Lecture 4: Microbial Growth Flashcards
_____: measured as an increase in the
number of cells
Growth
______: cell division following enlargement of a cell to twice its minimum size
Binary fission
_______: time required for microbial cells to double in number
Generation time
In _____ & ______, growth in cell size, chromosome replication and even
septum formation typically occur simultaneously
contrary to Eukaryotic cells where there is an interphase AND mitosis
bacteria and Archaea
T/F: Mitosis does not occur in bacteria and Archaea
true!
T/F: Most bacteria have shorter generation times than eukaryotic microbes
true!
what does generation time depend on?
growth medium and incubation conditions! carbon source, pH, temp. etc.
______: Growth of a microbial population in which cell numbers double at a constant and
specific time interval
exponential growth
A relationship exists between the initial number of cells present in a
culture and the number present after a period of exponential growth: what is this equation?
Nt = No x 2^n
Nt: final cell #
No: initial cell #
n: # generations during period of exponential growth
on what scale do we always plot exponential growth?
log scale
T/F: When growth is unlimited it is called exponential growth because
it generates a curve whose slope increases continuously
true!
growth rate (k)= ?
rate of increase in population # or biomass
Since bacteria and archaea grow by binary fission, the growth rate is
expressed as the number of _______ per hour
doublings
generation time (g): ?
time it takes for each cell to become 2 cells
specific growth rate formula?
k= (Log Nt - Log N0)/ 0.301 (delta t)
generation time (g) formula?
g= 1/k
For each organism there is a specific growth rate that is the fastest growth rate in the best growth medium at optimal temperature, are all of them the same?
no! different for each individual organism
______: a closed-system microbial culture of fixed volume
Batch culture
what are the four phases of a typical growth curve of a closed system?
Lag phase, Exponential phase, Stationary phase, Death phase
_____: Interval between inoculation of a culture and beginning of growth
Lag phase
____: Cells in this phase are typically in the healthiest state
Exponential phase
_______: Cells metabolically active, but growth rate of population is zero
Either an essential nutrient is used up, or waste product of the organism accumulates in the medium
Stationary Phase
_______: If incubation continues after cells reach stationary phase, the cells will
eventually die
Not all bacteria die, some bacteria (gram +!!)
form spores/cysts or dormant stages
that allow a significant proportion of
cells to survive for a long time
Death phase
________: an open-system microbial culture of fixed volume
Continuous culture
T/F: there is no stationary phase in a continuous culture
true! conditions stay ideal to the bacteria
what is the most common type of continuous culture device?
chemostat
in a _____, Both growth rate and population density of
culture can be controlled independently and
simultaneously
chemostat
______: rate at which fresh medium is
pumped in and spent medium is pumped out
Dilution rate
T/F: Concentration of a limiting nutrient controls the
population size and the growth rate
true!
______: Microbial cells can be enumerated by direct microscopic observations using a Petroff-Hausser counting chamber
microbial counts, each square corresponds to a calibrated volume
why are microscopic bacterial counts unreliable?
Cannot distinguish between live and dead cells without special stains
Small cells can be overlooked
Precision is difficult to achieve (need a lot of counts)
Phase-contrast microscope required if a stain is not used
Cell suspensions of low density (<106 cells/ml) hard to count
Motile cells need to immobilized
Debris in sample can be mistaken for cells
Cells may move (Brownian motion), some form clumps Based on random
distribution and dispersal of the cells
________ is an
alternative method that can
be used to count the total
number of cells
Flow Cytometry
what does flow cytometry use to count cells?
laser beams, fluorescent dyes, electronics
________: measure only living cells
viable cell counts, only counts cells capable of growing to form a population
what are the two main ways to perform a viable cell plate count?
spread-plate
pour-plate
bacteria show _____ growth when using the spread-plate method
surface
bacteria show _____ growth when using the pour-plate method
3D
what are the three main issues with viable cell count methods?
requires lots of prep
plate counts can be highly unreliable
selective culture media and growth conditions only target particular species
the great plate anomaly
T/F: Modern genomic techniques suggest that only 1-10% of microbial diversity is
culturable from most environmental samples (including the diversity of
organisms in our own microbiomes)
true! direct microscopic counts of samples reveal far more organisms than those recoverable on plates
why do microscopic counts reveal so many more organisms?
Microscopic methods count dead cells, whereas viable methods do not
Different organisms may have vastly different requirements for growth
We do not know the specific requirements for all organisms
how do we use spectrophotometry to count bacteria?
turbidity is measured with a
spectrophotometer, and measurement is
referred to as optical density (OD)
the more cells, the more absorption, the lower the light reading on the spec
what is the one caution of using a spec to count cells?
absorbance does not distinguish
dead cells from living cells!
_______: Typically do not require destruction or
significant disturbance of sample (some spectrophotometers are specifically designed to use growth tubes as cuvette)
Turbidity measurements, can use the sample again!
To relate a direct cell count to a turbidity value, a _____ must
first be established to another counting
method
standard curve
what are the four problems with using optical density for cell counts?
has a finite linear range of measurement
Only works if the cells are evenly distributed throughout the medium (no
clumps or biofilms)
Cuvette must not have scratches
Culture may need to be diluted when the
cells are at very high density
what are the two other counting techniques not already described?
Total mass of cells (dry cell weight): a specific aliquot (volume) cells are
concentrated, washed to remove media components, concentrated
and dried
There are other spectrometric techniques to measure specific components of the cell: protein, DNA etc. which are proportional to
the whole mass of cells
(have to account for error)
