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Microbial Growth
Chapter 4
Objectives
• What are the factors that influence bacteria growth?
• Oxygen requirements
• Nutrient requirements– Carbon, Nitrogen, Sulfur, Phosphorous
• Counting microogranisms
Microbial Growth
• Growth= an increase in the number of cells, not an increase in size
• Generation=growth by binary fission
• Generation time=time it takes for a cell to divide and the population to double
Bacteria divide by binary fission
Biofilms are communities of bacteria
Examples of Biofilms
Generation times vary for each organism
• E. coli– 20 minutes (optimal conditions)
• Mycobacterium tuberculosis– 24 hours– 6 week incubation for clinical diagnosis
Bacterial growth curve
Questions:Antibiotics would have the greatest impact on which phase?Why do microbes enter the stationary phase?At which phase(s) would spores be formed?
Primary and Secondary metabolites
Metabolites are required for growth Metabolites are required for survival
Requirements for bacterial growth
• Environmental factors that influence– Temperature, pH, osmotic pressure, oxygen
• Nutritional factors– Carbon, nitrogen, sulfur, and phosphorous– Trace elements
Optimum Growth Temperatures• Psychrophiles-
– optimum -5 to 15oC
• Psychrotrophs- – range from 20-30oC
• Mesophiles- – range from 25-45oC
• Thermophiles-– range from 45-70oC
pH influences the growth
• Bacteria-prefer a pH range of 6.5-7.5
• Molds prefer a pH range of 5.0-6.0
Osmotic environment influences growth
Oxygen preferences of organisms
• Obligate aerobes
• Facultative anaerobes
• Obligate anaerobes
Why can some organisms grow in the presence of oxygen?
• Reactive Oxygen Species are byproducts of Aerobic Respiration
• They are extremely toxic to both Prokaryotic and Eukaryotic cells
• DNA Damage Oxidize Amino Acids Inactivate Enzymes Disrupt Membranes Activate Immune System
Why can some organisms grow in the presence of oxygen?
• Toxic forms of oxygen need to be neutralized by enzymes– Superoxide dismutase
• Accepts electrons to make H2O2
– Catalase• Converts to 4H2O2 to 2H2O + 2O2
– Peroxidase
Oxygen Preferences of Bacteria
Candle jars increase CO2 levels for growing capnophiles
Anaerobic jars eliminate the oxygen for anaerobes to grow.
Chemical requirements: Carbon
• What are possible sources of carbon?
• Bacteria are classified based on the source of carbon as either heterotrophs or autotrophs
Classification based on Energy and Carbon sources
Chemical Requirements: Nitrogen
• Why do bacteria need nitrogen?
Chemical requirements: Sulfur and Phosphorous
• Why do bacteria need sulfur?
• Why do bacteria need phosphorous?
Chemical requirements: trace elements
• Small amounts of minerals– Cobalt, zinc, copper, manganese, etc.
• Usually function as cofactors
Culture Media
• Chemically defined– GSA
Chemically defined media
Culture Media
• Complex– Nutrient Agar
– TSA
– BHI
Culture Media
• Selective • Differential
– Blood
• Selective and Differential– EMB
Ways to measure bacterial growth
• Direct Count
• Plate Count/Viable Cell Count
• Measure Turbidity
Direct Counts
Plate counts require dilutions
Two techniques for plate counts
Turbidity gives a rough estimate
Standard Curves
You may determine the precise concentration of suspension by running knowns amounts and extrapolating data
Nor
mal
ized
abs
orba
nce
Concentration, mg