Streptococcus mitis bv. 1 is a pioneer colonizer of the human oral cavity. Studies of its population dynamics within parents and their infants and within neonates have shown extensive diversity within and between subjects. We examined the genetic diversity and clonal turnover of S. mitis bv. 1 isolated from the cheeks, tongue, and primary incisors of four infants from birth to 1 year of age. In addition, we compared the clonotypes of S. mitis bv. 1 isolated from their mothers' saliva collected in parallel to determine whether the mother was the origin of the clones colonizing her infant. Of 859 isolates obtained from the infants, 568 were unique clones. Each of the surfaces examined, whether shedding or nonshedding, displayed the same degree of diversity. Among the four infants it was rare to detect the same clone colonizing more than one surface at a given visit. There was little evidence for persistence of clones, but when clones were isolated on multiple visits they were not always found on the same surface. A similar degree of clonal diversity of S. mitis bv. 1 was observed in the mothers' saliva as in their infants' mouths. Clones common to both infant and mothers' saliva were found infrequently suggesting that this is not the origin of the infants' clones. It is unclear whether mucosal immunity exerts the environmental pressure driving the genetic diversity and clonal turnover of S. mitis bv. 1, which may be mechanisms employed by this bacterium to evade immune elimination.Streptococcus mitis bv. 1, S. oralis and S. salivarius are species of viridans streptococci that are pioneer colonizers of the human oral cavity and remain numerically significant throughout life (17,23,32). However, the origin of these bacteria remains to be determined. Despite the abundance of commensal bacteria present in the birth canal, none of these are able to successfully colonize the mouth of the infant suggesting that they do not have tropism for the oropharyngeal mucosa. It has been proposed that commensal bacteria are transferred from the primary care-giver (27,29,33,42), external environment (6), and from other areas of the respiratory tract (22,23).Successful colonization depends on the ability of the bacteria to circumvent host innate and acquired immunity in order that they can adhere to oral surfaces and avoid removal via the flushing action of saliva and mastication. Neonatal saliva has been shown to contain secretory immunoglobulin A (SIgA) antibodies that react with these bacteria (9, 10) but these antibodies appear insufficient to completely block adherence and subsequent colonization. Several species of viridans streptococci including the pioneers, S. mitis bv. 1 and S. oralis, produce IgA1 protease (11, 26) which may inactivate SIgA1 antibodies in saliva. In this context, it is interesting that over 90% of SIgA in the saliva of neonates belongs to subclass 1 (16). Furthermore, viridans streptococci elaborate extracellular polysaccharide (4) and bind salivary macromolecules (40) which may mask them from host immunit...

SUMMARYObjective-The purpose of the study was to explore the physiological and antigenic diversity of a large number of S. mitis biovar 1 isolates in order to begin to determine whether these properties contribute to species persistence.Design-S. mitis biovar 1 was collected from four infants from birth to one year of age. At each of 8-9 visits 60 isolates each were obtained from the cheeks, tongue and incisors (once erupted) yielding 4,440 in total. These were tested for production of neuraminidase, β1-N-acetylglucosaminidase, β1-N-acetylgalactosaminidase, IgA1 protease, and amylase-binding. Antigenic diversity was examined by ELISA and Western immunoblotting using antisera raised against S. mitis biovar 1 NCTC 12261 T and SK145.

To define the relationship between salivary SIgA antibodies and commensal oral bacteria, we examined the reactivity of SIgA antibodies from the saliva of four infants with their own colonizing Streptococcus mitis biovar 1 (S. mitis bv 1) clones (ribotypes). Immunoblot analysis was used to examine reactivity of these antibodies with persistent ribotypes isolated from the mouths of the infants over the first year postpartum. Results showed that the pattern of SIgA antibody reactivity with the majority of clones increased in complexity after colonization but that most additional bands were common to other clones, indicating that they represented shared antigens. However, unique bands were identified in 75% of the selected persistent clones. We hypothesized that if strainspecific SIgA antibody was induced in response to colonization of a particular clone and contributed to its elimination from the mouth, then the appearance of unique bands would immediately precede the disappearance of the strain. Seventy-three percent of all unique bands identified in the study fulfilled this criterion. Because the mouth is an open, dynamic environment and multiple factors are believed to play a role in the immune response at mucosal surfaces, it may not be possible to conclusively define the relationship between SIgA antibody and commensal bacteria. However, our data provide evidence that SIgA antibody, reactive with unique antigens of their own colonizing strains, is produced in infants and may point to a role of this antibody in regulating colonization by S. mitis bv 1.

SummaryObjective-To determine which cell fraction(s) of Streptococcus mitis biovar 1 serve as the best source of antigens recognized by salivary SIgA antibodies in infants.Design-Whole cells of 38 reference and wild-type isolates of Streptococcus mitis, S. oralis, S. gordonii, Enterococcus casseliflavus, and E. faecalis were fractionated into cell walls CW), proteasetreated cell walls (PTCW), cell membranes (CM) and cell protein (CP). Whole cells and these fractions were tested for binding by rabbit anti-S. mitis SK145 and anti-S. oralis SK100 sera, and also by salivary SIgA antibodies from infants and adults.Results-Anti-SK145 and anti-SK100 sera bound whole cells and fractions of all strains of S. mitis and S. oralis variably. Cluster analysis of antibody binding data placed the strains into S. mitis, S. oralis and 'Non-S. mitis/non-S. oralis' clusters. Antigens from CW and CM best discriminated S. mitis from S. oralis. CM bound the most infant salivary SIgA antibody and PTCW bound the least. In contrast, adult salivary SIgA antibody bound all of the cell fractions and at higher levels.Conclusions-Presumably the relatively short period of immune stimulation and immunological immaturity in infants, in contrast to adults, result in low levels of salivary SIgA antibody that preferentially bind CM of S. mitis but not PTCW. By utilizing isolated cell walls and membranes as sources of antigens for proteomics it may be possible to identify antigens common to oral streptococci and dissect the fine specificity of salivary SIgA antibodies induced by oral colonization by S. mitis.

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