Seroclassification of hitherto nontypeable Actinobacillus actinomycetemcomitans strains: evidence for a new serotype e.

Serotyping of 89 Actinobacillus actinomycetemcomitans isolates by indirect immunofluorescence with serotype-specific monoclonal antibodies yielded 10 nontypeable strains. The aim of the present study was to re-evaluate the seroclassification of these strains with a different assay. Electroblotted antigens of nontypeable isolates and selected reference strains were immunostained with 37 human sera containing high antibody titers to A. actinomycetemcomitans. Such immunoblots typically showed an immunodominant, serotype-specific antigen smear. Twelve human sera were found to recognize specifically serotype a, 10 serotype b and 9 serotype c. Three sera contained antibodies to 2 or all 3 serotype antigens; 3 sera had no activity to any reference strain smear antigen. There was complete agreement between the seroclassification of the reference strains with the monoclonal and the polyclonal reagents. Seven of the 10 strains, nontypeable with monoclonal antibodies, could also not be typed by the human sera. Of the remaining 3 strains, one belonged to a previously unrecognized serovar of serotype a, and 2 isolates expressed a novel smear antigen detected by the 3 human sera containing no antibodies to the serotype a, b, or c antigens. We propose that these 2 strains be assigned to a new A. actinomycetemcomitans serotype, designated serotype e.