Microorganisms can be classified based on their nutritional requirements and environmental tolerances. There are three main ways they are classified: 1. By their carbon and energy sources. Organisms are either heterotrophs that require organic carbon or autotrophs that can use carbon dioxide. They also differ in their energy sources as either phototrophs using light or chemotrophs using chemicals. 2. By their oxygen requirements. Microbes are aerobic and require oxygen, anaerobic and live without oxygen, or facultative and can live with or without oxygen. 3. By their temperature, pH, and other environmental tolerances. Organisms have optimal and minimum conditions for growth and are classified

1. Classification on the basis of Nutrition
• CO2
• Energy
• Electrons

2. 1. CO2
Carbon forms the central component of proteins,
carbohydrates, nucleic acids and lipids; indeed, the living world
is based on carbon, this is the most abundant element in all
living cells, microbial or otherwise.
Carbon incorporated into biosynthetic pathways may be
derived from organic or inorganic sources, some organisms can
derive it from CO2, while others require their carbon in ‘ready-
made’, organic form.

3. Heterotroph
• A heterotroph must use one or more organic molecules as its source
of carbon.
• Most microorganisms obtain carbon in the form of organic molecules,
derived directly or indirectly from other organisms. This mode of
nutrition is the one that is familiar to us as humans (and all other
animals); all the food we eat is derived as complex organic molecules
from plants or animals. Microorganisms that obtain their carbon in
this way are described as heterotrophs.
• Examples: include all the fungi and protozoans as well as most types
of bacteria.

4. Autotroph
• An autotroph can derive its carbon from carbon dioxide.
• A significant number of bacteria and all of the algae do not, take up
their carbon preformed as organic molecules, but derive it instead
from carbon dioxide. These organisms are called autotrophs, and
again we can draw a parallel with higher organisms, where all
members of the plant kingdom obtain their carbon in a similar
fashion.

5. 2.Energy
We can also categorise microorganisms nutritionally by the way they
derive the energy they require to carry out essential cellular reactions.
1. A chemotroph obtains its energy from chemical compounds.
2. A phototroph uses light as its source of energy.

6. 3.Electrons
There is one final subdivision of nutritional categories in
microorganisms! Whether organisms are chemotrophs or phototrophs,
they need a molecule to act as a source of electrons (reducing power)
to drive their energy generating system.
1. Those able to use an inorganic electron donor such as H2O, H2S or
ammonia are called lithotrophs.
2. While those requiring an organic molecule to fulfil the role are
organotrophs.

7. Nutrirional Types of microorganisms
1. Photolithoautotroph
2. Photoorganoheterotroph
3. Chemolithoautotroph
4. Chemolithoheterotroph
5. Chemoorganoheterotroph

8. Photolithoautotroph
• Carbon source: CO2
• Energy source: Light
• Electron Source: Inorganic e donor
• Examples: Purple and green sulpher bacteria, Cynobacteria

9. Photoorganoheterotroph
• Carbon source: organic carbon
• Energy Source: Light
• Electron Source: Organic electron donor
• Examples: purple non sulpher bacteria and green non sulpher
bacteria

10. Chemolithoautotroph
• Carbon source: CO2
• Energy Source: inorganic chemicals
• Electron Source: inorganic electron donor
• Examples: Sulpher oxidizing bacteria, hydrogen oxidizing bacteria,
Nitrifying bacteria

11. Chemolithoheterotroph( mixotroph)
• Carbon source: Organic carbon but CO2 may also be used
• Energy Source: inorganic cehmicals
• Electron Source: inorganic e donors
• Example: Beggiatoa

12. Chemoorganoheterotroph
• Carbon source: organic carbon
• Energy source: organic chemicals
• Electron source: organic electron donor
• Examples: Nonphotosynthetic bacteria including pathogens, fungi and
archea

13. Classification on the basis of Oxygen
requirement
Oxygen is present as a major constituent (20%) of our atmosphere, and
most life forms are dependent upon it for survival and growth. Such
organisms are termed aerobes. Not all organisms are aerobes,
however; some anaerobes are able to survive in the absence of oxygen,
and for some this is actually a necessity.

