Saliva contains a substance that promotes growth of L. acidophilus and expresses resistance or susceptibility to caries. Resistance to caries and to oral presence of L. acidophilus may be, to a certain extent, a racial characteristic.
Studies of the capacity of saliva (to which dextrose has been added) to sustain or prevent growth of implanted L. acidophilus indicate that saliva contains a factor which activates the growth, in vitro, of this organism. The intensity of this unknown factor varies, and its presence or absence is consistent with presence or absence of caries in the corresponding persons. Time and increases in temperature do not destroy, and dialyzation does not eliminate, this unknown factor. It can be removed by adsorption into the bodies of dead L. acidophilus.
Navajo Indians living on high-protein diet, and Maya Indians living on high- carbohydrate diet, have similarly low incidences of caries. Among the Jamaica group on high-carbohydrate diet (largely sugar), incidence of caries is very high in both Negroes and Whites. There is direct correlation between kind of diet and location of caries, cavities on smooth surfaces being more common in races living on high-carbohydrate diets. The percentage of Maya Indians whose mouths are free from L. acidophilus is greater, and the average for L. acidophilus in the mouths of Maya Indians is lower, than for White people in the United States.
References: J. Den. Res. (with Morris Steggerda), 1936; Annual Report, Carnegie Institution of Washington, 1937; J. Ant. Den. Assoc., 1939.