Etiology of secondary caries 31 Microbiology of secondary caries

Dental caries is determined by the dynamic balance between pathological factors that lead to demineralization and protective factors that lead to remineralization [Featherstone, 2004]. As a major pathological factor, oral bacteria, especially acidogenic bacteria, can dissolve the tooth mineral. Those acidogenic bacteria are also aciduric and can live preferentially under acid conditions [Loesche, 1986].

Hitherto, it is unclear about the microbiology of secondary caries yet. Although secondary caries is described alike primary caries in histopathology, whether the etiology of secondary caries is the same as that of primary caries is a matter in dispute. Kidd et al. found no significant differences between the microflora in samples from cavity walls involving primary and secondary caries next to the amalgam [Kidd et al., 1993]. However, Thomas et al. investigated bacterial composition in relation to primary and secondary caries via an in situ model, and found a phenomenon of higher proportion of caries-associated bacteria on composite surfaces. Then they indicated that the microbiology on the surface of the primary caries differs from that on the surface of lesion around composite, and secondary caries around composite may differ from the primary lesions process [Thomas et al., 2008]. In addition, some studies focused on the ecology under the restorations. Mejare et al. found the bacterial colonization beneath composite similar to that observed in dental plaque mainly including Streptococci and Actinomyces spp. [Mejare et al., 1979]. Nevertheless, according to the experiment conducted by Splieth et al., it was the other way around [Splieth et al., 2003]. They compared the microbial spectrum under composite and amalgam fillings with special attention to the anaerobic flora. The results showed that bacterial composition under amalgam was similar to the flora of carious dentin and carious plaque, with anaerobic and facultative anaerobic gram-positive rods dominating. On the contrary, huge amounts of Bacteroides and Prevotella spp. were detected under many composite fillings, similar to the microflora of infected root canals with potentially pulpopathogenic microbes. Thus, the study suggested that the types of restorative materials seemed to have an effect on the composition of the microflora on the surface of secondary caries, and that beneath the restorations, and then the differences might exist between the microbial flora of secondary caries and primary caries. In this study, it was indicated that inadequate composite fillings might stimulate the growth of cariogenic as well as obligate anaerobic and potentially pulpopathogenic bacteria. This could be explained as follows: 1) The microspace between the restoration and the cavity floor favors the obligate anaerobic, and then leads to the detection of those bacteria; 2) It is not surprising to discover many obligate anaerobic normally colonize in human oral, even in oral of people without obvious endodontic diseases. 3) It doesn't mean that people without clinical symptoms of toothaches or pulpitis don't have chronic or arrested tooth diseases, so it is possible to detect those anaerobic bacteria. However, existence does not mean participation, so it is necessary to certify the participation of those obligate anaerobic in the progress of secondary caries in further studies.

According to the viewpoint of Marsh that any species with the ability producing acids and tolerating the cariogenic environment can contribute to the dental caries process [Marsh, 2006]. Streptococci mutans (S. mutans), Lactobacilli and Actinomyces naeslundii have been used by various models in vitro studying secondary caries for a long time. S. mutants and lactobacilli can produce a series of acid and stay in a low pH environment for a long time, leading to demineralization of teeth and caries lesion. It has been shown that the three bacteria were widely present and might play an important role in the development of secondary caries around amalgam [González-Cabezas, 1999]. However, in a recent in situ study, S. mutants were not detected in each sample, but Lactobacilli. Meanwhile, A. Odontolyticus and Candida spp. were also found in most samples [Thomas et al., 2008]. In addition, in recent years, Beighton put forward a point of view — S. mutants might be good makers of secondary caries but not necessarily the etiological agents [Beighton, 2005]. The experiment by Thomas et al. described before, in which S. mutants was not found in every sample, but Lactobacilli and A. Odontolyticus, seemed to support Beighton's view. And scientists conjectured that there might be unknown caries-associated bacteria, which can not grow on blood agar [Thomas et al., 2008]. In the past decade, the detection of A. Odontolyticus and Candida spp. has caused the serious concern to researchers. It has been found that Candida albicans can dissolve hydroxyapatite in a liquid culture at a 20-fold higher rate than S. mutants, despite the much lower growth rate [Nikawa et al., 2003]. Klinke et al. assumed that Candida albicans might make a significant contribution to caries pathogenesis in caries-active children, and it could be taken into account Candida albicans as an appraisal of caries pathogenicity [Klinke et al, 2009]. Besides, it should be noted that some people may have serious caries activities without S. mutants dominating in dental plaque. Therefore, further research need to be carried out to determine the microbiology of secondary caries, such as the role of S. mutants, A. Odontolyticus and Candida spp. in the development of secondary caries and the relationship between restorations and microorganism of secondary caries.

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