. Examination of Z. bailii cell populations showed that extreme resistance to sorbic acid, benzoic
acid and acetic acid was limited to a few cells within the population, numbers decreasing with concentration
ofweak acid to b1 in 1000. Re-inoculation of resistant sub-populations into weak-acid-containingmedia showed
that all cells now possessed extreme resistance. Resistant sub-populations grown in any weak-acid preservative
also showed ~100% cross-resistance to other weak-acid preservatives. Tests using 14C-acetic acid showed that
weak-acid accumulation was much lower in the resistant sub-populations. Acid accumulation is caused by
acid dissociation in the higher pH of the cytoplasm. Tests on intracellular pH (pHi) in the resistant
sub-population showed that the pHwasmuch lower, ~ pH 5.6, than in the sensitive bulk population. The hypothesis
is proposed that extreme resistance toweak-acid preservatives in Z. bailii is due to population heterogeneity,
with a small proportion of cells having a lower intracellular pH. This reduces the level of accumulation of any
weak acid in the cytoplasm, thus conferring resistance to all weak acids, but not to other inhibitors.