Microbial Growth Flashcards
What is binary fission?
- Starts with a young cell at the early phase of the cycle.
- A parent cell prepares for division by enlarging its cell wall, cell membrane, and overall volume. The chromosome replicates it self.
- The septum begins to grow inward as the chromosomes move toward opposite ends of the cell. Other cytoplasmic components are distributed to the two developing cells.
- The septum is synthesized completely through the cell center, and the cell membrane patches itself so that there are two separate cell chambers.
- The identical daughter cells are divided. Some species can remain attached to form chains, doublets, or other cell arrangements.
What is generation time?
- the time required for the population to double in number of cells
- varies depending on species and environmental conditions
- during exponential growth, the cell number doubles within a fixed time period
How is the number of generations calculated?
n=3.3 (logNt-logN0)
Nt: number of cells at time t
N0: number of cells you start with
n: number of generations
How is the generation time calculated?
g= t/n
g: generation time
n: number of generations
t: time
What are the four phases of bacterial growth observed in a batch culture?
- lag phase
- exponential phase (log phase)
- stationary phase
- death phase
What happens during the lag phase?
- cells are synthesizing new components
- replenishing spent materials
- adapting to a new medium or other conditions
- can be very short or in some cases absent
What happens during the exponential phase?
- rate of growth is constant
- maximum rate of growth
- population is the most uniform in terms of chemical and physical properties
- cells in the mid-exponential phase are considered the healthiest
What happens during the stationary phase?
- total number of viable cells remains constant
- either the metabolically active cells stop reproducing or the reproductive rate is balanced by the death rate
- reasons for this phase: nutrient limitation, limited oxygen, toxic waste accumulation, critical population density reached (10^9 bacteria/mL)
What happens during the death phase?
- total number of viable cells is decreasing
- removal of critical nutrients is below a threshold level
- metabolic end product reaches a toxic level
- irreversible loss of ability to reproduce is considered death
- lysis may occur
How are cell numbers measured directly?
- total cell counts: counting the number of cells observed in the medium
- uses a counting chamber or an electric counter
- viable cell counts: counting the number of CFUs through plating technique or membrane filter
How are cell numbers measured indirectly?
- dry weight
- turbidity (absorbance)
How are counting chambers used?
- sample added to a slide with a grid
- under a microscope, the cells are counted and the numbers are averaged
- you may be counting dead cells that have not lysed (tend to get an overestimate)
- you can stain to differentiate between living and dead cells
How are plating methods used?
- plate dilutions of a population on a suitable solid medium
- count the number of colonies (30-300 is a countable plate)
- calculate the number of cells in the original population (CFUs/mL) by multiplying the number of colonies by the dilution factor
- can get an underestimate pretty easily
- dead cells will not make a colony
How is a membrane filter used?
- membrane filter is placed on a filter support
- water sample is filtered through the membrane filter
- the membrane filter is placed in the plate containing the appropriate medium
- incubated for 24 hours
- colonies will form
How is turbidity used?
- indirect method of getting a cell count
- if there are more cells in the sample, they will absorb more light (higher absorbance in spectrophotometer)
- must establish a standard curve to go from turbidity to cell count
- tend to get an overestimate
How does temperature affect microbial growth?
- microbes can not regulate their internal temperature
- exhibit distinct cardinal growth temperatures
- have a minimum, maximum, and optimum temperature
What are the different temperature classes?
- psychrophile
- psychrotolerant
- mesophile
- thermophile
- hyperthermophile
psychrophile
optimum temp of less than 15 C
psychrotolerant
- optimum temp of 20-40 C
- able to grow at 0 C
mesophile
- optimum temp of 20-40 C (body temp)
- unable to grow at 0 C
thermophile
optimum temp of 45-80 C
hyperthermophile
- optimum temp of over 80 C
- most likely archaeons
- likely have a lipid monolayer which is more stable at higher temperatures
How do cells adapt to cold environments?
- more flexible enzymes
- modified cytoplasmic membranes (high unsaturated fatty acid content)
How do cells adapt to warm environments?
- thermally stable enzymes (ex: Taq polymerase)
- modified cytoplasmic membrane (high saturated fatty acid content)
acidophile
optimum pH less than 5.5
neutrophile
optimum pH between 5.5 and 8
alkaliphile
optimum pH over pH 8
How does pH affect microbial growth?
- most microbes maintain an internal pH near neutrality
- acid shock proteins
- sodium motive force can be used instead of PMF
- some microbes buffer the pH of their habitat by producing acidic or basic waste products
What is water activity (aw)?
- amount of water available to organisms
- ranges from 0-1
- water associated with solutes is unavailable to microorganisms
- higher concentration of solute= lower aw
How do cells adapt to hypotonic solutions?
use mechanosensitive (MS) channels in the membrane to allow solutes to leave (less water comes into the cell)
How do cells adapt to hypertonic solutions?
increase internal solute concentration with compatible solutes (more water comes into the cell)
nonhalophile
- does not require NaCl
- can grow if the concentration of NaCl is less than 1%
halotolerant
can grow in low salt concentrations
halophile
- requires NaCl for growth
- grows optimally at 1-15% NaCl
extreme halophile
requires 15-30% NaCl for growth
aerobe
grows in the presence of atmospheric oxygen (21% O2)
obligate aerobe
- requires O2 for growth
- will be seen at the top of the tube
anaerobe
grows in the absence of O2
obligate anaerobe
- killed in the presence of O2
- will be seen at the bottom of the tube
microaerophile
- requires 2-10% O2
- will be seen concentrated in the middle of the tube
facultative aerobe/anaerobe
- does not require O2 but grows better in its presence
- will see more growth at the top of the tube
aerotolerant anaerobe
- grows with or without O2
- will be spread out in the tube
What is ROS?
- oxygen can be easily reduced to toxic reactive oxygen species (ROS)
- superoxide: O2-
- hydrogen peroxide: H2O2
- neutral hydroxyl: OH (most toxic)
How do microorganisms protect themselves from ROS?
- through protective enzymes:
- superoxide dismutase
- catalase
- peroxidase
What does catalase do?
H2O2 + H2O2 -> 2 H2O + O2 (bubbles)
What does peroxidase do?
H2O2 + NADH + H+ -> 2 H2O + NAD+
What does superoxide dismutase do?
O2- + O2- + 2H+ -> H2O2 + O2
What does superoxide dismutase/catalase in combination do?
4 O2- + 4H+ -> 2H2O + 3O2
What does superoxide reductase do?
O2- + 2H+ + reduced rubredoxin -> H2O2 + oxidized rubredoxin
