Background: Streptococcus mutans is among the principal etiologic agents involved in the onset and progression of dental caries. One important virulence trait of this organism is biofilm formation. This behavior is regulated in part by the HK11-RR11 two-component signal transduction system since mutants of this pathway produce defective biofilms. Objectives: The genes flanking the HK11-RR11 genetic locus in the S. mutans genome, namely SMU485 and PpiB, were identified and their contribution to biofilm maturation in this organism were explored. Methods: SMU485 (an uncharacterized gene) was co-transcribed with HK11 while PpiB (a cis-trans peptidyl-prolyl isomerase) was co-transcribed with RR11. Mutants defective in each of these two genes were constructed in S. mutans strain UA159 using PCR-ligation mutagenesis. Mutants were confirmed using PCR amplification and sequencing and their growth rates were analyzed under both planktonic and biofilm growth conditions. Results: SMU485 mutants appeared to co-aggregate and precipitate in liquid broth culture resulting in a slightly thicker biofilm relative to the parent strain. On the other hand, PpiB mutants had a thin biofilm compared to wild-type UA159 indicating that this gene has a role in biofilm formation. Conclusion: Further study of the biofilm architecture of the mutants compared to wild-type will give insight into their role in S. mutans biofilm formation.

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