<prompt> Colonization of a new smooth surface of a tooth will occur by S. mutans when its level in the oral cavity reaches:105ml4.5 x 104ml3.5 x 103ml5.5 x 102ml  <response>: 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 4.5×104ml for smooth surface and 103ml 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 