**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

Generation times vary for each organism E. coli Mycobacterium tuberculosis

**Bacterial growth curve** Know the phases of the bacterial growth curve what the X and Y axes represent. Be able to explain why the curve is shaped the way it is – i.e., what is happening in each of the phases and how that relates to bacterial growth. Also know the changes in production of primary and secondary metabolites at different stages in the growth curve.

*Primary and Secondary metabolites I will ask you about secondary metabolites – know what they are, when they are produced and why.

**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

Growth rates in different temperatures

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 require oxygen for growth Facultative anaerobes: can grow in aerobic or anaerobic environments Obligate anaerobes: unable to grow in the presence of oxygen

**Why can some organisms grow in the presence of oxygen?** Because they have the ability to neutralize toxic oxygen molecules (superoxide, oxygen radicals) Toxic forms of oxygen need to be neutralized by enzymes Superoxide dismutase Catalase

*Oxygen Preferences of Bacteria

Candle jars increase CO2 levels for growing capnophiles How does a candle jar create a CO2-enriched/O2-depleted environment? Is it an anaerobic environment?

Anaerobic jars eliminate the oxygen for anaerobes to grow. How does the hydrogen gas generator create an anaerobic environment?

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 A chemically defined medium is one where we know EXACTLY what the ingredients are. An undefined medium is one where we don’t know the exact chemical makeup (for instance, it may contain “protein digest”

Chemically defined media

Culture Media Complex Nutrient Agar TSA BHI

*Culture Media Selective Differential Selective and Differential

*Ways to measure bacterial growth Direct Count Plate Count/Viable Cell Count Measure Turbidity

Direct Counts

Plate counts require dilutions to obtain colonies

Plate Counts: pour plate

Plate Count-spread plate

*Turbidity gives a rough estimate