Multiple phase variation in haemolytic, adhesive and antigenic properties of Streptococcus gordonii

Streptococcus gordonii gave rise to β-haemolytic variants (Bhp⁺ for beta-haemolysin production) at frequencies of 10^-4-10^-3 on agar medium containing washed horse erythrocytes. Bhp⁺ variants reverted to the wild-type α-haemolytic phenotype (Bhp^-) at the same frequencies. There was a significant probability (≥ 0.1) that phase variation in Bhp and phase variation in the previously described Spp (sucrose promoted phenotype) would occur concomitantly, but there was no correlation between these phenotypes. There was evidence also of independent phase variation in adhesion to saliva-coated hydroxyapatite (Asp for adhesion to salivary pellicles), in lactose-sensitive coaggregation (Cis for coaggregation, lactose-sensitive) and in the concentrations of particular cell surface antigens (Cap for cell antigen profile) in strains that had undergone phase changes in Spp and/or Bhp. Phase variation in all these phenotypes were transitions between high and low levels of activity and each appeared to occur as an independent event. Significant associations (P < 0.0001 by contingency table analysis) between particular phenotypes such as Bhp and Asp and between Asp, Cls and Cap phenotypes, however, were apparent. The results suggest that S. gordonii cells become predisposed to phase variation and that the resulting independent phenotypic changes may give rise to phenotypically diverse streptococcal populations able to accommodate rapid and transient environmental changes in the mouth.