FIG. 2.

The effects of pH decreases on S. pyogenes survival time. S. pyogenes M49-CS101 was inoculated into TH broth, grown to stationary phase, and aged 1 week. Exogenous HCl or lactic acid was then added from 2 M stocks to final concentrations of 25.0, 12.5, or 6.3 mM in these aged cultures. Survival was assayed by spotting culture aliquots on TH agar plates at 24-h time intervals after addition of exogenous acid. The formation of any S. pyogenes colonies within the culture spot was scored positive for survival. Since 20 μl of culture was plated to assay for survival, the lower limit of detection for this assay was 50 CFU ml⁻¹. Bars in the graph represent the mean duration of survival for each culture condition. For this graph, T₀ is at the point of addition of exogenous acid to 1-week-old cultures. The terminal pH range, after addition of exogenous acid, is noted next to each culture condition. Each data set is the mean of results from eight independent experiments. Error bars represent the standard deviation between these experiments.