The mutans streptococci comprise a group of seven species, of which Streptococcus mutans and Streptococcus sobrinus are the predominant species isolated from human saliva and dental plaque (Loesche, 1986). Experiments with gnotobiotic hamsters revealed these to be the main initiator microorganisms in dental caries disease (Fitzgerald and Keyes, 1960). Dental caries is a common infectious disease world-wide. The aetiology of the disease is multifactorial, life habits and mutans streptococcus infection being the most important factors (Johnson, 1991; Bratthall, 1997). In the disease process, the calcified tissues of the tooth are demineralized and the organic substance is broken down.
In the western world, the prevalence of caries disease has declined, but 5 - 20% of the population age groups remain at high risk (Paunio, 1993; Bolin, 1997; Watt and Sheiham, 1999). In developing countries, the rate of dental caries is rising, and because more than 80% of the world's children live in these countries, dental caries disease is considered to be a major public health problem (Cirino and Scantlebury, 1998). Dental caries disease causes many people to experience a great deal of continuous discomfort through impaired function and aesthetics as well as inconvenient treatment. Dental caries may even lead to life-threatening infections, and the costs for operative dental treatment are significant both for individuals and society. Therefore, a need exists to identify individuals at risk for the disease, and to target preventive measures and active treatment for these individuals. Because mutans streptococci are the main initiator microorganisms in dental caries disease, individuals heavily colonized by the bacteria were earlier thought to automatically be at high risk for the disease, but it became evident that on the individual level the caries risk rate could not be accurately predicted on the basis of how heavily a child or adolescent was infected (Alaluusua, 1993; van Houte, 1993; Tenovuo, 1997). Thus, research on the colonization pattern and the virulence traits of mutans streptococci is essential.
The hypothesis inspiring this study was that individual oral isolates of mutans streptococci can be qualitatively different, expressing varying degrees of virulence characteristics, and that some strains are more likely to be associated with a high probability of transmission between individuals and/or dental caries disease than are others. Previous reports have shown that the oral cavity of an individual can be colonized by one or by multiple clonal types of mutans streptococci (Bowden and Hamilton, 1998). Using a large study population and numerous intra-individual mutans streptococcal isolates, we wanted to clarify this clonal diversity. The goal of this study was to improve on the basic information needed for research and for preventive dental treatment approaches.