Overview of Bacteriology (page 5)
(This chapter has 6 pages)
© Kenneth Todar, PhD
ECOLOGY OF BACTERIA AND ARCHAEA
Bacteria and Archaea are present in all environments that support life.
They may be free-living, or living in associations with "higher forms"
and animals), and they are found in environments that support no other
form of life. Procaryotes have the usual nutritional requirements for
of cells, but many of the ways that they utilize and transform their
are unique. This bears directly on their habitat and their ecology.
Nutritional Types of Organisms
In terms of carbon utilization a cell may be heterotrophic or
obtain their carbon and energy for growth from organic compounds in
use C02 as a sole source of carbon for growth and obtain
energy from light (e.g. photoautotrophs) or from the oxidation
compounds (e.g. lithoautotrophs).
Most heterotrophic bacteria are saprophytes, meaning that
obtain their nourishment from dead organic matter. In the soil,
bacteria and fungi are responsible for biodegradation of
material. Ultimately, organic molecules, no matter how complex, can be
degraded to CO2 (plus H2 and H2O).
Probably no naturally-occurring organic
cannot be degraded by the combined activities of the bacteria and
Hence, most organic matter in nature is converted by heterotrophs to CO2,
only to be converted back into organic material by autotrophs that die
and nourish heterotrophs to complete the carbon cycle.
Lithotrophic procaryotes have a type of energy-producing metabolism
which is unique. Lithotrophs
(also called lithoautotrophs
or chemoautotrophs) use
compounds as sources of energy, i.e., they oxidize compounds such as H2
or H2S or NH3 to obtain electrons to feed in to
electron transport system and to produce ATP. Lithotrophs are found in
soil and aquatic environments wherever their energy source is present.
Most lithotrophs are autotrophs so they can grow in the absence of any
organic material. Lithotrophic species are found among the Bacteria and
the Archaea. Sulfur-oxidizing lithotrophs convert H2S
and So to SO4. Nitrifying bacteria convert NH3
to NO2 and NO2 to NO3; methanogenic
electrons off of H2 as a source of energy and add them
to CO2 to form CH4 (methane). Lithotrophs have an
obvious impact on the sulfur, nitrogen and carbon cycles in the
Photosynthetic bacteria convert light energy into chemical energy
growth. Most phototrophic bacteria are autotrophs so their role in the
carbon cycle is analogous to that of plants. The planktonic
are the "grass of the sea" and their form of oxygenic photosynthesis
a substantial amount of O2 in the biosphere. However, among
the photosynthetic bacteria are types of photosynthetic metabolism not
eucaryotes, including photoheterotrophy (using light as an
assimilating organic compounds as a source of carbon), anoxygenic
and unique mechanisms of CO2 fixation (autotrophy).
Photosynthesis has not been found to occur among the Archaea,
but one archaeal species employs a light-driven non photosynthetic
of energy generation based on the use of a chromophore called bacteriorhodopsin.
Adaptations to Environmental Conditions
Most procaryotes, whether they have been cultured and studied in the
laboratory, or observed growing in their natural habitats, seem to be
highly adapted to their specific environment by means of their
macromolecular structure and/or their physiologic (metabolic)
capabilities. The nutritional quality of the environment determines
whether a particular organism will be present, but so do various
physical parameters such as the availability of light and O2,
as well as the pH, temperature and salinity of the environment. As
examples, the range of procaryotic responses to oxygen and temperature
are discussed below.
Procaryotes vary widely in their response to O2
oxygen). Organisms that require O2 for growth are called obligate
aerobes; those which are inhibited or killed by O2, and
which grow only in its absence, are called obligate anaerobes;
which grow either in the presence or absence of O2 are
anaerobes. Whether or not a particular organism can exist in the
of O2 depends upon the distribution of certain enzymes such
as superoxide dismutase and catalase that are required to detoxify
oxygen radicals that are always generated by living systems in the
Procaryotes also vary widely in their response to temperature. Those
that live at very cold temperatures (0 degrees or lower) are called psychrophiles;
those which flourish at room temperature (25 degrees) or at the
of warm-blooded animals (37 degrees) are called mesophiles;
that live at high temperatures (greater than 45 degrees) are thermophiles.
The only limit that seems to be placed on growth of certain procaryotes
in nature relative to temperature is whether liquid water exists.
growing procaryotic cells can be found in supercooled environments (ice
does not form) as low as -20 degrees and superheated environments
does not form) as high as 120 degrees. Archaea have been detected
thermal vents on the ocean floor where the temperature is as high as
The biomass of procaryotic cells in the biosphere, their metabolic
and their persistence in all habitats that support life, ensures that
these microbe will play a crucial role in the cycles of elements and
of the world ecosystem. However, the procaryotes affect the world
in another significant way through their inevitable interactions with
plants and animals. Some bacteria are required to associate with
animals or plants for the latter to survive. For example, the sex of
of certain insects is determined by endosymbiotic bacteria. Ruminant
(cows, sheep, etc.), whose diet is mainly cellulose (plant material),
have cellulose-digesting bacteria in their intestine to convert the
to a form of carbon that the animal can assimilate. Leguminous plants
poorly in nitrogen-deprived soils unless they are colonized by
bacteria which can supply them with a biologically-useful form of
Some bacteria are parasites of plants or animals, meaning
they grow at the expense of their eucaryotic host and may damage, harm,
or even kill it in the process. Such bacteria that cause disease in
or animals are pathogens. Human diseases caused by bacterial
include tuberculosis, whooping cough, diphtheria, tetanus, gonorrhea,
pneumonia, cholera and typhoid fever, to name a few. The bacteria that
cause these diseases have special structural or biochemical properties
that determine their virulence or pathogenicity. These include: (1)
to colonize and invade their host; (2) ability to resist or withstand
antibacterial defenses of the host; (3) ability to produce various
substances that damage the host. Plant diseases, likewise, may be
by bacterial pathogens. More than 200 species of bacteria are
with plant diseases, but a very small handful of genera are involved.
Figure 14. Borrelia
This spirochete is the bacterial parasite that causes Lyme disease. CDC.