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Dental caries is a multifactorial disease with many associated microbial taxa, but only a few are notably contributing to acidogenicity. The ribosome number and the corresponding 16S ribosomal RNA (rRNA) concentration are considered a molecular indicator for general metabolic activity of bacteria, as they are elevated with increased anabolic and catabolic activities. We hypothesize that the activity of aciduric/acidogenic bacterial taxa, reflected by a rise in ribosomal counts, could resolve differences between plaque biofilm from sound surfaces and caries lesions. The included subjects were allocated to two groups: caries-free (CF) or caries-active (CA). CF subjects presented one donor site, namely one sound surface (CFS, n = 10), whereas CA subjects presented two donor sites: a cavitated lesion with an ICDAS score of 5-6 (CAC, n = 13), and a sound reference surface (CAS, n = 13). Four aciduric/acidogenic bacterial taxa (Streptococcus mutans, lactobacilli, Bifidobacterium dentium, and Scardovia wiggsiae) and one asaccharolytic taxon (fusobacteria) as a contrast were selected. 16S rRNA and 16S rRNA genes were quantified by quantitative PCR. Based on these parameters, bacterial and ribosomal counts, as well as relative activities were calculated as the quotient of relative ribosomal abundance and relative genome abundance. Caries-associated bacteria showed the highest relative activity in caries lesions (e.g. lactobacilli CAC: 177.5 ± 46.0%) and lower activities on sound surfaces (e.g. lactobacilli CAS: 96.3 ± 31.5%), whereas asaccharolytic fusobacteria were most active on sound surfaces and less active in caries lesions (CFS: 275.7 ± 171.1%; CAS: 205.8 ± 114.3%; CAC: 51.1 ± 19.0%). Thus, the present study suggests different activity patterns for biofilms from CF and CA individuals.