A Conceptual Model for the Co-existence of Streptococcus Spp. and Actinomyces Spp. in Dental Plaque

Abstract

One of the most important questions in ecology is how to explain the co-existence of the variety of physiologically related organisms in the same habitat. A model is presented for the co-existence of Streptococcus species and Actinomyces species in dental plaque. The hypothesis is that these organisms co-exist because they simultaneously utilize several carbon and energy substrates. The hypothesis follows from the observation that the growth yield of oral streptococci and actinomyces in saliva is limited by carbohydrate. Preliminary experiments were undertaken to test the hypothesis using mixed chemostat cultures and gnotobiotic rats. Competition between S. mutans K1R and A. viscosus Ut2 in mixed chemostat cultures on glucose and asparagine was hampered by the early appearance of high-glucose-affinity variants of A. viscosus. From the physiological characteristics of S. sanguis and S. milleri, it might be predicted that simultaneous utilization of carbohydrate and arginine would enable these organisms to co-exist with S. mutans in an ecosystem. To test this mechanism under natural conditions, germ-free rats were inoculated with a combination of S. mutans K1R and S. sanguis P4A7 or the combination S. mutans K1R and S. milleri B448. The rats were fed on three different diets: (1) 58% cornstarch ; (2) 48% cornstarch and 10% sucrose; and (3) 53% cornstarch and 5% arginine. The results of this experiment demonstrated that dietary arginine caused a significant decrease of the ratios K1R/P4A7 and K1R/B448 in dental plaque. Thus, arginine stimulated S. sanguis and S. milleri, whereas sucrose promoted S. mutans. It can be concluded that bacterial substrate is a major ecological factor for dental plaque. This factor can potentially be studied in mixed-chemostat cultures and in gnotobiotic rats.