Important words and
concepts from Chapter 6, Black, 1999 (3/28/2003):
by Stephen T. Abedon (abedon.1@osu.edu)
for Micro 509
at the Ohio State University
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Course-external links are
in brackets Click [index] to access site index Click here to access
text’s website Vocabulary
words
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(1) Chapter Title: Growth and Culturing of Bacteria
(a)
"Because
individual cells grow larger only to divide into new individuals, microbial growth is defined not in
terms of cell size but as the increase in the number of cells, which occurs by
cell division."
(b)
This
emphasis has practical application since it is typically far easier to measure
increases in cell number than it is to measure increases in cell size
(c)
Furthermore,
unless cell division is synchronized, cells will typically vary in size across
an even homogeneous population, thus making measurement of cell size almost
irrelevant as a means of measuring microbial growth
(d)
Some
general external links: [microbial growth (Google Search)] [microbial growth
(with nice graphics) (David H. Demezas )] [Microbial Growth Dynamics (a book)] [microbial growth theory for
biotreatment] [effects of random motility on microbial growth and
competition in a flow reactor] [index]
(a)
The
majority of bacteria reproduce by a mechanism termed binary fission
(b)
Binary
fission is much simpler than the mechanisms of cell division seen in eucaryotic
cells
(c)
See Figure 6.1, Binary
fission
(d)
[binary transverse fission]
[binary fission
(nice cartoon illustration)] [index]
(e)
(if
there are two fish in a lake, and one of them is dead, that’s called binary fishin’)
(f)
[binary fission (Google Search)]
[index]
(a)
Tetrads
are a cell arrangement that is a consequence of binary fission not resulting in
complete separation of cells, and that occurs in two planes, thus producing a
square consisting of four cocci, one at each corner
(b)
[tetrad and bacteria
(Google Search)] [image, tetrad arrangement]
[index]
(a)
Sarcina
are a cell arrangement that is a consequence of binary fission that does not
result in complete separation of cells, and that occurs in three planes, thus
producing cubes consisting of eight cocci, one coccus at each corner
(b)
[sarcinae and bacteria
(Google Search)] [image, sarcina arrangement]
[index]
(6) Standard bacterial growth curve
(a)
Bacteria
added to fresh media typically go through four more-or-less distinct phases of
growth
(i)
Lag
phase (A)
(ii)
Log
(logarithmic or exponential) phase (B)
(iii)
Stationary
phase (C)
(iv)
Decline (death) phase (D)
(b)
(c)
[standard bacterial growth
curve, bacterial growth curve
(Google Search)] [index]
(d)
See Figure 6.3, A standard
bacterial growth curve
(a)
Transfers
of bacteria from one medium to another, where there exist chemical differences
between the two media, typically results in a lag in cell division
(b)
This
lag in division is associated with a physiological adaptation to the new
environment, by the cells, prior to their resumption of division
(c)
That
is, cells may increase in size during this time, but simply do not undergo binary
fission
(d)
[lag phase (Google Search)]
[index]
(8) Log phase (logarithmic phase, exponential phase)
(a)
Lag
phase is followed by log phase during which binary fission
occurs
(b)
This
phase of growth is called logarithmic or exponential because the rate of
increase in cell number is a multiplicative function of cell number
(c)
This
can be seen in a graph of cell number versus time where cell numbers increase
at ever increasing rates with time or generation; that is, the rate of increase
is a function of absolute cell number such that the more cells present, the
faster the population of cells increases in size (at least, during log phase)
(d)
See Figure 6.4,
Nonsynchronous growth
(e)
When
graphed on semi-log graph paper (Figure 6.3, i.e., log cell number versus
time), log-phase growth produces a straight line
(f)
[log phase, exponential phase, logarithmic phase (Google Search)] [illustration, exponential growth] [exponential growth rate
(a student activity)] [index]
(9) Continuous culture (serial transfer)
(a)
A
means of keeping cultures in log phase can be accomplished either by
employing a chemostat or via serial transfer
(b)
