Intermicrobial binding plays an important role in the ecology of the oral cavity because itrepresents one mechanism by which specific bacteria colonize dental plaque. The formation of“corncobs”, a morphologically distinct microbial unit composed of Streptococcus crista andFusobacterium nucleatum, is a highly specific binding interaction that depends on the presence ofpolar tufts of fimbriae on the streptococci. We have used a genetic approach to examine the role ofstreptococcal cell surface components involved in the binding of S. crista to F. nucleatum. Suchbinding may be an important component of corncob formation. A method for the genetictransformation of S. crista was used to transfer the broad host range transposon, Tn916, into thebacteria. Cells were grown to early log phase in brain heart infusion broth containing 10% fetalcalf serum. The competent cells were mixed with purified DNA from pDL916, a plasmidconstruct consisting of Tn916 and the streptococcal/Escherichia coli shuttle vector pDL278. Over300 transformants were screened for a reduction in binding to F. nucleatum. Five of thetransformants showed a change in binding ranging from 59% to 29% of the positive controlvalues. Southern blots revealed that the binding-deficient transformants contained the Tn916element integrated into one of 4 different sites in the chromosome. The transposon, integrated into4 different sites, appeared to be stable in the absence of selective pressure. Based on thesefindings, it appears that some strains of S. crista are naturally competent and that insertionalinactivation methods can be used to facilitate the study of binding receptors in this group of oralstreptococci.