I. Microbial Growth A. Phases of Growth - A microbial lab culture typically passes through 4 distinct, sequential phases of growth that form the standard bacterial growth curve: (Not all growth phases occur in all cultures). See graph; be able to draw & label. 1. Lag Phase - In the lag phase, the number of cells doesnt increase. However, considerable metabolic activity is occurring as the cells prepare to grow. (This phase may not occur, if the cells used to inoculate a new culture are in the log phase & provided conditions are the same). 2. Log Phase (logarithmic or exponential phase) - cell numbers increase exponentially; during each generation time, the number of cells in the population increases by a factor of two). The number of microbes in an exponentially increasing population increases slowly at first, then extremely rapidly. Organisms in a tube of culture medium can maintain log growth for only a limited time, as nutrients are used up, metabolic wastes accumulate, microobes suffer from oxygen depletion. 3. Stationary Phase - The number of cells doesnt increase, but changes in cells occur: cell become smaller and synthesize components to help them survive longer periods without growing (some may even produce endospores); the signal to enter this phase may have to do with overcrowding (accumulation of metabolic byproducts, depletion of nutrients, etc.). 4. Death Phase - In this phase, cells begin to die out. Death occurs exponentially, but at a low rate. Death occurs because cell have depleted intracellular ATP reserves. Not all cells necessarily die during this phase!
Posted on: Wed, 06 Aug 2014 18:18:56 +0000