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Summary Introduction and microbial growth
- Instelling
- Rijksuniversiteit Groningen (RuG)
Summary about the lecture and corresponding literature about introductionary microbiology and microbial growth.
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HC3 Introduction and Microbialgrowth (BOOK)
Chapter 5.1 – 5.7
CH5 Microbial Growth and Regulation
5.1 Binary Fission, Budding, and Biofilms
In microbiology, growth is defined as an increase in the number of cells.
Binary fission means that an cell elongates to about twice its original length and then a septum (kind
of ‘insnoering’) forms and finally completes and the cell divides.
The time required for a cell to divide and thus form a new generation, is called the generation time.
Most bacteria divide by binary fission, but there are other forms of cell division. Budding divisions
form a totally new daughter cell, with the mother cell retaining its original identity. The big difference
between budding bacteria and bacteria that divide by binary fission is that the formation of new cell
wall material in budding division occur at one point (polar growth) while in binary fission it happens
in the whole cell (intercalary growth). Note that for polar growth, large internal membrane
complexes are not partitioned and also need to be formed de novo in the developing bud.
Biofilms
Microbial cells can grow either in suspension or attached to surfaces. The suspended live style (swim
or float in liquid medium) is called planktonic growth. Microorganisms that grow attached to a
surface (sessile growth) can develop into biofilms which are attached polysaccharide matrixes
containing embedded bacterial cells. Biofilms form in stages:
1. Attachment of planktonic cells
2. Production of sticky matrix
3. Further growth and development to form the tenacious and nearly impenetrable mature
biofilm
Some biofilms form multiple layers with different organisms inside the individual layers these
biofilms are called microbial mats.
5.2 Quantitative aspects of microbial growth
In the generation time, both total cell number and cell mass double.
When the cell number of exponential growth is plotted on a logarithmic scale as a function of time
(semilogarithmic graph) the points fall on a straight line. This reflects the fact that the cells are
growing exponentially and that the population is doubling in a constant time interval.
The relationship between the initial number of cells in a culture and the number present after a
period of exponential growth: N = No x 2n
Where N is the final number, N0 the initial number and n the number of generations.
Generation time: g = t / N
The instantaneous growth rate constant (k) expresses the rate at which the population is growing at
any instant, expressed in units of reciprocal hours (h -1). dN/dt = kN by integrating natural logarithms;
N = No x ekt
, 5.3 The microbial growth cycle
A culture of organisms growing in an enclosed vessel, is called a batch culture.
Growth when a microbial culture is inoculated into a fresh growth media only begins after a period of
time, this time is called the lag phase. When a growing culture is tranferred into the same medium
under the same conditions, there will be no lag phase.
In general, prokaryotic cells grow faster than eukaryotes, and small eukaryotes tend to grow faster
than large ones.
Stationary and death phases
When exponential growth ceases (due to essential nutrient depletion or waste products
accumulation), the population enters stationary phase (there is no increase or decrease in cell
number —> growth rate = 0).
Cryptic growth means that some individuals divide, while others die —> net decrease/increase = 0.
During the death phase, the growth number < 0.
5.4 Continuous culture
The environment in batch cultures are constantly changing because of nutrient consumption and
waste production. These limitations can be circumvented in a continuous culture device.
The most common type of continuous culture is the chemostat, a device in which growth rate and
cell density can be controlled independently.
Two factors govern the specific growth rate (the dilution rate (D) = F / V where F is the flow rate and
V culture volume) and cell density (concentration of a limiting nutrient present in the sterile medium
entering the chemostat vessel).
In steady state, the specific growth rate equals D; the rate of increase in cell numbers due to growth
is equal to the rate of decrease in cell numbers due dilution (outflow).
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