Growth of E. coli at pH 5 protected the bacteria against the lytic effect of beta lactam antibiotics typically observed when the cells are grown at pH 7 or 7.5, i.e., the pH values routinely used in laboratory experiments. In contrast, the typical effects of beta lactam antibiotics on cellular shape and elongation and cell division appeared to be similar in cultures grown under neutral and acid pH conditions. The pH-dependent antibiotic tolerance can also be demonstrated with pneumococci, staphylococci, streptococci, and Bacillus subtilis. We suggest that the mechanism of the pH-dependent antibiotic tolerance may involve either the production of a more stable plasma membrane or the suppression of the activity of a murein hydrolase(s) that catalyzes the antibiotic-induced lysis; at least a fraction of these enzyme molecules may be localized at the cell surface and be accessible to experimental manipulation.

Goodell, E. W., Lopez, R., & Tomasz, A. (1976). Suppression of lytic effect of beta lactams on Escherichia coli and other bacteria. Proceedings of the National Academy of Sciences, 73(9), 3293–3297.