**Question:** The following micro-organism constitutes the normal flora of oral cavity -

A. Streptococcus salivarius
B. Staphylococcus aureus
C. Escherichia coli
D. Bacteroides fragilis

**Correct Answer:** A. Streptococcus salivarius

**Core Concept:**
The normal flora refers to a group of microorganisms that inhabit a specific body site without causing disease. In the oral cavity, Streptococcus salivarius is one of the predominant members of the normal flora, along with other bacteria and yeast species. These microorganisms help maintain oral health by competing with pathogenic bacteria for resources, producing antimicrobial substances, and modulating the host immune response.

**Why the Correct Answer is Right:** Streptococcus salivarius is part of the normal oral flora due to its ability to adhere to oral tissues and withstand the acidic environment of the mouth. It is a facultative anaerobe that can grow under both aerobic and anaerobic conditions, making it well-adapted to the oral cavity's fluctuating microenvironment. S. salivarius produces antimicrobial substances, such as bacteriocins, which inhibit the growth of potentially pathogenic bacteria like Streptococcus pyogenes, a major cause of oral infections like gingivitis and tonsillitis.

**Why Each Wrong Option is Incorrect:**

B. Staphylococcus aureus (Staphylococcus aureus) is typically not considered part of the normal oral flora. It is known for causing skin and respiratory tract infections but can occasionally colonize the oral cavity, particularly in individuals with weakened immune systems.

C. Escherichia coli (Escherichia coli) is primarily a bowel bacterium, not a common inhabitant of the oral cavity. Although some strains might transiently colonize the oral cavity, they are not considered part of the normal flora.

D. Bacteroides fragilis (Bacteroides fragilis) is an anaerobic bacterium predominantly found in the gastrointestinal tract. It is not a common member of the oral microbiota.

**Clinical Pearl:**

Normal oral flora plays a crucial role in maintaining oral health and preventing oral infections. Understanding the composition of the oral microbiome can help in recognizing abnormal microbial imbalances associated with oral diseases like dental caries, periodontitis, and infectious diseases like Streptococcal infections. Oral health professionals should be familiar with the normal oral flora to provide appropriate guidance for patients and recognize potential signs of oral pathobionts and pathogens.