Fig. 2.

Survival of purA and purB mutants at pH 2.5. After the cells had been grown in EG medium at pH 7.5 until the OD₆₀₀ reached 0.3, the cells were harvested and suspended in EG medium at pH 5.5. After the cells had been grown for 4 h at pH 5.5 under anaerobic conditions, the cells were challenged at pH 2.5 for 1 and 2 h under anaerobic conditions, and the viable cells were counted. Adenine (0.1 mM) was added to EG medium at pH 7.5 for the growth of purA and purB mutants. Adenine also was added at pH 5.5 but not at pH 2.5 for bars labeled adenine. Isopropyl-β-d-thiogalactopyranoside (0.25 mM) was added when the cells containing plasmids were cultured. Symbols: open bars, W3110; gray bars, W3110 purA; black bars, W3110 purB; vertical striped bars, W3110 purA containing ppurA; horizontal striped bars, W3110 purB containing ppurB; gray dotted bars, W3110 purA with the addition of adenine; black dotted bars, W3110 purB with the addition of adenine. The average values and standard deviations obtained from three experiments using separate cultures are represented.