Lecture 7
Download as PPTX, PDF1 like211 views
- Bacterial growth involves an increase in cell number through cell division rather than an increase in cell size. Growth requires both physical factors like temperature, pH, and osmotic pressure as well as chemical nutrients including carbon, nitrogen, phosphorus, trace elements, and vitamins. - Bacteria are cultured in growth media that provide necessary nutrients. Growth is studied using growth curves that track changes in cell number over time through lag, exponential, stationary, and death phases as nutrients are consumed in a closed system. Exponential phase shows maximum growth rate until stationary phase when growth ceases due to nutrient depletion or waste accumulation.
Lecture 7
- 1. Growth & Culture of Bacteria
- 2. • Microbial growth = increase in number of cells, not cell size
- 3. The Requirements for Growth • Physical requirements • Temperature • pH • Osmotic pressure • Chemical requirements • Carbon • Nitrogen, sulfur, and phosphorous • Trace elements • Oxygen • Organic growth factor
- 4. The Requirements for Growth: Physical Requirements • Temperature • Minimum growth temperature • Optimum growth temperature • Maximum growth temperature
- 6. The Requirements for Growth: Physical Requirements • pH • Most bacteria grow between pH 6.5 and 7.5
- 7. The Requirements for Growth: Physical Requirements • Osmotic Pressure – Hypertonic environments, increase salt or sugar, cause plasmolysis – Extreme or obligate halophiles require high osmotic pressure – Facultative halophiles tolerate high osmotic pressure
- 8. The Requirements for Growth: Physical Requirements Figure 6.4
- 9. The Requirements for Growth: Chemical Requirements • Carbon • Structural organic molecules, energy source • Nitrogen • In amino acids, proteins • Sulfur • In amino acids, thiamine, biotin • Phosphorus • In DNA, RNA, ATP, and membranes
- 10. • Trace Elements • Inorganic elements required in small amounts • Usually as enzyme cofactors
- 11. The Requirements for Growth: Chemical Requirements • Organic Growth Factors • Organic compounds obtained from the environment • Vitamins, amino acids, purines, pyrimidines
- 12. Culture Media • Culture Medium: Nutrients prepared for microbial growth • Sterile: No living microbes • Inoculum: Introduction of microbes into medium • Culture: Microbes growing in/on culture medium
- 13. Agar • Complex polysaccharide • Used as solidifying agent for culture media in Petri plates • Generally not metabolized by microbes • Liquefies at 100°C • Solidifies ~40°C
- 14. Culture Media • Chemically Defined Media: Exact chemical composition is known
- 15. • A pure culture contains only one species or strain • A colony is a population of cells arising from a single cell or spore or from a group of attached cells • A colony is often called a colony-forming unit (CFU)
- 17. THE GROWTH CURVE
- 18. Phases of Growth • Lag • Adapt to nutrients • Log • Active growth • Stationary • Death = Growth rate • Death • Nutrients consumed
- 20. • Binary fission and other cell division processes bring about an increase in the number of cells in a population. Population growth is studied by analyzing the growth curve of a microbial growth.
- 21. Closed system • When microorganisms are cultivated in liquid medium, they usually are grown in a closed system—that is, they are incubated in a closed culture vessel with a single batch of medium. • Because no fresh medium is provided during incubation, nutrient concentrations decline and concentrations of wastes increase.
- 22. Lag Phase • When microorganisms are introduced into fresh culture medium, usually no immediate increase in cell number occurs, so this period is called the lag phase. • Although cell division does not take place right away and there is no net increase in mass, the cell is synthesizing new components.
- 23. Lag Phase • A lag phase prior to the start of cell division can be necessary for a variety of reasons. • The cells may be old and depleted of ATP, essential cofactors, and ribosomes; these must be synthesized before growth can begin. • The medium may be different from the one the micro organism was growing in previously.
- 24. Lag Phase • Here new enzymes would be needed to use different nutrients. • Possibly the microorganisms have been injured and require time to recover. • Whatever the causes, eventually the cells retool, replicate their DNA, begin to increase in mass, and finally divide.
- 25. Lag Phase • The lag phase varies considerably in length with the condition of the microorganisms and the nature of the medium. This phase may be quite long if the inoculum is from an old culture or one that has been refrigerated. • Inoculation of a culture into a chemically different medium also results in a longer lag phase. • On the other hand, when a young, vigorously growing exponential phase culture is transferred to fresh medium of the same composition, the lag phase will be short or absent.
- 26. Exponential Phase • During the exponential or log phase, microorganisms are growing and dividing at the maximal rate possible given their genetic potential, the nature of the medium, and the conditions under which they are growing. • Their rate of growth is constant during the exponential phase; that is, the microorganisms are dividing and doubling in number at regular intervals.
- 27. Exponential phase • The population is most uniform in terms of chemical and physiological properties during this phase; therefore exponential phase cultures are usually used in biochemical and physiological studies.
- 28. Stationary Phase • Because this is a closed system, eventually population growth ceases and the growth curve becomes horizontal (figure 6.6). This is called stationary phase.
- 29. Stationary Phase • In the stationary phase the total number of viable microorganisms remains constant. • This may result from a balance between cell division and cell death, or the population may simply cease to divide but remain metabolically active.
- 30. Stationary Phase • Microbial populations enter the stationary phase for several reasons. One obvious factor is nutrient limitation; if an essential nutrient is severely depleted, population growth will slow. • Population growth also may cease due to the accumulation of toxic waste products.
- 31. Senescence and Death • For many years, the decline in viable cells, following stationary phase, was described simply as the “death phase.” • It was assumed that detrimental environmental changes like nutrient deprivation and the build up of toxic wastes caused irreparable harm resulting in loss of viability.