A mutant of wild-type Streptococcus sobrinus 6715-13 has been isolated which resists aggregation by exogenous dextran. This variant is able to form adherent plaque deposits in vitro when cultured in the presence of sucrose and has dextranase activity. In these respects it is the complement of previously described isolates which are plaque formation defective but aggregation normal. Measurements of the incorporation of glucose from glucosyl-labeled sucrose into glucan by cell-associated glucosyltransferase enzyme activity and the thermal labilities of catalytic and receptor functions, as well as the binding of labeled dextrans to the cells, provide evidence that neither dextranase nor glucosyltransferase is the receptor involved in dextran-induced aggregation. Bloctkage of such bactenal aggregation by anti-glucosyltransferase or anti-dextranase sera suggests crossreactivity between the antigenic determinants of proteins which recognize a(1-6) glucan linkages. A model is proposed, consistent with these and previous findings, in which enzymatic function precedes dextran receptor activity in emergence from the cell. It is also proposed that dextran receptor components of the multireactive glucosyltransferase enzyme(s) and dextranase(s) are spatially separate from, although functionally and antigenically related to, the receptors on the bacterial surface involved in dextran-induced aggregation. Multisurface cariogens of rodents, primates, and humans, collectively called Streptococcus mutans, comprise at least four species (6), bind exogenously supplied, preformed a(1-6) linkagerich glucan (dextran) in vitro, and, as a result, aggregate or agglutinate into cohesive, macroscopic clumps. This phenomenon, which varies with both species and glucan, was described by Gibbons and Fitzgerald (15) and studied and confirmed by others (23, 24, 39). Guggenheim and Schroeder (19) have previously reported the rapid clumping of S. mutans OMZ176 cells when MATERIALS AND METHODS Cell strains and culture conditions. Wild-type (WT) S. sobrinus 6715-13, formerly S. mutans serogenetic group lIlg (6, 20), and mutants derived from it were used for all experiments. Cell stocks were maintained at 4°C and were transferred monthly in fluid thioglycolate medium supplemented with meat extract and excess CaCO3 (12). Bacterial cultures were grown, after several passages wherein inocula were diluted at least 1:100, in the defined medium FMC (41) supplemented with glucose at 0.1% or sucrose at 5% (final concentration). The 20% glucose stock, adjusted to pH 5, was treated with invertase at 37°C for 24 h (grade X, 10 U/mI; Sigma Chemical Co.) and dialyzed, and its concentration was measured by glucose oxidase (10). The invertase did not appear to contain dextran or promote cell aggregation. The sucrose was purified of high-molecular-weight contaminants by dialysis. Medium and sugars were separately filter sterilized (HA, 0.45-rim pore, Millipore Corp.).
