Bacterial Generation Time Microbial Growth Growth an increase

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 • 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. Mohammed laqqan

Generation Times Bacterium Escherichia coli Bacillus megaterium Streptococcus lactis Medium Glucose-salts Sucrose-salts Milk Lactose broth Mohammed laqqan Generation Time (minutes) 17 25 26 48

What occurs during binary fission? Mohammed laqqan

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. Mohammed laqqan

Typical growth curve for a bacterial population Mohammed laqqan

Phases of Growth Mohammed laqqan

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. Mohammed laqqan

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. Mohammed laqqan

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 O 2 depletion, critical population level reached. Mohammed laqqan

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. Mohammed laqqan

The End Mohammed laqqan