenes can cause wound infections but is less frequently the leading pathogen in SSIs.