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

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Concentration, mg