Previous studies have shown that invasion of bacteria into the enamel may take place at a very early phase of the caries process. The present study in originally germ‐free rats was undertaken to observe the ability ofStreptococcus mutansto invade rat fissure enamel in initial caries without cavitation. At 24 days, 42 rats were orally infected withS. mutans, while 15 rats were kept uninfected. After feeding on a cariogenic diet for 10 or 21 days the rats were killed, and the upper molars bisected, stained, photographed, and studied for carious changes. The lower molars were split and the cracked surface examined by SEM. In the enamel, subsurface spread and colonization by bacteria was found in all but one case of initial caries. Numerous colonies were seen especially along the dentino‐enamel junction. Thus,S. mutansappears capable of invading rat fissure enamel before carious collapse of the enamel surf
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