Chapter 5: Microbial Growth and its Control Flashcards
Growth
-increases in the number of cells
Binary fission
-cell division following enlargement of cell to twice its minimum size
One generation
- cell elongation
- septum formation
- completion of septum; formation of walls; cells separate
Septum
-partition between dividing cells, pinches off between two daughter cells
Generation time
- time required for microbial cells to double in number
- depends on nutritional and generic factors and temperature
During cell division
-each daughter cell receives a chromosome and sufficient copies of all other constituents to exist as an independent cell
Budding
- results from unequal cell growth and forms totally new daughter cell
- form cytoplasmic extensions such as stacks (caulobacter), hyphae, (hyphomicrobium), and appendages (ancalomicrobium)
Cytoplasmic structure
-are not partitioned and remain in old cell
Cell division in bacteria
- equal products of cell division
- binary fission: most bacteria
- unequal products of cell division
- simple budding: pirellula, blastobacter
- budding from hyphae: hyphomicrobium, rhodomicrobium, pedomicrobium
- cell division of staked organism: caulobacter
- polar growth without differentiation of cell size: rhodopseudomonas, nitrobacteria, methylosinus
Planktonic growth
-growth as suspension
Sessile growth
- attached to surface
- can develop into biofilms
- attached polysaccharide matrix containing embedded bacteria
Biofilms form in stages
- planktonic cells attach
- sticky matrix forms
Microbial mats
-multilayered sheets with different in each layer
BiofIlms
- prevent harmful chemicals from penetrating
- prevent protists from grazing
- prevent washing away of cells
- affect human health
- water distribution systems
- fuel storage
Exponential growth
-growth of a microbial population in which cell numbers double within a specific time interval
N=N_02^n (2^0→2^1→2^2)
- a relationship exists between the initial number of cells present in a culture and the number present after a period of exponential growth
- N is the final cell number
- N_0 is the initial cell number
- n is the number of generations during the period of exponential growth
g=t/n
- generation time (g) of the exponentially growing population
- t is the duration of exponential growth (days/hours/minutes)
- n is the number of generations during the period of exponential growth
- can be calculated from slope of the straight-line logarithmic growth plot
Instantaneous growth rate constant
-(k) expresses rate of growth at any instant (measure in h^-1)
Instantaneous growth rate
- (k) is calculated as k=0.693/g
- useful for optimizing culture conditions for microorganism growth
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
Lag phase
- interval between inoculation of a culture and beginning growth
- time needed for biosynthesis of new enzymes and to produce required metabolites before growth can begin
Exponential phase
-cells in this phase are typically in the healthiest state
Stationary phase
- growth rate of population is zero
- either an essential nutrient is used up or waste products accumulate
- some cells grow while others die, balancing each other
Death phase
- is inoculation continues after cells reach stationary phase, the cells will eventually die
- typically much slower than exponential growth
- re-adjust to dormant stage
- viable cells remain for months or years
Continuous culture
-an apen system, microbial culture of fixed volume
Chemostat
-most common type of continuous culture device
Dilution rate
- F/V
- F is the flow of adding fresh medium and removing spent medium
- V is the culture volume
- concentration of a limiting nutrient
- both growth rate and population density of culture can be controlled independently and simultaneously
Steady state
- cell density and substrate concentration do not change over time
- specific growth rate (D)= decrease in cell numbers due to dilution
- experimental uses:
- can maintain exponential growth phase for weeks/months
- used for study physiology, microbial ecology and evolution, enriched and isolate of bacteria from nature
- growth rate controlled by dilution rate
Culture media
- nutrient used to grow microbes in the laboratory
- typically sterilized in an autoclave
Defined media
-exact chemical composition known
Complex media
-composed of digests of microbial, animal, or plant products
Enriched media
- contain complex media plus highly nutritious materials
- used to culture fastidious (nutritionally demanding) microbes
Selective media
-contain compounds that selectively inhibit growth of some microbes but not other
Differential media
-contain an indicator, usually a dye, that detects particular metabolic but not during growth
Microbes
- sterilization of media is critical
- to prevent contamination, aseptic techniques should be followed
Total cell count
-counting chambers with squares etched in a slide for liquid samples
Microscopic cell count
-observing and enumerating cells present dried on slides or on liquid samples
Limitations of microscopic cell counts
- cannot distinguish between live vs dead cells without special stains
- precision is difficult to achieve
- phase-contrast microscope required if a stain is not used
- cell suspensions of low density hard to count
- motile cells need to immobilized
- debris in sample can be mistaken for cells
Microscopic cell counts in microbial ecology
-often used to natural samples
-use stains to visualize and provide phylogenetic information or metabolic properties
-DAPI (blue) reacts with DNA
-other fluorescent stains differentiate live and dead cells
phylogenetic stains can determine properties of bacteria and archaea
