This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Dental
Colonization of a new smooth surface of a tooth will occur by S. mutans when its  level in the oral cavity reaches: 
Select and explain. 
A. >105/ml, B. 4.5 x 104/ml, C. 3.5 x 103/ml, D. 5.5 x 102/ml
B: 4.5 x 104/ml
Answer: B: 4.5 x 104/ml 
Explanation: Levels of S.mutans greater than 105 are indicative of an acceptable cariogenic challenge because colonization does not occur until the level of S. mutans reaches 4.5×104/ml for smooth surface and 103/ml of occlusal ﬁssures. 
Several clinical studies have corroborated the ﬁnding that the percentage of S.mutans in saliva increases signiﬁcantly as a function of the DMFS score. 
There is thus substantial evidence for associating high S.mutans counts with caries activity.

Shafers textbook of Oral Pathology – page 468