Effect of changing intracellular turgor pressure on the antibacterial activity of LysMK34 against A. baumannii MK34 (a colistin-sensitive strain) and A. baumannii Greek47 (a colistin-resistant strain). Exponentially grown bacterial cells were suspended in high tonicity, corresponding to a low intracellular turgor pressure (20 mM HEPES-NaOH, 150 mM NaCl [pH 7.4]) and then treated with 500 μg/ml LysMK34 for 2 h at room temperature. The treated cells were washed twice using high-tonicity buffer and then either diluted prior to plating in high-tonicity (20 mM HEPES-NaOH, 150 mM NaCl [pH 7.4]; black bar) or low-tonicity (20 mM HEPES-NaOH [pH 7.4]; white bars) buffer, the latter condition causing an osmotic shock. The antibacterial activity was calculated as the ratio of reduction in log units [log10(N0/Ni)], where N0 is the number of LysMK34 untreated cells, and Ni is the number of cells after treatment. Each value represents the mean ± the SD of three independent replicates. Asterisks represent statistical differences compared to cells diluted in in high tonicity (20 mM HEPES-NaOH, 150 mM NaCl [pH 7.4]). *, P < 0.05; **, P < 0.01; ***, P < 0.001 (Student t test).