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
Normalized absorbance
You may determine the precise concentration of suspension by
running knowns amounts and extrapolating data
Concentration, mg