Nutrition and Growth of Bacteria (page 4)
(This chapter has 6 pages)
© Kenneth Todar, PhD
Physical and Environmental Requirements for
The procaryotes exist in nature under an enormous range of physical
conditions such as O2 concentration, Hydrogen ion
(pH) and temperature. The exclusion limits of life on the planet, with
regard to environmental parameters, are always set by some
most often a procaryote, and frequently an Archaeon. Applied to all
is a vocabulary of terms used to describe their growth (ability to
within a range of physical conditions. A thermophile grows at high
an acidiphile grows at low pH, an osmophile grows at high solute
and so on. This nomenclature will be employed in this section to
the response of the procaryotes to a variety of physical conditions.
The Effect of Oxygen
Oxygen is a universal component of cells and is always provided in
amounts by H2O. However, procaryotes display a wide range of
responses to molecular oxygen O2 (Table 6).
Obligate aerobes require O2 for growth; they use O2
as a final electron acceptor in aerobic respiration.
Obligate anaerobes (occasionally called aerophobes)
not need or use O2 as a nutrient. In fact, O2 is
a toxic substance, which either kills or inhibits their growth.
anaerobic procaryotes may live by fermentation, anaerobic respiration,
bacterial photosynthesis, or the novel process of methanogenesis.
Facultative anaerobes (or facultative aerobes) are
that can switch between aerobic and anaerobic types of metabolism.
anaerobic conditions (no O2) they grow by fermentation or
respiration, but in the presence of O2 they switch to
Aerotolerant anaerobes are bacteria with an exclusively
(fermentative) type of metabolism but they are insensitive to the
of O2. They live by fermentation alone whether or not O2
is present in their environment.
Table 6. Terms used to
O2 Relations of Microorganisms.
||Required (utilized for aerobic respiration)
||Growth if level not too high
||Required but at levels below 0.2 atm
|Facultative Anaerobe (Facultative Aerobe)
||Not required for growth but utilized when
||Not required and not utilized
The response of an organism to O2 in its environment depends
upon the occurrence and distribution of various enzymes which react
O2 and various oxygen radicals that are invariably generated
by cells in the presence of O2. All cells contain enzymes
of reacting with O2. For example, oxidations of
by O2 invariably result in the formation of H2O2
(peroxide) as one major product and small quantities of an even more
free radical, superoxide or O2.-. Also,
and other pigments in cells can react with O2 in the
of light and generate singlet oxygen, another radical form of oxygen
is a potent oxidizing agent in biological systems.
In aerobes and aerotolerant anaerobes the potential for lethal
of superoxide is prevented by the enzyme superoxide dismutase (Figure
All organisms which can live in the presence of O2 (whether
or not they utilize it in their metabolism) contain superoxide
Nearly all organisms contain the enzyme catalase, which decomposes H2O2.
Even though certain aerotolerant bacteria such as the lactic acid
lack catalase, they decompose H2O2 by means of
enzymes which derive electrons from NADH2 to reduce peroxide
to H2O. Obligate anaerobes lack superoxide dismutase and
and/or peroxidase, and therefore undergo lethal oxidations by various
radicals when they are exposed to O2. See Figure 3 below.
All photosynthetic (and some nonphotosynthetic) organisms are
from lethal oxidations of singlet oxygen by their possession of
pigments which physically react with the singlet oxygen radical and
it to its nontoxic "ground" (triplet) state. Carotenoids are said to
singlet oxygen radicals.
Figure 3. The action of
dismutase, catalase and peroxidase. These enzymes detoxify oxygen
that are inevitably generated by living systems in the presence of O2.
The distribution of these enzymes in cells determines their ability to
exist in the presence of O2
Table 7. Distribution of
dismutase, catalase and peroxidase in procaryotes with different O2
|Obligate aerobes and most facultative
|Most aerotolerant anaerobes (e.g.
|Obligate anaerobes (e.g. Clostridia,
The Effect of pH on Growth
The pH, or hydrogen ion concentration, [H+], of natural
varies from about 0.5 in the most acidic soils to about 10.5 in the
alkaline lakes. Appreciating that pH is measured on a logarithmic
the [H+] of natural environments varies over a billion-fold
and some microorganisms are living at the extremes, as well as every
between the extremes! Most free-living procaryotes can grow over a
of 3 pH units, about a thousand fold change in [H+]. The
of pH over which an organism grows is defined by three cardinal
the minimum pH, below which the organism cannot grow, the maximum
pH, above which the organism cannot grow, and the optimum pH,
at which the organism grows best. For most bacteria there is an orderly
increase in growth rate between the minimum and the optimum and a
orderly decrease in growth rate between the optimum and the maximum pH,
reflecting the general effect of changing [H+] on the rates
of enzymatic reaction (Figure 4).
Microorganisms which grow at an optimum pH well below neutrality
are called acidophiles. Those which grow best at neutral pH are
called neutrophiles and those that grow best under alkaline
are called alkaliphiles. Obligate acidophiles, such as some Thiobacillus
species, actually require a low pH for growth since their membranes
and the cells lyse at neutrality. Several genera of Archaea, including
and Thermoplasma, are obligate acidophiles. Among eukaryotes,
fungi are acidophiles, but the champion of growth at low pH is the
alga Cyanidium which can grow at a pH of 0.
In the construction and use of culture media, one must always
the optimum pH for growth of a desired organism and incorporate buffers
in order to maintain the pH of the medium in the changing milieu of
waste products that accumulate during growth. Many pathogenic bacteria
exhibit a relatively narrow range of pH over which they will grow. Most
diagnostic media for the growth and identification of human pathogens
a pH near 7.
Figure 4. Growth rate vs pH
for three environmental classes of procaryotes. Most free-living
grow over a pH range of about three units. Note the symmetry of the
below and above the optimum pH for growth.
Table 8. Minimum, maximum
optimum pH for growth of certain procaryotes.