Proton motive force may regulate cell wall-associated enzymes of Bacillus subtilis

Bacterial metabolism excretes protons during normal metabolic processes. The protons may be recycled by chemiosmosis, diffuse through the wall into the medium, or bind to cell surface constituents. Calculations by Koch (J. Theor. Biol. 120:73-84, 1986) have suggested that the cell wall of gram-positive bacteria may serve as a reservoir of protons during growth and metabolism, causing the wall to have a relatively low pH. That the cell wall may possess a pH lower than the surrounding medium has now been tested in Bacillus subtilis by several independent experiments. When cultures of B. subtilis were treated with the proton conductors azide and carbonylcyanide m-chlorophenylhydrazone, the cells bound larger amounts of positively charged probes, including the chromium (Cr3+) and uranyl (UO2(2+) ions and were readily agglutinated by cationized ferritin. In contrast, the same proton conductors caused a decrease in the binding of the negatively charged probe chromate (CrO4(2-)). Finally, when levansucrase was induced in cultures by the addition of sucrose, the enzyme was inactive as it traversed the wall during the first 0.7 to 1.0 generation of growth. The composite interpretation of the foregoing observations suggests that the wall is positively charged during metabolism, thereby decreasing its ability to complex with cations while increasing its ability to bind with anions. This may be one reason why some enzymes, such as autolysins, are unable to hydrolyze their substrata until they reach the wall periphery or are in the medium.