The influence of pH value on the antibacterial action of subtilin A has been demonstrated by survivor counts, inhibition of respiration, and pH gradient plates. is more sensitive to subtilin as the pH value increases; is more sensitive as the pH value decreases. The results are analogous to those obtained by other investigators working with cationic detergents, and are consistent with the hypothesis that the basic surface-active antibiotics kill bacteria by the same general mechanism as do the quaternary ammonium germicides. A survey of the growth inhibition of other organisms on pH gradient plates indicates that behaves like , while , and behave like . A pH/mobility curve for subtilin A, obtained by paper electrophoretic studies, indicates no striking changes in the charge of the molecule over the pH range 4–9, although the presence of at least one free α-amino group is indicated by inflexion points in the range of pH 6–7. The isoelectric point of subtilin A at ionic strength 0·1 is approximately 6·7. The use of sectored, square-shaped Petri dishes for pH-gradient plate studies with germicides is described.


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