During the exponential or log phase, cells divide at a constant rate and their numbers increase by the same percentage during each time interval. The generation time is measured during this period of active multiplication. Because bacteria are most susceptible to antibiotics and other chemicals during this time, the log phase is important medically.
During the initial phase of exponential growth, all the cells' activities are directed toward increasing cell mass. Cells produce compounds such as amino acids and nucleotides, the respective building blocks of proteins and nucleic acids. Cells are remark ably precise in their ability to regulate the synthesis of these compounds, ensuring that each is made in the appropriate relative amount for efficient assembly into macromolecules. Compounds synthesized during this period of active multiplication are called primary metabolites. A metabolite is any product of a chemical reaction in a cell and includes compounds required for growth, as well as waste materials. Some primary metabolites are commercially valuable as flavoring agents and food supplements. Understandably, industries that harvest these compounds are working to develop methods to manipulate bacteria to overproduce certain primary metabolites. ■ regulation of gene expression, p. 183 Cells' activities shift as they enter a stage called late log phase, which marks the transition to stationary phase. This change occurs in response to multiple factors that are inevitable in a closed system, such as depletion of nutrients and buildup of waste products. If the cells are able to form endospores, they initiate the process of sporulation. If they cannot, they still "hunker down" in preparation for the starvation conditions ahead. The cells become rounder in shape and more resistant to harmful chemicals and radiation. Changes in the composition of their cell wall and cytoplasmic membrane also occur. As their surrounding environment changes, cells begin synthesizing different enzymes and other proteins, which collectively give rise to a new group of metabolites, termed secondary metabolites (figure 4.18). Commercially, the most important secondary metabolites are antibiotics. These compounds are produced by many members of the genus Streptomyces, and inhibit the growth of or kill other organisms.
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