Common oral diseases such as periodontitis and dental caries can be prevented effectively by passive immunization. The present invention provides human single chain Fv (scFv) and diabody antibody fragments based on the binding characteristics of the murine monoclonal antibody Guy's 13. Like the parent antibody, these derivatives bind specifically to SAI/II, the surface adhesin of Streptococcus and the human diabody derivative is capable of aggregating streptococcal cells, making it a useful candidate therapeutic agent for passive immunization against oral diseases.