Microbial nutrient requirements (part 2)

1. Microbial nutrient requirements
By
Mr. K. VIJAY,
Assistant Professor,
Sacred Heart College,
Tirupattur District.

2. Requirements for Carbon, Hydrogen, and Oxygen
The requirements for carbon, hydrogen, and oxygen
often are satisfied together.
Carbon is needed for the skeleton or backbone of
all organic molecules, and molecules serving as carbon
sources normally also contribute both oxygen and
hydrogen atoms.
They are the source of all three elements. Because
these organic nutrients are almost always reduced and
have electrons that they can donate to other molecules,
they also can serve as energy sources.

3. Indeed, the more reduced organic molecules are, the
higher their energy content (e.g., lipids have a higher
energy content than carbohydrates).
This is because, as we shall see later, electron transfers
release energy when the electrons move from reduced
donors with more negative reduction potentials to
oxidized electron acceptors with more positive
potentials.
Thus carbon sources frequently also serve as energy
sources.

4. One important carbon source that does not supply
hydrogen or energy is carbon dioxide (CO2). This is
because CO2 is oxidized and lacks hydrogen.
Probably all microorganisms can fix CO2 - that is, reduce
it and incorporate it into organic molecules.
However, by definition, only autotrophs can use CO2 as
their sole or principal source of carbon. Many
microorganisms are autotrophic, and most of these carry
out photosynthesis and use light as their energy source.
Some autotrophs oxidize inorganic molecules and derive
energy from electron transfers.

5. A most remarkable nutritional characteristic of
microorganisms is their extraordinary flexibility with
respect to carbon sources.
Laboratory experiments indicate that there is no
naturally occurring organic molecule that cannot be used
by some microorganism.
Actinomycetes will degrade amyl alcohol, paraffin, and
even rubber. Some bacteria seem able to employ almost
anything as a carbon source; for example, Burkholderia
cepacia can use over 100 different carbon compounds.

6. In contrast to these bacterial omnivores, some bacteria
are exceedingly fastidious and catabolize only a few
carbon compounds.
Cultures of methylotrophic bacteria metabolize methane,
methanol, carbon monoxide, formic acid, and related
one-carbon molecules.
Parasitic members of the genus Leptospira use only
long-chain fatty acids as their major source of carbon
and energy.

7. In natural environments complex populations of
microorganisms often will metabolize even relatively
indigestible human-made substances such as
pesticides.
Indigestible molecules sometimes are oxidized and
degraded in the presence of a growth promoting nutrient
that is metabolized at the same time, a process called
cometabolism.
The products of this breakdown process can then be
used as nutrients by other microorganisms.

8. Carbon Sources
The extracellular source of carbon as opposed to the
intracellular function of carbon compounds.
Although a distinction is made between the type of
carbon compound cells absorb as nutrients (inorganic or
organic), the majority of carbon compounds involved in
the normal structure and metabolism of all cells are
organic.

9. A heterotroph is an organism that must obtain its
carbon in an organic form. Because organic carbon
originates from the bodies of other organisms,
heterotrophs are dependent on other life forms.
Among the common organic molecules that can satisfy
this requirement are proteins, carbohydrates, lipids, and
nucleic acids.
In most cases, these nutrients provide several other
elements as well. Some organic nutrients available to
heterotrophs already exist in a form that is simple
enough for absorption (for example, monosaccharides
and amino acids), but many larger molecules must be
digested by the cell before absorption.

10. Moreover, heterotrophs vary in their capacities to use
various organic carbon sources. Some are restricted to a
few substrates, whereas others (certain Pseudomonas
bacteria, for example) are so versatile that they can
metabolize more than 100 different substrates.
An autotroph is an organism that uses CO2, an
inorganic gas, as its carbon source. Because autotrophs
have the special capacity to convert CO2 into organic
compounds, they are not nutritionally dependent on
other living things. In a later section, we enlarge on the
topic of nutritional types as based on carbon and energy
sources.

11. Nitrogen Sources
The main reservoir of nitrogen is nitrogen gas (N2),
which makes up about 79% of the earth’s atmosphere.
This element is indispensable to the structure of
proteins, DNA, RNA, and ATP.
Such nitrogenous compounds are the primary source for
heterotrophs, but to be useful, they must first be
degraded into their basic building blocks (proteins into
amino acids; nucleic acids into nucleotides).

