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
Colonization of the Mouth in the Newborn
When a child is born his or her mouth is usually sterile, but will very quickly become colonized by microorganisms, particularly from the mother, but also from other sources such as milk, food, water, etc. The first micro-organisms to colonize the mouth of a newborn are termed pioneers.34 Further development or microbial succession is dependent on the conditions offered to or changed by these pioneers, for example, nutrition and local pH. Eventually, a climax community develops, which is a stable, complex microbial community of great species diversity. In the period before the first teeth appear, the microflora consists mainly of Streptococcus and in particular Streptococcus salivarius. However, plaque development does not occur on oral soft tissues in the same way as on teeth, owing to the continual shedding of the outer cells harboring the microorganisms. When the first teeth appear, a change in the microflora is noted, as types which can adhere to dental hard tissues such as S. mutans and S. sanguini become established.
Table 1.3 Biological classification of the Streptococcus mutans and Lactobacillus acidophilus
| Streptococcus mutans | Lactobacillus acidophilus | |
| Kingdom | Bacteria | Bacteria |
| Division/Phylum | Firmicutes | Firmicutes |
| Class | Bacilli (coccoid) | Bacilli (rod) |
| Order | Lactobacillales | Lactobacillales |
| Family | Streptococcaceae | Lactobacillaceae |
| Genus | Streptococcus | Lactobacillus |
| Species | Mutans, salivarius, etc. | Acidophilus, casei, etc. |
Plaque: Development and Metabolic End Products
Professional cleaning as well as tooth brushing, if done properly, removes plaque and the pellicle on the teeth leaving the enamel naked. When saliva moistens the teeth a new pellicle will start to form. During the first couple of hours after the cleaning procedure microorganisms in the saliva will adhere to the pellicle on the teeth by means of weak biological as well as electrostatic forces such as van der Waal's interaction.39 Such microorganisms are also called pioneers, as mentioned above, when a newborn child's mouth is colonized. The pioneers are mostly Streptococcus sanguinis, S. oralis and S. mitis biovar1, but genera such as Actinomyces, Haemophilus, and Neisseria are also present.40 The mechanism of this initial adherence of microorganisms to the pellicle is complex and not fully understood. However, it seems that microorganisms have a kind of recognition system in their cell membrane which fits to receptors in the pellicle.39 In addition, microorganisms can shelter in development defects and in the groove–fossa system without physicochemical interactive forces.18,19
Fig. 1.15 Acidic metabolic end products and change of pH in dental plaque before and after intake of a lump of sugar [2]. L: lactic acid; A: acetic acid; P: propionic acid.
The microorganisms need energy for their survival and replication. They can use many different methods for obtaining energy, which is influenced by the substrate available in the mouth that comes from saliva and the host's diet. The pioneers accumulated on the teeth after 3–6 hours are arranged in a monolayer. The pioneers, which primarily are aerobes or facultative anaerobes, will most likely use oxygen from the surrounding salivary film, which enters via the cell membrane, and the tricarboxylic acid cycle of Krebs (see Ref. [1] or other biochemical textbooks) to get intracellular energy. The end products leaving the cells are CO2 and water, which are not harmful to the teeth.
Through multiplication of the pioneers and arrival of newcomers, over the next few hours a rapid increase in the number of microorganisms accumulating on the teeth is seen (6–12h). Thus, the monolayer of microorganisms is replaced by multiple layers.40 As the thickness of the layers increases (at a certain stage it becomes visible, and thus, as plaque), the oxygen tension in the inner layer