Lecture 4: Microbial Growth

Question 1

_____: measured as an increase in the
number of cells

Answer 1

Growth

Question 2

______: cell division following enlargement of a cell to twice its minimum size

Answer 2

Binary fission

Question 3

_______: time required for microbial cells to double in number

Answer 3

Generation time

Question 4

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

Answer 4

bacteria and Archaea

Question 5

T/F: Mitosis does not occur in bacteria and Archaea

Answer 5

true!

Question 6

T/F: Most bacteria have shorter generation times than eukaryotic microbes

Answer 6

true!

Question 7

what does generation time depend on?

Answer 7

growth medium and incubation conditions! carbon source, pH, temp. etc.

Question 8

______: Growth of a microbial population in which cell numbers double at a constant and
specific time interval

Answer 8

exponential growth

Question 9

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?

Answer 9

Nt = No x 2^n

Nt: final cell #
No: initial cell #
n: # generations during period of exponential growth

Question 10

on what scale do we always plot exponential growth?

Answer 10

log scale

Question 11

T/F: When growth is unlimited it is called exponential growth because
it generates a curve whose slope increases continuously

Answer 11

true!

Question 12

growth rate (k)= ?

Answer 12

rate of increase in population # or biomass

Question 13

Since bacteria and archaea grow by binary fission, the growth rate is
expressed as the number of _______ per hour

Answer 13

doublings

Question 14

generation time (g): ?

Answer 14

time it takes for each cell to become 2 cells

Question 15

specific growth rate formula?

Answer 15

k= (Log Nt - Log N0)/ 0.301 (delta t)

Question 16

generation time (g) formula?

Answer 16

g= 1/k

Question 17

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?

Answer 17

no! different for each individual organism

Question 18

______: a closed-system microbial culture of fixed volume

Answer 18

Batch culture

Question 19

what are the four phases of a typical growth curve of a closed system?

Answer 19

Lag phase, Exponential phase, Stationary phase, Death phase

Question 20

_____: Interval between inoculation of a culture and beginning of growth

Answer 20

Lag phase

Question 21

____: Cells in this phase are typically in the healthiest state

Answer 21

Exponential phase

Question 22

_______: 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

Answer 22

Stationary Phase

Question 23

_______: 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

Answer 23

Death phase

Question 24

________: an open-system microbial culture of fixed volume

Answer 24

Continuous culture

Question 25

T/F: there is no stationary phase in a continuous culture

Answer 25

true! conditions stay ideal to the bacteria

Question 26

what is the most common type of continuous culture device?

Answer 26

chemostat

Question 27

in a _____, Both growth rate and population density of culture can be controlled independently and simultaneously

Answer 27

chemostat

Question 28

______: rate at which fresh medium is pumped in and spent medium is pumped out

Answer 28

Dilution rate

Question 29

T/F: Concentration of a limiting nutrient controls the population size and the growth rate

Answer 29

true!

Question 30

______: Microbial cells can be enumerated by direct microscopic observations using a Petroff-Hausser counting chamber

Answer 30

microbial counts, each square corresponds to a calibrated volume

Question 31

why are microscopic bacterial counts unreliable?

Answer 31

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

Question 32

________ is an alternative method that can be used to count the total number of cells

Answer 32

Flow Cytometry

Question 33

what does flow cytometry use to count cells?

Answer 33

laser beams, fluorescent dyes, electronics

Question 34

________: measure only living cells

Answer 34

viable cell counts, only counts cells capable of growing to form a population

Question 35

what are the two main ways to perform a viable cell plate count?

Answer 35

spread-plate pour-plate

Question 36

bacteria show _____ growth when using the spread-plate method

Answer 36

surface

Question 37

bacteria show _____ growth when using the pour-plate method

Answer 37

3D

Question 38

what are the three main issues with viable cell count methods?

Answer 38

requires lots of prep plate counts can be highly unreliable selective culture media and growth conditions only target particular species

Question 39

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)

Answer 39

true! direct microscopic counts of samples reveal far more organisms than those recoverable on plates

Question 40

why do microscopic counts reveal so many more organisms?

Answer 40

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

Question 41

how do we use spectrophotometry to count bacteria?

Answer 41

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

Question 42

what is the one caution of using a spec to count cells?

Answer 42

absorbance does not distinguish dead cells from living cells!

Question 43

_______: Typically do not require destruction or significant disturbance of sample (some spectrophotometers are specifically designed to use growth tubes as cuvette)

Answer 43

Turbidity measurements, can use the sample again!

Question 44

To relate a direct cell count to a turbidity value, a _____ must first be established to another counting method

Answer 44

standard curve

Question 45

what are the four problems with using optical density for cell counts?

Answer 45

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

Question 46

what are the two other counting techniques not already described?

Answer 46

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)