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.

Generation Times Bacterium Medium Generation Time (minutes) Escherichia coli Glucose-salts 17 Bacillus  megaterium Sucrose-salts 25 Streptococcus lactis Milk 26 Lactose broth 48 Staphylococcus aureus Heart infusion broth 27-30 Lactobacillus acidophilus 66-87 Rhizobium japonicum Mannitol-salts-yeast extract 344-461 Mycobacterium tuberculosis Synthetic 792-932 Treponema pallidum Rabbit testes 1980

What occurs during binary fission?

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.

Typical growth curve for a bacterial population

Phases of Growth

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.

Balanced and Unbalanced Growth Balanced (exponential) growth occurs when all cellular components are synthesized at constant rates relative to one another. Unbalanced growth occurs when the rates of synthesis of some components change relative to the rates of synthesis of other components. This usually occurs when the environmental conditions or nutrient levels change.

Measurement of Bacterial Growth Cell numbers Metabolic rate

Cell numbers 1- Direct microscopic count accomplished by direct microscopic observation on specially etched slides (such as Petroff-Hausser chambers or hemocytometers) or by using electronic counters. 2- Viable count (colony forming units)

Cell mass / numbers Turbidity or optical density it is one of the optical methods for counting cells; can estimate cell numbers accurately by measuring visible turbidity. Light scattered is proportional to number of cells. Use a spectrophotometer to accurately measure absorbance, usually at wavelengths around 400-600 nm. Accurate measure of cells when concentration not too high. Easy and quick to measure (can measure a sample in less than a minute). This technique measures the total mass of organisms and does not distinguish between dead and viable organisms.

02 uptake, C02 and ATP production Metabolic rate 02 uptake, C02 and ATP production

Turbidity measurements of microbial growth Optical Density (Counting by Spectrophotometer)

End of lecture