Bacteria species concept

The presence of S. mutans and other cariogenic bacteria contributes towards the formation of a biofilm known as dental plaque, and their metabolism of fermentable carbohydrates in the diet leads to the formation of acids [12]. Dental caries has been described as a complex imbalance in physiologic equilibrium between tooth mineral and biofilm [13]. Biofilms imply the involvement of microbiological species [14], but the key concept included within this definition is that the bacteria involved are native to the body, not a group of specific invasive bacteria causing infection [14]. 

To study and quantify microbial diversity one has to define what a species is. This is not a trivial objective since bacteria reproduce asexually and interbreeding cannot be applied for traditional species definition. While individual species of higher organisms may differ by as little as 1 % in their respective DNA, bacterial strains differ by up to 30%. Beside the so called species problem in microbiology, it is a widely accepted concept that bacteria with 16S rRNA gene sequences with similarity more than 98.7% (checked by DNA DNA hybridisation) belong to the same species. 

The enormous diversity of microorganisms is a factor that must be kept in mind for future drug development. Only a minor proportion of bacteria and fungi have been cultured or examined for secondary metabolite production. For example, only 5% of the total number of fungal species have been described. Of those described (69,000), only 16% (11,500) have been cultured. It has been estimated that 1 gram of soil contains 1000 to 10,000 species of undiscovered prokaryotes The extensively used concept of isolation of microbial strains fixim different geographical and climatic locations around the world still gathers support. ... 

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