3. Microbial growth and nutrition Flashcards
Bacterial growth
Bacteria grow by binary fission (division into 2 identical daughter cells)
Generation Time:
time required for population to double
Depending on species and nutritional factor (quicker in lab culture)
Bacterial growth
Binary fission: 2n
0 hours:
1 Cell
1 hour:
4 cells
1.5 hours:
8 cells
4 hours:
256 cells
Every half an hour, the cell count doubles to the previous count.
Bacterial growth = logarithmic (log) or exponential growth
Growth curve - Batch culture
Log phase:
- No cell division occurs.
- Interval between when a culture is inoculated and when growth begins
Log or exponential phase:
- Cells are typically in the healthiest state
- Doubling is at a constant rate
- Cells are also dying but division is occuring at a faster rate -> net gain in population
Exponential growth phase:
* Generation time constant
Stationary phase:
- Reproduction and death are balanced
- growth rate of population is zero (no net increase or decrease in number of cells)
- Either an essential nutrient is used up or waste product of the organism accumulates in the medium and inhibits growth
Death phase:
- Death rate exceeds reproduction
- If inocubation continues after cells reach stationary phase, the cells will eventually die
-> Microorganisms can sense population density (sensing, communication)
Metabolites, signal molecules, etc.
Continuous culture (Chemostat)
- system in balanced state
- Volume, cell number and nutrient supply remain constant
- growth medium reservoir with fresh media
- metering pump with the metered growth medium
- culture vessel with the culture in exponential growth phase for longer, including a stirring motor. Constant oxygenation
- Collection vessel where the culture is removed
Measuring microbial growth
- Direct measurement
direct vs. indirect
- Direct measurement:
* Viable cell count / total cell count
* serial dilutions
* each viable cell grows into a single colony counted
* doesn’t count dead cells
* microscope and counting chamber (Petroff-Hausser, Neubauer)
-> no discrimination between living or dead cells
- Sample added
- care must be taken to not overflow
- space between coverslip and slide is 0.02 mm
- whole grid has 25 large squares - total area of 1 mm2 - total volume of 0.02 mm3
- microscopic observation
- all cells are counted in large squares
Measuring microbial growth
- Indirect measurement
- Indirect measurements:
* Turbidity / Optical density
* Turbidity is proportional to cell number - within limits
* Tubidity / cloudiness measured by using a spectrophotometer
* quick calculation of generation time
-> does not discriminate between living or dead cells
Optical density measurement less reliable than viable count
Requirements for microbial growth
- Microbes live in virtually every environmental niche on earth
- Adaptation to different environments
- Evolution -> diverse nutritional types utilising a broad array of substrates
- growth defined by physical (environmental) and nutritional requirements
- > Temperature
- > Oxygen
- > Osmotic pressure
- > pH
- > Light
- Temperature
Minimum: (8 C)
Membrane gelling - transport processes so slow that growth cannot occur
Enzymatic reactions occuring at increasingly rapid rates
Optimum: (39 C)
Enzymatic reactions occuring at maximal possible rate
Maximum: (48 C)
Protein denaturation - collapse of the cytoplasmic membrane - thermal lysis
-> The cardinal Temperatures and temperature range (40 C) vary by organism
Microbial temperature ranges
- > Psychophile (4 C) (e.g. Polaromonas vacuolata)
- > Mesophile (39 C) (e.g. E. Coli)
- > Thermophile (60 C)
- > Hyperthermophile (88 C)
- > Hyperthermophile (106 C)
Temperature range: Membrane Fluidity
Proper function of the membrane depends on its fluidity
More unsaturated (membrane more fluid) more saturated (membrane more rigid)
Fatty acid composition reflects growth temperature
Psychrophiles
- > extremophiles
- > found in polar regions
- > liquid water must be available
- > grow very slow (T below 15 C)
- > Cold - active enzymes
- > Bacteria, fungi, algae, protozoa
- > can grow in refrigerator
Mesophiles
- > most common microorganisms
- > Temperate and tropical environments
- > Optimum 36 - 38 C (body temp)
- > Also found in warm-blooded animals
- > e.g. E. Coli
Psychrotolerant
- > can grow at 0 C, but have optima between 20 C and 40 C
- > grow very slow at 0 C, normally higher temperature optimum
- > Tend to come from temperate environments (soils, food)
- > often cause food spoilage in fridge
Thermophiles
- > found in compost heaps, hot springs, hot water heaters, thermal gradients
- > heat-stability of proteins
Hyperyhermophiles
- > found in geothermal vents, volcanic areas
- > heat-stability of proteins
- > prokaryotes, some Eubacteria and mostly Archaea bacteria
- > no fatty acids in membrane but contains C 40 hydrocarbons
- > composed of repeating units of the compound isoprene
