Text II. Microbiological aspects of caries prevention

The history of oral microbiology can be traced right back to the origins of microbiology as a science. Antonie van Leeuwenhoek, a seventeenth century Dutch draper who was developing his microscopes so that he could check the quality of his textiles, examined scrapings from his teeth out of curiosity. He was fascinated by the seething activity he could observe, and took great delight in the variety of shapes, sizes, and movement of the 'little animalcules' that he saw. However, not everyone shared his enthusiasm for these tiny fellow-travellers and, indeed, many were horrified at the thought of such monsters in their mouths. From his drawings and descriptions, we now know that van Leeuwenhoek saw spheres, rods, and spirals, which we now recog­nize as streptococci, fusobacteria, and spirochaetes. Some of van Leeuwenhoek's other observations set the scene for subsequent developments in oral microbiology—for example, he found that everyone he examined harboured a similarly diverse collection of organisms in the dental plaque, but that the mixture of types was not constant. But he didn’t make any connection between what he observed and dental disease.

The fundamentals of our understanding of the pathogenesis of caries date back to the period when specific bacteria were being linked to specific diseases, and researchers such as Pasteur and Koch were developing the 'germ theory' of disease. So, the latter half of the nineteenth century saw rapid advances in the quest to identify the causative organisms of infectious diseases. Over a few decades, the bacteria responsible for diseases such as anthrax, plague, dysentery, cholera, and tuberculosis were isolated and cultured in the labora­tory. It was naturally, therefore, that interest was aroused in deter­mining the specific aetiology of other diseases, including dental caries. At the International Medical Congress held in London in 1881 two London dentists, Underwood and Milles, presented a paper describing the microscopic observa­tion of 'germs' in decaying teeth, and the experiments in which they incubated extracted teeth in test tubes and looked for damage to the enamel. They found that enamel dissolution occurred only if there was both a source of carbohydrate and live germs. Their concept of the three-way interaction between bacteria, carbohydrate, and teeth was developed and firmly established by W. D. Miller, an American working in the same institute as Koch in Vienna. Miller proposed his 'chemicoparasitic theory' of caries in a book published in 1890, identifying the essential conjunction of bacte­ria and fermentable carbohydrate to generate the acid that resulted in the demineralization of enamel. Miller's book was an important step in the description and classification of the variety of bacteria found in dental plaque, and helped establish two of the main themes that have dominated research in caries microbiology—what bacteria are involved and how they use carbohydrate.

Now it is estimated that some 500 different types of bacteria can be isolated from dental plaque. These comprise our normal oral flora, for the most part living in harmony as commensals. The vast majority of these are not directly implicated as causative agents of caries. They may influence the properties of plaque and the conditions conducive to caries.

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