FIG. 5.
Inhibition of Stx production but not STEC proliferation at higher AA concentrations and lower pH values in vitro. The pH and concentration of AA were adjusted in the growth medium so that the conditions were the same as those in the control cecum (•) (pH 7.15; AA concentration, 28 mM) or the B. breve-colonized cecum (○) (pH 6.75; AA concentration, 56 mM). STEC was added to each medium at a final concentration of 105 CFU/ml and cultivated at 37°C for 8 h. Cultures were then divided into two groups, and either 20 μl of fresh medium or MMC at a final concentration of 1 μg/ml in 20 μl of medium was added to each group, and the tubes were incubated for an additional 8 h. (A) Viable bacterial counts were determined at the indicated periods during incubation. The straight line and the dotted line show growth without (−) and with (+) MMC, respectively. (B) Stx2 concentrations were determined after incubation for 16 h. Columns: black, control, white, B. breve. Results are expressed as the means and standard deviations of the results from triplicate cultures. ***, significant differences are shown for growth under the B. breve colonization conditions versus growth under control conditions (P < 0.001).