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Progression of established carious lesions, especially within dentine, is driven mainly by highly acidogenic and acid-tolerant organisms such as Lactobacillus spp.; these bacteria continue to produce organic acids that deepen and extend the lesion, whereas Streptococcus mutans is more important for initiation [Harper & Loesche, 1984, PMID 6594096; Byun et al., 2004, PMID 15243071].