Question

A 30 year old patient who had a laparotomy recently developed an intraperitoneal abscess (peritonitis). The causative organism is a Gram positive coccus that is resistant to vancomycin and to bacitracin, grows well in 6.5% NaCl broth, and shows a positive optochin reaction. Which organism is the likely cause?

• A. Enterococcus faecalis
• B. Streptococcus pneumoniae
• C. Staphylococcus aureus
• D. Peptostreptococcus

Hint:

Growth in 6.5% NaCl broth plus bacitracin resistance is the classic identification pattern for Enterococcus.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The clinical picture is a post laparotomy intraperitoneal abscess, and the lab clues describe a Gram positive coccus. The task is to match those biochemical clues to one organism among the four options.

Step 2: Key Concept:
Three simple bedside tests separate the common Gram positive cocci from each other: the bacitracin disc test, growth in 6.5% NaCl broth, and vancomycin susceptibility. Group A strep (Streptococcus pyogenes) is bacitracin sensitive, while enterococci and most other streptococci are bacitracin resistant. Only enterococci reliably grow in 6.5% NaCl broth, a feature other streptococci and staphylococci do not share in the same way.

Step 3: Detailed Explanation:
Growth in 6.5% NaCl broth points straight to Enterococcus, since this high salt tolerance is a classic identifying feature of the genus and is not shared by Streptococcus pneumoniae or Peptostreptococcus.
Resistance to bacitracin fits Enterococcus as well, since bacitracin resistance is expected for this genus, unlike Streptococcus pyogenes which is bacitracin sensitive.
Resistance to vancomycin is a well known and clinically important trait of some Enterococcus strains, called VRE (vancomycin resistant enterococcus), and it is a recognised cause of hospital acquired peritonitis after abdominal surgery.
Streptococcus pneumoniae is ruled out because it does not tolerate 6.5% NaCl broth and is a common cause of pneumonia and meningitis, not typically a post laparotomy abdominal abscess.
Staphylococcus aureus is a catalase positive coccus, and while it can cause abscesses, it does not match the salt tolerant, bacitracin resistant profile described.
Peptostreptococcus is an anaerobic coccus and would not grow in the aerobic conditions implied by the biochemical work up, and it is not associated with vancomycin resistance in this way.

Step 4: Final Answer:
The vancomycin resistant, bacitracin resistant, salt tolerant profile fits Enterococcus faecalis, so that is the causative organism.