The number of surviving cells, N, after each test was determined by pour plate method in Petri dishes, after a correct dilution of the treated samples in distilled water. The colonies count, grown on agar slants at 32 C for 48 h (for L. delbrueckii and S. cerevisiae cells) or 37 C for 24 h (for E. coli cells), is expressed as cfu/ml (colony forming units per ml of sample). For each test, the survival fractions, S = N/N0, reported in a logarithmic scale, were evaluated.