The sputum specimen of a 70 year old male was cultured on a 5% sheep blood agar. The culture showed the presence of a-haemolytic colonies next day. The fuher processing of this organism is most likely to yield –
**Core Concept**
The question tests the ability to identify and process a bacterial specimen based on its growth characteristics on a specific medium, in this case, 5% sheep blood agar. The presence of **a-haemolytic** colonies suggests a partial breakdown of red blood cells, which is characteristic of certain bacterial species. This is a key concept in **microbiology** and **infectious diseases**.
**Why the Correct Answer is Right**
Given the **a-haemolytic** nature of the colonies, the most likely organism is *Streptococcus pneumoniae*, which is known for causing partial hemolysis on blood agar. This bacterium is a common cause of **pneumonia**, especially in the elderly, and its identification is crucial for guiding antibiotic therapy. The **a-haemolysis** is due to the production of **hydrogen peroxide**, which partially breaks down the red blood cells in the agar.
**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because it does not align with the typical presentation of *S. pneumoniae* or the expected outcome of culturing an **a-haemolytic** organism.
**Option B:** Similarly, this option does not match the expected identification of an **a-haemolytic** colony on 5% sheep blood agar.
**Option C:** This choice is also incorrect as it does not correspond with the characteristics of the organism described.
**Option D:** This option is incorrect because, although *S. pneumoniae* can cause infections, the description provided does not specifically point to this option as the correct answer without further context.
**Clinical Pearl / High-Yield Fact**
It's crucial to remember that *Streptococcus pneumoniae* is a significant pathogen, particularly in elderly patients and those with compromised immune systems, and its identification requires the observation of **a-haemolytic** colonies on blood agar. The use of **optochin** sensitivity and **bile solubility** tests can further confirm the identity of *S. pneumoniae*.
**Correct Answer:** D. Streptococcus pneumoniae