Habituation to normally lethal acidity by prior growth of Escherichia coli at a sub-lethal acid pH value

Both Col^- and ColV, I-K94⁺ strains of Escherichia coli , grown at pH 7–0, failed to grow after relatively short periods of exposure to pH 3·0 or 3·5. After growth in exposure medium initially at pH 5·0, both strains were almost unaffected by exposure to such acid pH values. Addition of catalase to nutrient agar only slightly increased plating efficiency after acid treatment and very slightly reduced the difference in survival, after acid treatment, between organisms grown from pH 5·0 and those grown from pH 7·0. Accordingly, acid resistance of organisms grown from pH 5·0 is not chiefly due to greater resistance to hydrogen peroxide already present in nutrient media.