**Question:** 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 further processing of this organism is most likely to yield.

A. Staphylococcus aureus
B. Streptococcus pneumoniae
C. Streptococcus viridans
D. Haemophilus influenzae

**Correct Answer:** C. Streptococcus viridans

**Core Concept:**

The identification of colony types on a blood agar plate is based on the characteristic appearance and biochemical reactions of the bacteria. In this case, the presence of a-haemolytic colonies indicates that the organism is part of the Streptococcus group, which is further divided into various species and groups.

**Why the Correct Answer is Right:**

Streptococcus viridans is a group of Gram-positive, catalase-negative, β-hemolytic streptococci that are commonly found in the oral flora and on the skin. These organisms can cause various infections, particularly in immunocompromised individuals. The a-haemolytic colonies are consistent with the characteristics of S. viridans.

**Why Each Wrong Option is Incorrect:**

A. Staphylococcus aureus (B. Streptococcus pneumoniae, D. Haemophilus influenzae) cannot be ruled out based on colony type alone, as these organisms can also exhibit similar a-haemolytic colonies. However, considering the patient's age, S. viridans is a more likely pathogen, especially as it is part of the normal oral flora.

**Clinical Pearl:**

In elderly patients, the presence of a-haemolytic colonies on a blood agar plate should raise suspicion for an oral or skin flora-related infection, such as that caused by Streptococcus viridans. Further laboratory testing (e.g., biochemical tests, serotyping) may be needed for definitive identification and differentiation of the specific S. viridans species.