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The bacterium Streptococcus mutans was first implicated in causing dental caries in 1924, when an English dentist named J. Kilian Clarke found this microorganism at the scene of a cavity and declared it the culprit. Because the bacterium was easy to culture and study outside the mouth, scientists in subsequent decades were able to gather more and more evidence to support the guilty verdict: S. mutans was adept at attaching itself to hard tooth surfaces; it loved dietary sugars; and it churned out acid — in fact, it thrived in the kind of acidic milieu that carved holes in tooth enamel. By 1960, many dentists considered S. mutans the cause of dental caries (tooth decay) and by the mid-1970s, scientists were developing a caries vaccine from whole bacterial cells.