Isolation and characterization of Actinomyces viscosus mutants defective in binding salivary proline-rich proteins

Recent studies have provided evidence for human salivary proline-rich proteins (PRPs) serving as potential receptors in the acquired pellicle for Actinomyces viscosus type 1 fimbriae. We report here the isolation of mutants derived from A. viscosus T14V-J1 which are defective in binding to PRPs partially purified from parotid gland saliva. Mutagenesis with ethyl methanesulfonate preceded enrichment for cells nonreactive with PRPs by successive adsorptions with PRP-treated latex beads. Screening was accomplished by random selection of 250 isolated colonies from each of four enrichment cycles and reaction with PRP-treated latex beads in microtiter plates. Two mutants of independent origin were examined for adherence to hydroxyapatite treated with either PRPs, proline-rich glycoproteins, deglycosylated proline-rich glycoproteins, or whole saliva. Additional surface properties that were examined included agglutination with polyclonal antisera to type 1 and type 2 fimbriae, agglutination by a monoclonal antibody to type 1 fimbriae that inhibits adherence of the parent strain to saliva-treated hydroxyapatite, the ability to bind monoclonal antibody to the type 1 fimbrial subunit, and lactose-reversible coaggregation with Streptococcus sanguis 34. Both mutants exhibited reduced binding to hydroxyapatite treated with whole saliva or salivary protein preparations but were still capable of reaction with antiserum to type 1 and type 2 fimbriae. In addition, these mutants possessed the ability to bind monoclonal antibody to the type 1 fimbrial subunit in amounts comparable to the amount bound by the parent strain but were not agglutinated by the adherence-inhibiting monoclonal antibody. When considered with previously published data, these results suggest that an adhesive molecule is probably associated with type 1 fimbriae and allows for the interaction of A. viscosus with constituents in the salivary pellicle.