SUMMARY Antimicrobial peptides (AMPs) are among the repertoire of host innate immune defenses. In the oral cavity, several AMPs are present in saliva and have antimicrobial activities against oral bacteria, including Streptococcus mutans, a primary etiologic agent of dental caries. In this study, we hypothesized that unique S. mutans strains as determined by DNA fingerprinting from sixty 13 year-old subjects with or without caries experience would have different susceptibilities to α-defensins-1-3 (HNP-1-3), β-defensins-2-3 (HBD-2-3) and LL-37. The salivary levels of these peptides in subjects also were measured by enzyme-linked immunosorbent assays (ELISA). We found that S. mutans strains from caries-active subjects showed greater resistance to salivary HNP-1-2, HBD-2-3 and LL-37 at varying concentrations than those from caries-free subjects. In addition, combinations of these peptides increased their antimicrobial activity against S. mutans either additively or synergistically. The salivary levels of these peptides were highly variable among subjects with no correlation to host caries experience. However, the levels of a number of these peptides in saliva appeared to be positively correlated within an individual. Our findings suggest that the relative ability of S. mutans to resist host salivary AMPs may be considered a potential virulence factor for this species such that S. mutans strains that are more resistant to these peptides may have an ecological advantage to preferentially colonize within dental plaque and increase the risk of dental caries.

Genotypic analyses of Streptococcus mutans using fingerprinting methods depend on a few genetic loci being different but do not reveal the underlying genome-wide differences between strains. We used comparative genomic hybridization (CGH) with 70-mer oligonucleotide microarrays containing open reading frames (ORFs) from S. mutans UA159 to examine the genetic diversity of 44 isolates from nine children selected from a local study population in Eastern Iowa. Unique strains (clones) within each child initially identified by AP-PCR were confirmed by CGH. There was a wide range of variation in the hybridization patterns of the 1948 ORFs among test isolates examined. Between 87 and 237 ORFs failed to give a positive signal among individual isolates. A total of 323 of the UA159 ORFs were absent from one or more of the test strains. These 323 variable genes seemed to be distributed across the entire UA 159 genome and across all the predicted functional categories. This set of very close geographically and temporally collected S. mutans isolates had a degree of gene content variations as high as a global set of strains examined in a previous publication (Waterhouse et al., 2007). Comparing the frequency of these variable genes, the majority of which have unknown function, among strains of different origins (i.e. different caries status) could help determine their relevance in S. mutans cariogenicity.

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