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Bacterial nutrition and Growth
introduction
Microorganisms use chemicals called nutrients for growth and
development. They need these nutrients to build molecules and cellular
structures. The most important nutrients are carbon, hydrogen, nitrogen,
and oxygen. Microorganisms get their nutrients from sources in their
environment. When these microorganisms obtain their nutrients by living
on or in other organisms, they can cause disease in that organism by
interfering with their host’s nutrition, metabolism, and, thus disrupting
their host’s homeostasis, the steady state of an organism. Organisms can
be classified in two groups depending on how they feed themselves.
1- Organisms that use carbon dioxide (CO2) as their source of carbon are
called autotrophs. These organisms “feed themselves,” auto- meaning
“self” and -troph meaning “nutrition.” Autotrophs make organic
compounds from CO2 and do not feed on organic compounds from other
organisms.
2- Organisms that obtain carbon from organic nutrients like proteins,
carbohydrate, amino acids, and fatty acids are called heterotrophs.
Heterotrophic organisms acquire or feed
on organic compounds from other organisms. Organisms can also be
categorized according to whether they use chemicals or light as a source
of energy. Organisms that acquire energy from redox reactions involving
inorganic and organic chemicals are called chemotrophs.Organisms that
use light as their energy source are called phototrophs A microorganism
requires two chemical elements in order to grow. These elements are
carbon and oxygen.
1-Carbon
Carbon is one of the most important requirements for microbial growth.
Carbon is the backbone of living matter. Some organisms, such as
photoautotrophs, get carbon from carbon dioxide (CO2).
2-Oxygen
Microorganisms that use oxygen produce more energy from nutrients
than microorganisms that do not use oxygen. These organisms that
require oxygen are called obligate aerobes. Oxygen is essential for
obligate aerobes because it serves as a final electron acceptor in the
electron transport chain, which produces most of the ATP in these
organisms. An example of an obligate aerobe is micrococcus. Some
organisms can use oxygen when it is present, but can continue to grow by
using fermentation or anaerobic respiration when oxygen is not available.
These organisms are called facultative anaerobes. An example of a
facultative anaerobe is E. coli bacteria, which is found in the large
intestine of vertebrates, such as humans. Some bacteria cannot use
molecular oxygen and can even be harmed by it. Examples include
Clostridium botulinum, the bacterium that causes botulism, and
Clostridium tetani, the bacterium that causes tetanus. These organisms
are called obligate anaerobes.
Anaerobic growth
Because anaerobic organisms can be killed when exposed to
oxygen, they must be placed in a special medium called a reducing
medium. Reducing media contain ingredients like sodium thioglycolate
that attaches to dissolved oxygen and depletes the oxygen in the culture
medium. In health clinics and hospitals, it is necessary to detect
microorganisms that are associated with disease. Selective and
differential media are therefore used. Selective media are made to
encourage the growth of some bacteria while inhibiting others. An
example of this is bismuth sulfite agar. Bismuth sulfite agar is used to
isolate Salmonella typhi from fecal matter. Salmonella typhi is a
gramnegative bacterium that causes salmonella. Differential media make
it easy to distinguish colonies of desired organisms from nondesirable
colonies growing on the same plate. Pure cultures of microorganisms
have identifiable reactions with different media. An example is blood
agar. Blood agar is a dark red/brown medium that contains red blood cells
used to identify bacterial species that destroy red blood cells. An example
of this type of bacterium is streptococcus pyogenes, the agent that causes
strep throat. MacConkey agar is both selective and differential.
MacConkey agar contains bile salts and crystal violet, which inhibit the
growth of gram-positive bacteria,
Bacteria normally reproduce by a process called binary fission:
1. The cell elongates and chromosomal DNA is replicated.
2. The cell wall and cell membrane pinch inward and begin to divide.
3. The pinched parts of the cell wall meet, forming a cross wall
completely around the divided DNA.
4. The cells separate into two individual cells.
Some bacteria reproduce by budding. A small outgrowth or bud emerges
from the bacterium and enlarges until it reaches the size of the daughter
cell. It then separates, forming two identical cells. Some bacteria, called
filamentous bacteria (or actinomycetes), reproduce by producing chains
of spores located at the tips of the filaments. The filaments fragment and
these fragments initiate the growth of new cells.
Generation time
The generation time is the amount of time needed for a cell to
divide. This varies among organisms and depends upon the environment
they are in and the temperature of their environment. Some bacteria have
a generation time of 24 hours, although the generation time of most
bacteria is between 1 to 3 hours. Bacterial cells grow at an enormous rate.
For example, with binary fission, bacteria can double every 20 minutes.
In 30 generations of bacteria (10 hours), the number could reach one
billion
Phases of growth
There are four basic phases of growth: the lag phase, the log phase,
the stationary
phase, and the death phase.
A- Lag phase
In the lag phase there is little or no cell division. This phase can
last from one hour to several days. Here the microbial population is
involved in intense metabolic activity involving DNA and enzyme
synthesis. This is like a factory “shutting down” for two weeks in the
summer for renovations. New equipment is replacing old and employees
are working, but no product is being turned out.
B- Log phase
In the log phase, cells begin to divide and enter a period of growth
or logarithmic increase. This is the time when cells are the most active
metabolically. This is the time when the product of the factory must be
produced in an efficient matter. In this phase, however, microorganisms
are very sensitive to adverse conditions of their environment.
C- Stationary phase
This phase is one of equilibrium. The growth rate slows, the
number of dead microorganisms equals the number of new
microorganisms, and the population stabilizes. The metabolic activities of
individual cells that survive will slow down. The reasons why the growth
of the organisms stops is possibly that the nutrients have been used up,
waste products have accumulated, and drastic harmful changes in the pH
of the organisms environment have occurred. There is a device called a
chemostat that drains off old, used medium and adds fresh medium. In
this way a population can be kept in the growth phase
indefinitely.
D- Death phase
Here the number of dead cells exceeds the number of new cells.
This phase continues until the population is diminished or dies out
entirely.
Nutritional types in bacterial metabolism
Nutritional
type
*Phototrophs
* Lithotrophs
Source of
energy
Sunlight
Source of carbon
Examples
Organic compounds Cyanobacteria, Green
(photoheterotrophs) sulfur bacteria,
or carbon fixation Chloroflexi, or Purple
(photoautotrophs) bacteria
Organic compounds
Thermodesulfobacteria,
Inorganic (lithoheterotrophs)
Hydrogenophilaceae, or
compounds or carbon fixation
Nitrospirae
(lithoautotrophs)
Organic Organic compounds Bacillus, Clostridium
*Organotrophs
compounds (chemoheterotrophs) or Enterobacteriaceae
or carbon fixation
(chemoautotrophs)
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