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Bacterial Generation Time. Microbial Growth. Growth: an increase in the number of cells, not an increase in size Bacterial species only maintained if population continues to grow Generation: growth by binary fission Growth rate: cell number/time or cell mass/time
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Microbial Growth • Growth:an increase in the number of cells, not an increase in size • Bacterial species only maintained if population continues to grow • Generation:growth by binary fission • Growth rate: cell number/time or cell mass/time • Generation time:time it takes for a cell to divide and the population to double; most are 1-3 hours. • Generation times vary markedly with the species of microorganism and environmental conditions; they can range from 10 minutes for a few bacteria to several days with some eucaryotic microorganisms.
Generation Times Bacterium Medium Generation Time (minutes) Escherichia coli Glucose-salts 17 Bacillus megaterium Sucrose-salts 25 Streptococcus lactis Milk 26 Streptococcus lactis Lactose broth 48 Staphylococcus aureus Heart infusion broth 27-30 Lactobacillus acidophilus Milk 66-87 Rhizobium japonicum Mannitol-salts-yeast extract 344-461 Mycobacterium tuberculosis Synthetic 792-932 Treponema pallidum Rabbit testes 1980
The Growth Cycle • The population growth is studied by analyzing the growth curve of a microbial culture. • The standard bacterial growth curve describes various stages of growth a pure culture of bacteria will go through, beginning with the addition of cells to sterile media and ending with the death of all of the cells present.
Lag phase • The period of apparent inactivity in which the cells are adapting to a new environment and preparing for reproductive growth. • Cells are usually synthesizing new components. • In practice, bacteria from one medium to another, where there are chemical differences between the two media, typically results in a lag in cell division. • This lag in division is associated with a physiological adaptation to the new environment. • Cells may increase in size during this time, but simply do not divide (by binary fission). • Lag phase varies considerably in length depending upon the condition of the microorganisms and the nature of the medium.
Log (exponential) phase • The period in which the organisms are growing at the maximal rate possible given their genetic potential, the nature of the medium, and the conditions under which they are growing. • Generation time can be easily obtained from the exponential phase of a growth curve • The population is most uniform in terms of chemical and physical properties during this period.
Stationary phase • Eventually population growth decrease, and the growth curve becomes horizontal. • rate of cell growth = rate of cell death • Cell death may result from Nutrient limitation & Toxic waste accumulation (e.g. acid buildup from fermentation); as well as O2 depletion, critical population level reached.
Death phase • Stationary phase, in a standard bacterial growth curve, is followed by a die-off of cells, called Death phase. • It is the period in which the cells are dying at an exponential rate. • Some of the reasons are: continued accumulation of wastes, loss of cell's ability to detoxify toxins, etc.
