Acid tolerant streptococci in dentine caries using pH-selective agars

Objectives: To investigate the acid tolerant microflora in dentine caries, with special respect to oral streptococci cultivable on pH-selective agar media, as acid stress might be a major selective force in dentine caries. Methods: Five patients with primary dentine caries lesions (vital teeth, no symptoms) participated in the study. Caries lesions were excavated aseptically under rubber-dam using rose-burs. Dentine sampling was performed using sterile rose-burs at 3 levels: superficially, in the centre and in the cavity floor. Samples were incubated on neutral (blood agar) and pH-selective agars (Todd-Hewitt agar with citrate-phosphate buffer; pH 4.5, 5.0 and 5.5). Numbers of colony-forming units (CFUs) were determined, characterized morphologically, isolated to blood agar and then identified (Gram-reaction, cell- and colony morphology). Each colony type was frozen in skim milk (158 isolates). Isolates described as streptococci were revived, re-incubated (65 isolates) and further characterized by enzymatic- and sugar fermentation tests. Results: The same bacterial counts were recovered from superficial, center and cavity floor, respectively. Each dentine sample exhibited a unique microflora. There was no significant difference in proportions of aciduric microorganisms in different levels in the lesion. Approximately 38% (superficial), 5% (centre) and 38% (cavity floor) of the total cultivable microbiota was able to grow at pH 5.5, a pH value critical for demineralisation of dentine. The acid-tolerant streptococcal isolates included mutans streptococci, S. anginosus, S. constellatus and a group of unidentified streptococci. Lactate consuming taxa were found in one case, but only at pH 5.5. Conclusion: There was no significant difference in numbers of microorganisms on different levels in the lesion. Composition of the dentine caries microflora differs from lesion to lesion. The dentine caries microflora consortia differ from lesion to lesion. A common property of these bacteria was acid tolerance. Approved by the ethical committee Lund University. Funded by Faculty grants