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Chapter 5 Microbial Growth and Regulation (Growth cycle (Lag (Observed:…
Chapter 5
Microbial Growth and Regulation
Growth cycle
Lag
Time between inoculation and growth
Based on time to take up nutrients and perform biosynthesis
Observed:
When old cells are used to perform inoculation
When transferring from nutrient rich to nutrient poor medium
Exponential
Phase where cell population doubles at regular intervals
Stationary
Limits in available nutrients
Accumulation of waste products
No net increase or decrease of cells; growth rate = 0
Cell growth rate = cell death rate
Death
Occurs exponentially
Culture Types
Batch culture
Closed system
Organism growing in an enclosed vessel
Continuous culture
Open system
Process
Known volume of sterile medium is added at a constant rate
Equal volume of spent culture medium is removed at same rate
Equilibrium achieved when rates remain constant
Culture volume
Cell number
Nutrient/waste product status
Chemostat
Grow rate
Dilution rate
Flow rate (F) / Culture volume (V)
Cell density
Concentration of limiting nutrients entering the vessel
Benefit: Cell population can be maintained in exponential growth for long periods of time
Culture media
Classes
Complex media
Digests of microbial, animal, or plant products
Examples
Milk protein (casein)
Beef (beef extract)
Soybeans (tryptic soy broth)
Yeast cells (yeast extract)
Thioglycolate
#
Reducing agent for O2
Obligate aerobes grow at top
Facultative grow throughout
Microaerophiles grow near top
Anaerobes grow near the bottom
Exact composition is unknown
Commercially available
Enriched medium
Addition of high nutritious substances are added
Defined media
Exact composition is known
Addition of organic and inorganic chemicals to distilled water
Types
Selective
Compounds that inhibit growth of some microbials
Differential
Indicators, dye, which reveal metabolic reactions
Microbial Growh
Oxygen
Microorganism classes
Aerobes
Grow at full O2 tensions
Respire O2
Subtypes
Microaerophiles
Use O2 when present at reduced levels
Facultative
Can grow in absence of O2
Anaerobes
Cannot respire O2
Aerotolerant
Grow in presence of O2
Obligate
Inhibited/killed by O2
Toxicity
\(O_2\) converted to toxic by-products during reduction to \(H_2O\)
Superoxide anion \(O_2-\)
Hydrogen peroxide \(H_2O_2\)
Hydroxyl radical \(OH\bullet\)
Physical Methods of Controlling Microbial Growth
Heat
Sterilization
Decimal Reduction Time (D)
Time required for 10-fold reduction in viability at a given temperature
Exponential relationship with temperature
Independent of population size
Killing proceeds more rapidly as temp rises
Moist heat better than dry heat
Thermal death time
Time it takes to kill all cells at a given temp
Affected by population size
Influenced by medium
More rapid at low pH
Salt may inc. or dec. heat resistance
Heat resistance
when high concentrations
present
Sugar
Proteins
Fats
Autoclave
Sealed heating device that uses steam to kill microorganisms
Can kill endospores
Pasteurization
Reduces microorganisms found in liquid
Eliminates known pathogenic bacteria
Radiation
Ultraviolet radiation
220-300 nm
absorbed by DNA and leads to death
Disinfect surfaces or air
Poor penetrating power
Ionizing radiation
Electromagnetic radiation of sufficient energy
Produces ions and reactive molecular species
Roentgen
rads
Absorbed radiation dose
Produced by X-rays
Filter Sterilization
Filter with 0.2 - 0.45 um
Traps particles
Does not trap most viruses
HEPA filters
Not sufficient for sterilization
Removes particles > 0.3 um w/ 99.9% efficiency
Chemical
Antimicrobial agents
Kills or inhibits growth
Agents that kill -cidal
Fungicidal
Viricidal
Bactericidal
Agents that inhibit: -static
bacteriostatic
Fungistatic
Viristatic
Inhibit by blocking biochemical process
Sterilants
Destroy all microorganisms, including endospores
Disinfectants
Kill microorganisms but not endospores
Sanitizers
Reduce microbial numbers
Antiseptics
Kill or inhibit growth of microorganisms
Nontoxic to animals