A
chemostat involves adding fresh medium to a culture, mixing, and then allowing
an equal volume of culture to drain from the vessel; this is typically done
continuously (i.e., a steady stream of fresh medium is added)
(c)
Serial
transfer means taking a volume of culture and diluting that volume into fresh media
(d)
["continuous culture"
and bacteria, "serial transfer"
and bacteria (Google Search)] [index]
(a)
Generation
time it takes a bacterial population to double in size (number) during log-phase
growth
(b)
Note
that the time it takes for the population to double in size does not change
with cell number (so long as cells remain in log phase)
(c)
That
is, with exponential growth, the absolute increase in cell number increases
as cell number increases while the relative increase remains invariant
(d)
Typically,
generation times range from 20 minutes to 20 hours depending on the bacterial
species/strain and the conditions during which log-phase growth is occurring
(e)
["generation time"
and bacteria (Google Search)]
[index]
(a)
Stationary
phase is a steady-state equilibrium where the rate of cell growth (division)
is exactly balanced by the rate of cell death (i.e., increase in cell number
due to cell divisions exactly balanced by a decrease in cell number due to
death)
(b)
Cell
death (or, at least, lack of cell growth) occurs because of a loss of limiting
nutrients (due to their incorporation into cells during log-phase growth) or a
build-up of toxins (due to their release during log-phase growth, e.g.,
fermentative products)
(c)
Note
that the simplest conditions that will result in a stationary phase is when
both the rate of cell increase and the rate of cell death together equal zero
(i.e., cells neither die nor are born)
(d)
[stationary phase (Google Search)]
[index]
(12) Decline phase (death phase)
(a)
Stationary
phase, in a standard bacterial growth curve, is followed by a die-off of cells
(b)
Cell
death in bacteria cultures basically means that the cells are unable to resume
division following their transfer to new environments
(c)
Typically
this die-off occurs exponentially, i.e., such that cell number graphed against
time, using a semi-log scale for cell number, results in a straight line (i.e.,
see Figure 6.3)
(d)
This
death occurs because vegetative cells can survive exposure to harsh conditions
(few nutrients or too-many toxins) for only so long
(e)
["decline phase" and
bacteria, "death phase" and
bacteria (Google Search)]
[index]
Stop material covered this day; Start material below next day
(a)
Solid
media contains agar, which is a compound that goes into water solution at
temperatures approaching boiling, and then, once in solution, solidifies the
medium at room (<40ºC) temperature
(b)
Subsequent
exposure to high temperature (i.e., boiling) will melt the medium
(c)
Exposure
to relatively low temperatures (i.e., >40ºC), however, will not melt the
medium, thus allowing incubation of solid medium at various temperatures
(compare to gelatin which liquefies at 37ºC)
(d)
Once
boiled, agar-containing medium will stay liquid at 45ºC
(e)
This
allows solid medium to be poured into various vessels at temperatures that will
not kill most cells (nor melt vessels), followed by a solidification of the
medium
(f)
(g)
["solid medium" and
bacteria (Google Search)]
[index]
(a)
Colonies
represent piles of cells descended, assuming pure culture technique and
sufficiently few colonies on a single plate, from a single parent cell, all
growing on or in a solid medium
(b)
(c)
The
four phases of bacterial growth can be observed within a single colony, with
the edges displaying lag and log phases and the interior can display stationary
and then decline phases
(d)
Various
colony morphologies (no need to memorize):
(e)
(f)
(g)
[bacterial colony (Google Search)]
[index]
(a)
The
pour-plate method is employed for bacterial-cell enumeration and isolation
(b)
In
the pour-plate method of addition of cells to solid medium contained within a
petri dish, cells are added to melted (but not too hot) solid medium
(c)
The
melted solid medium is then poured into a petri dish and allowed to harden
(d)
Colonies
appear both within, beneath, and on top of the agar
(e)
See Figure 6.7, Calculation
of the number of bacteria per milliliter of culture using serial dilution
(