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
Generation times vary for each
organism
• E. coli
• Staphylococcus aureus
• Mycobacterium tuberculosis
Why is generation time important?
• Food?
• Infections
Bacterial growth curve
Primary and Secondary
metabolites
Requirements for bacterial
growth
• Environmental factors that influence
– Temperature, pH, osmotic pressure, oxygen
• Nutritional factors
– Carbon, nitrogen, sulfur, and phosphorous
– Trace elements
• metals
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
• Salt concentrations are important for the class of
bacteria termed Halophiles.
Oxygen preferences of organisms
• Obligate aerobes
• Facultative anaerobes
• Obligate anaerobes
Why can some organisms grow
in the presence of oxygen?
Why can some organisms grow
in the presence of oxygen?
• Toxic forms of oxygen need to be
neutralized by enzymes
– Superoxide dismutase
– Catalase
– 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
• 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