12. Some bacteria and algae utilize inorganic nitrogenous
nutrients (NO3 , NO2 , or NH3). Asmall number of
bacteria can transform N2 into compounds usable by
other organisms through the process of nitrogen fixation.
Regardless of the initial form in which the inorganic
nitrogen enters the cell, it must first be converted to NH3,
the only form that can be directly combined with carbon
to synthesize amino acids and other compounds.

13. Oxygen Sources
Because oxygen is a major component of organic
compounds such as carbohydrates, lipids, and proteins,
it plays an important role in the structural and enzymatic
functions of the cell.
Oxygen is likewise a common component of inorganic
salts such as sulfates, phosphates, nitrates, and water.
Free gaseous oxygen (O2) makes up 20% of the
atmosphere. It is absolutely essential to the metabolism
of many organisms

14. Hydrogen Sources
Hydrogen is a major element in all organic compounds
and several inorganic ones, including water (H2O), salts
(Ca[OH]2), and certain naturally occurring gases (H2S,
CH4, and H2). These gases are both used and produced
by microbes.
Hydrogen performs the following overlapping roles in the
biochemistry of cells:
(1) maintaining pH,
(2) forming hydrogen bonds between molecules, and
(3) serving as the source of free energy in oxidation-
reduction reactions of respiration

15. Phosphorus (Phosphate) Sources
The main inorganic source of phosphorus is phosphate
(PO4), derived from phosphoric acid (H3PO4) and found in
rocks and oceanic mineral deposits.
Phosphate is a key component of nucleic acids and is
thereby essential to the genetics of cells and viruses.
Because it is found in the nucleotide ATP, it also serves in
cellular energy transfers. Other phosphate-containing
compounds are phospholipids in cell membranes and
coenzymes such as NAD.
Phosphate can be so scarce in the environment that its
rarity severely limits growth, which explains why bacteria
such as Corynebacterium concentrate it in metachromatic
granules.

16. Sulfur Sources
Sulfur is widely distributed throughout the environment in
mineral form.
Rocks and sediments (such as gypsum) can contain
sulfate (SO4), sulfides (FeS), hydrogen sulfide gas
(H2S), and elemental sulfur (S). Sulfur is an essential
component of some vitamins (vitamin B1) and the amino
acids cysteine and methionine .
Cysteine contributes to the shape and structural stability
of proteins by forming unique linkages called disulfide
bonds.

17. Other Nutrients Important in Microbial Metabolism
The remainder of important elements are mineral ions.
Potassium is essential to protein synthesis and
membrane function.
Sodium is important for some types of cell transport.
Calcium is a stabilizer of the cell wall and endospores
of bacteria. It is also combined with carbonate (CO3) in
the formation of shells by foraminiferans and
radiolarians.
Magnesium is a component of chlorophyll and a
stabilizer of membranes and ribosomes.
Iron is an important component of the cytochrome
pigments of cell respiration.

18. Zinc is an essential regulatory element for eucaryotic
genetics. It is a major component of “zinc fingers”-
binding factors that help enzymes adhere to specific
sites on DNA.
Copper, cobalt, nickel, molybdenum, manganese,
silicon, iodine, and boron are needed in small amounts
by some microbes but not others.
A discovery with important medical implications is that
metal ions can directly influence certain diseases by
their effects on microorganisms.
For example, the bacteria that cause gonorrhea and
meningitis grow more rapidly in the presence of iron
ions.

19. Growth Factors: Essential Organic Nutrients
Few microbes are as versatile as Escherichia coli in
assembling molecules from scratch. Many fastidious
bacteria lack the genetic and metabolic mechanisms to
synthesize every organic compound they need for
survival.
An organic compound such as an amino acid, nitrogen
base, or vitamin that cannot be synthesized by an
organism and must be provided as a nutrient is a
growth factor.

20. For example, although all cells require 20 different
amino acids for proper assembly of proteins, many cells
cannot synthesize them all.
Those that must be obtained from food are called
essential amino acids. A notable example of the need
for growth factors occurs in Haemophilus influenzae, a
bacterium that causes meningitis and respiratory
infections in humans.
It can grow only when hemin (factor X), NAD (factor V),
thiamine and pantothenic acid (vitamins), uracil, and
cysteine are provided by another organism or a growth
medium.