This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Bacteriology, Subject name: Microbiology
Which of the following is the predominant flora of the mouth? 
Select and explain. 
A. Streptococcus mutans, B. Lactobacillus, C. Staphylococcus epidermidis, D. E.coli
A: Streptococcus mutans
Answer: A: Streptococcus mutans 
Explanation: The eruption of teeth at 6 to 9 months of age leads to colonization by organisms such as Streptococcus mutans and S. sanguis, both a-hemolytic streptococci. An edentulous person loses a-hemolytic streptococci as normal flora. The predominant bacteria on the skin surface are S. epidermidis and Propionibacterium, an anaerobic diphtheroid.