14. 1. Aerobe: An aerobe is an organism that grows in the presence of molecular
oxygen, which it uses as a terminal electron acceptor in aerobic respiration.
2. Anaerobe: An anaerobe is an organism that grows in the absence of
molecular oxygen.
3. Obligate anaerobes: cannot tolerate oxygen at all. They are cultured in
special anaerobic chambers, and oxygen is excluded from all liquid and solid
media.
4. Facultative anaerobes: are able to act like aerobes in the presence of
oxygen, but have the added facility of being able to survive when conditions
become anaerobic
5. Aerotolerant anaerobes: are organisms that are basically anaerobic;
although they are not inhibited by atmospheric oxygen, they do not utilise
it.
6. Microaerophiles: require oxygen, but are only able to tolerate low
concentrations of it (2–10%), and are harmed by higher concentrations.

16. Microorganisms have different oxygen
requirements.
In a static culture, microorganisms occupy different regions of the medium,
reflecting their pattern of oxygen usage.
(a) Obligate aerobes must grow at or near the surface, where oxygen is able
to diffuse.
(b) Facultative anaerobes are able to adjust their metabolism to the
prevailing oxygen conditions.
(c) Obligate anaerobes, in contrast, occupy those zones where no oxygen is
present at all.
(d) Aerotolerant anaerobes do not use oxygen, but neither are they inhibited
by it.
(e) Microaerophiles have specific oxygen requirements, and can only grow
within a narrow range of oxygen tensions.

17. How can oxygen be toxic?
It seems strange to us to think of oxygen as a toxic substance; however,
it can be converted by metabolic enzymes into highly reactive
derivatives such as the superoxide free radical (O2−), which are very
damaging to cells. Aerobes and most facultative anaerobes convert this
to hydrogen peroxide, by means of the enzyme superoxide dismutase.
This is further broken down by catalase. Obligate anaerobes do not
possess either enzyme, and so cannot tolerate oxygen.

18. pH
Microorganisms are strongly influenced by the prevailing pH of their
surroundings. As with temperature, we can define minimum, optimum
and maximum values for growth of a particular type.
Acidophilic = ‘acid-loving’; a term applied to organisms that show
optimal growth in acid conditions (pH < 5.5).

19. Classification on the basis of PH

20. Most microorganisms grow best around neutrality (pH 7). Many
bacteria prefer slightly alkaline conditions but relatively few are
tolerant of acid conditions, and fewer still are acidophilic. Fungi, on the
other hand, generally prefer slightly acid conditions and therefore tend
to dominate over bacteria when these prevail. The reason for the
growth rate falling away either side of the optimum value is again due
to alterations in three-dimensional protein structure.

21. Classification on the basis of Temperature
Microorganisms as a group are able to grow over a wide range of
temperatures, from around freezing to above boiling point. For any
organism, the minimum and maximum growth temperatures define the
range over which growth is possible; this is typically about 25–30◦C.
Growth is slower at low temperatures because enzymes work less
efficiently and also because lipids tend to harden and there is a loss of
membrane fluidity.

22. Growth rates increase with temperature until the optimum
temperature is reached, and then the rate falls again (Figure 5.4). The
optimum and limiting temperatures for an organism are a reflection of
the temperature range of its enzyme systems, which in turn is
determined by their three-dimensional protein structures. The
optimum temperature is generally closer to the maximum growth
temperature than the minimum. Once the optimum value is passed,
the loss of activity caused by denaturation of enzymes causes the rate
of growth to fall away sharply.

23. 1. Mesophiles: The majority of microorganisms achieve optimal growth at
‘middling’ temperatures of around 20–45◦C; these are called mesophiles.
2. Thermophiles: have become adapted not merely to surviving, but
thriving at much higher temperatures. Typically, these would be capable
of growth within a range of about 40–80◦C, with an optimum around 50–
65◦C.
3. Extreme thermophiles (hyperthermophiles) have optimum values in
excess of this, and can tolerate temperatures in excess of 100◦C. In 2003,
a member of the primitive bacterial group called the Archaea was
reported as growing at a temperature of 121◦C, a new world record!

24. 4. Psychrophiles occupy the other extreme of the temperature range;
they can grow at 0◦C, with optimal growth occurring at 15◦C or below.
Such organisms are not able to grow at temperatures above 25◦C or so.

25. Different species occupy different temperature ranges. Microorganisms can be
categorised according to the temperature range at which they grow.