Effect of indole derivatives on the heat-resistant CFU of P. alvei. The cells (an initial turbidity of 0.05 at 600 nm) were grown in spore forming DSM medium for 16 h. Exogenous indole derivatives (1 mM) and glucose (0.5% w/v) were added at the beginning of the culture. Tryptophan (Trp) was dissolved in water, and indole (Ind), 3-indolylacetonitrile (IAN), indole-3-carboxyaldehyde (I3C), 3-indoleacetic acid (IAA), indole-3-acetamide (I3A), tryptamine (TM), and 2-oxindole (OI) were dissolved in dimethyl sulfoxide (DMSO). DMSO (0.1% v/v) was used as a control (None). Each experiment was repeated three to four times and one standard deviation is shown. The structures of Trp, Ind, IAA, I3CA, IAN, I3A, TM, and OI are shown. The asterisk indicates statistical significance determined using a Student t test (P < 0.05).