Caries Management - Science and Clinical Practice. Группа авторов

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      Classifying Oral Microorganisms

      The Dutchman Antonie van Leeuwenhoek was the first to discover small organisms in dental plaque by means of simple microscopy. Actually what he saw was microorganisms of differing morphology—some small and round, some quite long, some lying still, and some moving. Since then, microorganisms in the oral cavity have been examined using simple and more complex light microscopes; sometimes the microorganisms are colored, other times not so (e.g. gram+ or gram−)(Table 1.2). The microorganisms have also been examined by means of electron microscopy, cultivation on different media, and more recently by means of genetic methods. Today, more than 700 different species have been identified in oral biofilms. The composition of species varies between individuals and various tooth sites, and even within different locations of the plaque.

      Table 1.3 shows the overall biological classification hierarchy of two of the most studied microorganisms relating to caries disease, namely Streptococcus mutans and Lactobacillus acidophilus. Both are Bacteria (Kingdom) and have a gram-positive cell wall structure (Firmicutes). S. mutans is a coccus and L. acidophilus a rod (Class), and the major metabolic end product of carbohydrate fermentation is lactic acid, making them Lactobacillales (Order); they belong to the families of Streptococcaceae and Lactobacillaceae, respectively. Microbes of the genus Streptococcus (Table 1.3)—which make up the majority of the micro-organisms in the oral cavity and include the species S. mutans—are thus facultative anaerobic gram-positive cocci occurring in chains, which do not move or produce spores (Table 1.2). Microbes of the genus Lactobacillus, of which L. acidophilus is a member (Table 1.3), are mainly facultative anaerobic gram-positive rods, which do not move or produce spores (Table 1.2).

      Differentiation within the individual species can be seen, for example, by means of the growth pattern on a range of selective and nonselective agar plates (Table 1.2). Concerning the streptococci, S. mutans can be differentiated from S. sanguinis by the pattern of colony formation when cultivated on Mitis Salivarius Agar. S. mutans appears as slimy granulated colonies, while S. sanguinis appears as small, firmly adhering colonies. Biochemical tests show in addition that S. mutans metabolizes sorbitol while S. sanguinis does not.

      By the use of other techniques introduced in the 1980s such as serological and genetic testing methods (checker-board DNA–DNA hybridization, polymerase chain reaction),^36,37 it has been suggested that S. mutans can be subdivided into subgroups, such as serotypes a–h, where the original S. mutans consists of serotypes c, e, and f. Serotypes d and g are called S. sobrinus. This differentiation is important because some serotypes produce more acid from sucrose than S. mutans.³⁸