Chapter 6 Microbial Growth
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Chapter 6
Microbial Growth
GrowthWhen we speak of bacterial growth, we are
talking about an increase in population size not an increase in the size of a single bacterium
Animation: Binary Fission
Animation: Overview of Bacterial Growth
Binary FissionBacterial cells replicate by a form of
asexual reproduction called binary fission
Generation TimeGeneration Time – time required to complete
fission cycle from parent cell to 2 daughter cells. (Doubling time). In terms of a population it is the amount of time needed to double the population.
The length of the generation time is a measure of the Growth Rate of the microbe.
It varies depending on environmental conditions.
Different microbes have different generation times.Mycobacterium leprae 10-30 daysStaphylococcus aureus 20-30 minutes
Exponential GrowthExponential Growth---# of cells doubles
during each unit of time.During exponential growth, the increase in
cell number is initially slow but increases at a faster rate
Cell population size can be represented by 2N (where N = the number of generations)
Predicting the Number of cells : Nfinal = (Ninitial) 2n
Growth of Microbial Populations
Arithmetic Versus Logarithmic Growth
Microbial Growth Cycle (batch culture)
Batch culture: a closed-system microbial culture of fixed volume
Typical growth curve for population of cells grown in a closed
system is characterized by four phases
Lag phase
Exponential phase
Stationary phase
Death phase
Animation: Bacterial Growth Curve
The Growth Curve
Microbial Growth Cycle (batch culture)
Lag phase Interval of time between when a culture is inoculated and when growth begins.
Exponential phase Cells in this phase are typically in the healthiest state. Growth is at maximal
rate.
Stationary phase Growth rate of population is zero.
Number new divisions=number of cells dying
Either an essential nutrient is used up or waste product of the organism
accumulates in the medium
Death phase Lack of nutrients and increasing accumulation of wastes lead to… number of cell
deaths > number of new divisions
Continuous Culture: The Chemostat
Continuous culture: an open-system microbial culture of
fixed volume
Chemostat: most common type of continuous culture
device
Measuring Growth (Direct Measurement)
Total Cell CountDirect Microscopic examination using special
slides
Automated counters (flow cytometry)
Measuring Growth (Direct Measurement)
Viable Count
Measurement of living, reproducing population
Two main ways to perform plate counts
Spread-plate method
Pour-plate method
To obtain the appropriate colony number, the sample
to be counted may need to be diluted (serial dilutions)
Spread-Plate Method for the Viable Count
Figure 6.15
Pour-Plate Method for the Viable Count
Procedure for Viable Counting Using Serial Dilutions
Measuring Growth (Direct Measurement)
Filtration
Most Probable Number
A statistical estimating technique
Indirect MethodsMetabolic activityDry weightTurbidity
Spectrophotometer-measures amount of light that passes through a sample. Absorbance is related to the number of bacteria
Measuring Growth (Indirect Measurement)
Turbidity
Temperature and Microbial Growth
Temperature is a major
environmental factor
controlling microbial
growth
Cardinal temperatures:
the minimum,
optimum, and
maximum
temperatures at which
an organism grows
Effects of Temperature on Microbial Growth
Microorganisms can be classified into groups by
their growth temperature optima
Psychrophile: low temperature
Psychrotolerant
Mesophile: midrange temperature
Thermophile: high temperature
Hyperthermophile (extreme thermophile): very high
temperature
Temperature and Growth Relations in Different Classes
Effects of Temperature on Microbial Growth
Mesophiles: organisms that have midrange
temperature optima; found in
Warm-blooded animals
Terrestrial and aquatic environments
Temperate and tropical latitudes
Microbial Growth at Cold Temperatures
Extremophiles
Organisms that have evolved to grow optimally under very
hot or very cold conditions
Psychrophiles
Organisms with cold temperature optima; the most extreme
representatives inhabit permanently cold environments
Psychrotolerant
Organisms that can grow at 0ºC but have optima of 20ºC to
40ºC; more widely distributed in nature than psychrophiles
Microbial Growth at HotTemperatures
Thermophiles
Hyperthermophiles
produce enzymes widely used in industrial
microbiology
(E.g., Taq polymerase; used to automate the repetitive
steps in the polymerase chain reaction (PCR) technique)
pH
Organisms sensitive to changes in acidity because H+ and OH- interfere with H bonding in proteins and nucleic acids
Most bacteria and protozoa grow best in a narrow range around neutral pH (6.5-7.5) – these organisms are called neutrophiles
Other bacteria and fungi are acidophiles – grow best in acidic habitatsAcidic waste products can help preserve foods by
preventing further microbial growthAlkalinophiles live in alkaline soils and water
up to pH 11.5
Physical Effects of WaterMicrobes require water to dissolve enzymes and
nutrients required in metabolismWater is important reactant in many metabolic
reactionsMost cells die in absence of water
Some have cell walls that retain waterEndospores and cysts cease most metabolic
activity in a dry environment for yearsTwo physical effects of water
Osmotic pressureHydrostatic pressure
Osmotic Pressure
Is the pressure exerted on a semipermeable membrane by a solution containing solutes that cannot freely cross membrane; related to concentration of dissolved molecules and ions in a solution
Hypotonic solutions have lower solute concentrations; cells placed in these solutions will swell and burst\
Hypertonic solutions have greater solute concentrations; cells placed in these solutions will undergo crenation (shriveling of cytoplasm)This effect helps preserve some foods
Restricts organisms to certain environmentsObligate halophiles – grow in up to 30% saltFacultative halophiles – can tolerate high salt
concentrations
Hydrostatic PressureWater exerts pressure in proportion to its depth
For every addition of depth, water pressure increases 1 atm
Organisms that live under extreme pressure are barophilesTheir membranes and enzymes depend on this
pressure to maintain their three-dimensional, functional shape
Oxygen and Microbial GrowthAerobes: require oxygen to live
Anaerobes: do not require oxygen and may even be killed by
exposure
• Facultative organisms: can live with or without oxygen
• Aerotolerant anaerobes: can tolerate oxygen and grow in its
presence even though they cannot use it
Microaerophiles: can use oxygen only when it is present at levels
reduced from that in air
Growth Versus Oxygen Concentration
Figure 6.27
Necessary enzymes for life with oxygen
Superoxide dismutase (SOD)
CatalasePeroxidase
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