Fig. 1.

Growth of S. pyogenes strains NZ131 and OK171. The growth of NZ131 and its small-colony variant OK171 in THY broth culture was monitored by OD₆₀₀ measurement. At each time point, the pH of the culture and the titre (c.f.u.) were determined. (a) OD₆₀₀ and pH of the cultures are shown. The lines are the OD₆₀₀ values for the cultures, and the areas filled with vertical and horizontal lines indicate the pH values (NZ131 and OK171, respectively). (b) NZ131 and OK171 titres at each time point are indicated. Even though the cultures differed in optical density, with NZ131 achieving a higher density, the c.f.u. values were essentially the same. For both experiments, the data (OD₆₀₀, pH and c.f.u.) are the average and standard error of two independent experiments for each strain (not all error bars are visible).