BACTERIAL GROWTH

Summary (i) The structure of the bacterial cell is simple: the various regions of enzyme activity may be macromolecular networks of specific pattern which grow by poly‐condensation reactions. Intermediate substrates diffuse from one enzyme to another in the cell, so that the activity of the bacterium is at least in part a sequence of chemical reactions linked both temporally and spatially in definite relations. Various growth phenomena can usefully be looked at from this point of view. A fundamental scheme of enzyme synthesis is considered. (2) The various phases of the bacterial growth cycle are considered. During the lag, among other events, concentrations of diffusible intermediates are built up: these intermediates may be lost from the cell (which accounts for various observations, including an effect of inoculum size on lag). The starting‐point of the reaction series varies with the species of bacterium. When all enzymes are active, and all the intermediates are present with stationary concentration gradients, the logarithmic phase begins. The conditions determining the validity of the logarithmic growth law, the factors governing the setting in of the stationary phase, and the law expressing the decline of the viable population are discussed in the light of experimental data. (3) Cell elongation and cell division are factors separately modifiable by media or by drugs, with resulting changes in morphology. In particular the formation of cells of abnormal length is considered. ‘Life cycles’ are not accepted. (4) The laws expressing the influence of drug concentration on bacterial growth are discussed. Drugs may attack specifically different points in the reaction sequence: they may influence lag and growth rate independently. The action of proflavine on a coliform organism is considered in some detail. (5) The adaptation of bacteria to use new sources or to resist drugs is thought to depend upon modifications induced. in individual cells during growth, and not to be explicable simply by a selectionof substrains. (6) Various groups of experimental facts are described and the hypothesis put forward that certain adaptation phenomena arise from a modification of the relative proportions of different enzymes, occurring during the expansion of the cell substance. The possibility that variants arise from multiple modes of cell division is also considered: change of enzyme balance and multiple modes of division are probably interrelated effects. (7) The spontaneous and induced reversion of drug adaptation is considered. (8) The limits of adaptation and the reason for the stability of the main species characters is discussed.