The L-form NC7, derived from K12, grew in a complex medium containing 0·2 M-CaCl as osmotic stabilizer, but not at pH values above 7·8. The cessation of growth at alkaline pH was not due to cell death. In complex media containing K or Na, the L-form grew over a wide pH range. Growth at alkaline pH was inhibited by 1 mM-amiloride, indicating that Na/H antiport activity was required for growth at alkaline pH. The internal pH (pH) of the L-form in media containing K, Na or Ca was constant at about 7·8 to 8·0 at external pH (pH) values of 7·2 and 8·2. The rates of O consumption by intact cells, lactate oxidation by membrane vesicles from cells grown in Ca-containing medium, and cell division were all strongly repressed under alkaline conditions.


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