5. microbial growth2-st
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5. Microbial Growth2-STTRANSCRIPT
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Microbial Growth
Chapter 6
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Objectives• Describe some techniques
used to grow microorganisms• Explain the requirements for
the growth of microorganisms• Explain the different phases of
bacteria growth
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Growth of Microbes• Bacteria growth is an increase in number of
cells, not cell size– One cell becomes colony of millions of cells
• Bacteria divide by – Binary fission– Alternative means
•Budding (yeast)•Conidiospores (filamentous bacteria,
actinomycetes)•Fragmentation
• Control of growth important in control of infection
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Figure 6.12a
Binary Fission
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Generation Time• Time required for cell to divide, for
population to double• Average for bacteria is 1-3 hours• E. coli generation time = 20 min
–20 generations (7 hours), 1 cell becomes 1 million cells!
–Exponential growth
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Figure 6.12b
Cell Division and Growth Curve
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Phases of Growth• Lag phase:
– making new enzymes in response to new medium
• Log phase (exponential growth):– Most sensitive to drugs
and radiation during this period
• Stationary phase:– nutrients becoming
limiting or waste products becoming toxic
– death rate = division rate
• Death phase:– death exceeds division
Standard Growth Curve
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Direct Measurement of Microbial Growth
• Several methods of measuring cell growth– Direct
• Plate count• Filtration• Microscopic count• Most probable number
– Indirect• Turbidity, Metabolic
activity, Dry weight
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Figure 6.16
Serial Dilutions• Reducing concentration to manageable levels• After incubation, count colonies on plates with 25–250 colonies
(CFUs)• Measures viable cells; require 24 h or longer; assume that each
bacterium produces a single colony
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Counting Bacteria by Filtration
• Water sample are passed through a thin filter whose pores are 0.22 m in size
• Used to detect coliform bacteria (indicators of fecal contamination) in drinking water
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Most Probable Number (MPN)• M/os being counted usually grown in liquid media
– Usually liquid differential medium is used
• You are making a statement that there is a 95% chance the bacterial population falls within a certain range
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Direct Microscopic Count of Bacteria
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Estimating Bacterial #s by Turbidity
•A practical way of measuring bacterial growth in a spectrophotometer
•A beam of light is transmitted through a bacterial suspension to a light-sensitive detector
•As bacterial #s increase, less light will reach the detector
•The turbidity is expressed as absorbance or optical density in a scale of 0 to 2
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Ex.• A piece of hamburger meat is left at room
temperature for 2 hrs. If the generation time for pathogenic E. coli (O157:H7) is 20 min, how many bacteria would be present at the end of the 2 and 7 hrs if you only had 2 bacterium at the beginning?
• A)128• B)4,194,304