The mechanism by which methionine relieves the growth inhibition of Escherichia coli K-12 that is caused by organic weak acid food preservatives was investigated. In the presence of 8 mM acetate the specific growth rate of E. coli Frag1 (in MacIlvaine's minimal medium pH 6 0) is reduced by 50 %.

During inhibition of cell growth by weak acids, there is substantial accumulation of the weak acid anions in the cytoplasm. This study was undertaken to determine the impact of anion accumulation on cellular pools. At pH 6, growth in the presence of 8 mM acetate led to an internal pool of greater than 240 mM acetate anion and resulted in reduced levels of glutamate in the cell, but there were no significant changes in K+ and Na+ levels. At low osmolarity, the change in the glutamate pool compensated for only a small fraction of the accumulated acetate anion. However, at high osmolarity, glutamate compensated for over half of the accumulated acetate. Recovery of the normal cytoplasmic pH after the removal of acetate was dependent on the synthesis of glutamate.

To determine the contribution of sigma B ( B ) to survival of stationary-phase Listeria monocytogenes cells following exposure to environmental stresses, we compared the viability of strain 10403S with that of an isogenic nonpolar sigB null mutant strain after exposure to heat (50°C), ethanol (16.5%), or acid (pH 2.5). Strain viabilities were also determined under the same conditions in cultures that had been previously exposed to sublethal levels of the same stresses (45°C, 5% ethanol, or pH 4.5). The ⌬sigB and wild-type strains had similar viabilities following exposure to ethanol and heat, but the ⌬sigB strain was almost 10,000-fold more susceptible to lethal acid stress than its parent strain. However, a 1-h preexposure to pH 4.5 yielded a 1,000-fold improvement in viability for the ⌬sigB strain. These results suggest the existence in L. monocytogenes of both a B -dependent mechanism and a pH-dependent mechanism for acid resistance in the stationary phase.B contributed to resistance to both oxidative stress and carbon starvation in L. monocytogenes. The ⌬sigB strain was 100-fold more sensitive to 13.8 mM cumene hydroperoxide than the wild-type strain. Following glucose depletion, the ⌬sigB strain lost viability more rapidly than the parent strain.B contributions to viability during carbon starvation and to acid resistance and oxidative stress resistance support the hypothesis that B plays a role in protecting L. monocytogenes against environmental adversities.