5. Microbial Growth and Its Control
5.1 Binary Fission, Budding, and Biofilms
growth is the result of cell division and is the ultimate process in the life of a microbial cell,
growth = an increase in
the number of cells
binary fission = cells
elongate to approximately
twice their original length
and then form a partition
that constricts the cell into
daughter cells
the partition that forms
between dividing cells is
called a septum and results
from the inward growth of
the cytoplasmic membrane
and cell wall from opposing
directions; septum formation continues until the two daughter cells are pinched off
binary fission allows rapid growth
when one cell eventually separates to form two cells, we say that one generation has
occurred, and the time required for this process is called the generation time
the generation time of a given bacterial species is variable and depends on nutritional and
genetic factors, and on temperature
generation times vary from 10 minutes to 10000 years
we do not know what limits the growth of microorganisms, but fast-growing cells tend to be
(sub)micrometer size
fast growth is possible when building blocks can be imported rather than synthesized de
novo
in nature, microbial cells probably grow much slower than their maximum rates observed in
the laboratory
not all cell division in bacteria occurs by binary fission, budding bacteria are cells that divide
as a result of unequal cell growth
budding division forms a totally new daughter cell, with the mother cell retaining its original
identity
a fundamental difference between budding bacteria and bacteria that divide by binary fission
is the formation of new cell wall material from a single point (polar growth) rather than
throughout the whole cell (intercalary growth) as in binary fission
some budding bacteria form cytoplasmic extensions such as stalks (Caulobacter), hyphae
(Hyphomicrobium), and appendages (Ancalomicrobium)
microbial cells can grow either in suspension or attached to surfaces
the suspended lifestyle, called planktonic growth, is the way many bacteria live in nature
many other microorganisms show sessile growth, meaning that they grow attached to a
surface, these attached cells can then develop into biofilms
,
,biofilm = attached polysaccharide matrix containing embedded bacterial cells
biofilms are a common growth form for bacteria in nature because the intensely interwoven
nature of the structure prevents harmful chemicals from penetrating
5.2 Quantitative Aspects of Microbial Growth
exponential growth = repetitive pattern where the number of cells doubles in a constant time
interval
a fixed relationship exists between the initial number of cells in a culture and the number
present after a period of exponential growth, and this relationship is expressed as N = N02n
(N = final cell number, N0 = initial cell
number, n = number of generations during
the period of exponential growth)
generation time g = t/n (t = duration of
exponential growth expressed in days,
hours or minutes)
Nt = N02t/g > N0eμt
lnNt - lnN0 = μ x t
doubling of number of cells: lnNt / N0 = ln2
μ = ln2 / td
5.3 The Microbial Growth Cycle
batch culture = a closed-system microbial
culture of fixed volume
exponential growth is only part of the
microbial growth cycle
an organism growing in an enclosed vessel (a batch culture) cannot grow exponentially
indefinitely, instead a typical growth curve for the population is obtained
the growth curve describes the entire growth cycle and is made up of four phases: lag,
exponential, stationary, and death
when a microbial culture is inoculated into fresh growth media, growth begins only after a
period of time called the lag phase
in order to grow, cells must have a complete complement of enzymes for synthesis of the
essential metabolites not present in that medium, hence, when transferred to a nutrient-poor
medium, time is needed for biosynthesis of new enzymes and for these to produce a
sufficient pool of required metabolites before growth can actually begin, these events occur
during the lag period
when a growing cell population doubles at regular intervals the cells are said to be in the
exponential phase of growth
exponential growth cannot be maintained indefinitely, growth becomes limited in such
cultures because either an essential nutrient in the culture medium is depleted or the
organism’s waste products accumulate
when exponential growth ceases the population enters stationary phase, in the stationary
phase there is no net increase or decrease in cell number and thus the growth rate of the
population is zero
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