L. bulgaricus was inoculated after B. subtilis was allowed to grow
for 48 h (Fig. 1B). The pH changes and growth of B. subtilis during
the fermentation period were similar to that of the two starter
cultures that were simultaneously inoculated. L. bulgaricus was
inoculated two days after the growth of B. subtilis because we expected
L. bulgaricus to grow fast in an anaerobic environment,
which was created once oxygen was depleted by the aerobic
growth of B. subtilis. Surprisingly, L. bulgaricus did not grow and
died quickly under these conditions. The pH changes and growth of
B. subtilis during the fermentation period were similar to those
when the two starter cultures were simultaneously inoculated. We
suspected that substances produced by B. subtilis might be toxic to
L. bulgaricus.