Purpose: Dentin caries constitute a complex ecosystem with a diverse microbiota adapted to fluctuations in nutrient concentration and acidity. However, knowledge about the acid tolerant microbiota at different levels in the lesion is rather poor. Hence, the purpose of this study was to investigate the acid tolerant microflora at different levels in established dentin caries lesions using solid pH-selective media.
Materials and methods: Primary dentin caries lesions were sampled with a bur at three levels (superficial, centre and the clinically caries-free floor of the lesion) in 10 patients. Samples were incubated on pH-neutral and pH-selective (pH 4.0, 4.5, 5.0, 5.5) agars. Numbers of colony-forming units (cfu) were determined and colonies were subsequently characterised morphologically and isolated.
Results: The total number of bacteria in the carious lesions, recovered using blood agar (BA), ranged from 5.88 x 103 to 5.85 x 105 (median 2.64 x 105; range 5.80 x 105) and numbers of recovered bacteria decreased with decreasing agar pH. Fewer bacteria were found in the clinically caries-free dentin (P = 0.042), but the mean number of cfu (BA) was still 5.88 x 103 in those samples.
Conclusion: Each of the 10 investigated dentin caries lesions harboured a unique microbial flora, indicating that various combinations of aciduric bacteria can colonise, survive in and probably propagate dentin caries. Solid pH-selective agars can be used successfully to select acid-tolerant microorganisms in dentin caries lesions. This could be used to describe this subset of the total microbiome from a phenotypic point of view, an objective that cannot be accomplished using molecular methods.