Why exponential phase?
• Cells in optimum growth state, divide repeatedly by binary fission at maximal rate. • Note: useful to calculate doubling time; can vary from 20 min to several days • Why stationary phase? • Can be due to exhaustion of some critical nutrient, or to accumulation of waste products that slow down growth (e.g. acid buildup from fermentation). • Why death phase? • Continued accumulation of wastes, exposure to oxygen, loss of cell’s ability to detoxify toxins, etc. Note that death is exponential; 90% of cells die in certain time, another 90% in same time period, etc. • Note: in practice, try to prolong stationary phase, reduce death phase. Don’t store cultures at room temperature on plates (ready exposure to oxygen, dessication, high temperature speeds oxidation reactions). Better, transfer to slants (tubes), store capped in refrigerator once grown. Still better, transfer to stab tube (soft nutrient agar), stopper and seal with airtight seal. Can recover viable culture even after a year of
