Colonization of a new smooth surface of a tooth will occur by S. mutans when its level in the oral cavity reaches:

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