A farmer presents with pustules, which show Gram Positive Cocci on smear. Culture shows Beta hemolysis, and organisms are Catalase negative. To show that the identified organism is group A streptococci, which of the following test should be done?
**Core Concept**
The identification of Group A Streptococci (GAS) involves a combination of morphological, biochemical, and immunological characteristics. Group A Streptococci are catalase-negative, Gram-positive cocci that exhibit beta-hemolysis on blood agar. The key to distinguishing GAS from other beta-hemolytic streptococci lies in their ability to produce specific enzymes or toxins.
**Why the Correct Answer is Right**
Bacitracin sensitivity is a critical test for identifying Group A Streptococci. GAS are sensitive to bacitracin, an antibiotic that inhibits bacterial cell wall synthesis. The sensitivity of GAS to bacitracin is due to the production of a specific enzyme, N-acetylglucosaminidase, which is responsible for the breakdown of bacitracin. This unique characteristic allows for the rapid identification of GAS in clinical specimens. By applying a bacitracin disk to a blood agar plate inoculated with the suspected organism, a clear zone of inhibition around the disk indicates the presence of Group A Streptococci.
**Why Each Wrong Option is Incorrect**
**Option B:** Optochin sensitivity is used to identify Streptococcus pneumoniae, not Group A Streptococci. S. pneumoniae exhibits a characteristic zone of inhibition when exposed to optochin.
**Option C:** Novobiocin sensitivity is used to differentiate between coagulase-negative staphylococci, particularly Staphylococcus saprophyticus, which is resistant to novobiocin.
**Option D:** Bile solubility is a characteristic of Streptococcus pneumoniae, which dissolves in bile due to the presence of a specific enzyme, autolysin.
**Clinical Pearl / High-Yield Fact**
When suspecting Group A Streptococci, remember to perform a rapid streptococcal antigen test (RSAT) or a throat culture to confirm the diagnosis, as GAS infections can lead to severe complications, such as rheumatic fever and post-streptococcal glomerulonephritis.
**✓ Correct Answer: A. Bacitracin sensitivity**