The effect of the pH of the medium during growth on the enzymic activities of bacteria (Escherichia coli and Micrococcus lysodeikticus) and the biological significance of the changes produced

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Abstract
E. coli can grow in casein digest adjusted to any pH between 4.5 and 9. Change in the external pH during growth is followed by an alteration in the enzyme content of the cells. The enzymes can be divided into 2 groups according to the type of their variation with growth pH: Group I[long dash]enzymes whose formation undergoes a variation so that their activity per cell is constant whatever the medium pH; Group II[long dash]enzymes whose formation is greatest when the growth pH approaches their optimum activity pH. The Group 1 enzymes appear to have a protective function in the cell in that they remove inhibitory substances, e.g., urease, catalase, formic dehydrogenase, etc.; the Group II enzymes include amino-acid decarboxylases and deaminases so that the former are produced in response to an acid growth environment and the latter to an alkaline growth pH. In this manner they act as neutralization mechanisms. Variations in enzyme content with pH do not occur in the absence of growth. Similar variations in the enzymes of M. lysodeikticus are not affected by lysis of the cells with lysozyme and consequently represent variations in actual enzyme content of the cells. The variations take place such that (1) an attempt is made to counter the external change and (2) certain essential activities are maintained at a constant level.