Quantitative Analysis of Diverse Lactobacillus Species Present in Advanced Dental Caries

Abstract

Our previous analysis of 65 advanced dental caries lesions by traditional culture techniques indicated that lactobacilli were numerous in the advancing front of the progressive lesion. Production of organic acids by lactobacilli is considered to be important in causing decalcification of the dentinal matrix. The present study was undertaken to define more precisely the diversity of lactobacilli found in this environment and to quantify the major species and phylotypes relative to total load of lactobacilli by real-time PCR. Pooled DNA was amplified by PCR with Lactobacillus genus-specific primers for subsequent cloning, sequencing, and phylogenetic analysis. Based on 16S ribosomal DNA sequence comparisons, 18 different phylotypes of lactobacilli were detected, including strong representation of both novel and gastrointestinal phylotypes. Specific PCR primers were designed for nine prominent species, including Lactobacillus gasseri, L. ultunensis, L. salivarius, L. rhamnosus, L. casei, L. crispatus, L. delbrueckii, L. fermentum, and L. gallinarum. More than three different species were identified as being present in most of the dentine samples, confirming the widespread distribution and numerical importance of various Lactobacillus spp. in carious dentine. Quantification by real-time PCR revealed various proportions of the nine species colonizing carious dentine, with higher mean loads of L. gasseri and L. ultunensis than of the other prevalent species. The findings provide a basis for further characterization of the pathogenicity of Lactobacillus spp. in the context of extension of the carious lesion.Dental caries continues to be a significant public health problem in many parts of the world. Although the bacteria responsible for caries initiation and early caries progression have been studied extensively, the microbiology of dentine caries has been reported to show considerable diversity and has not yet been fully characterized. Dissolution by acid of the surface enamel exposes the underlying avascular mineralized connective tissue matrix of dentine, which is prone to invasion. This occurs by migration of bacteria into the network of tubules occupied by processes of the pulpal odontoblasts. The early stage of invasion involves lactobacilli, Actinomyces spp., veillonellae, and mutans streptococci (for a review, see reference 19). This phase is followed by the invasion of a more diverse group of microorganisms including gram-negative anaerobes. There is evidence that interspecies cooperation enhances the migration of the mixed bacterial flora through the dentinal tubules (20,27).Lactobacilli have been reported to occur in high numbers in both superficial and deep caries (9), though they are not suspected of being involved in bacterial invasion of nonexposed dental pulp (12). Our previous analysis of lactobacilli by culture under microaerophilic conditions in 65 deep caries samples indicated that Lactobacillus acidophilus was numerically dominant, although Lactobacillus paracasei, Lactobacillus rhamnosus, and Lact...

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