2. • Microbial symbiotic associations, which are
pervasive, can be beneficial (mutualistic),
neutral (commensal), or harmful (pathogenic)
to the host animal.
• Within symbioses, microbes can exploit the
host for space and nutrients or as a vector for
dissemination to other environments.
• Both within and between hosts, microbes
experience changing environmental conditions
to which they must adapt for optimal fitness
and for maintenance of symbiotic associations.
3. • Microorganisms are adapted for optimum functioning in
their normal physiological environments.
• Any extreme change in environmental conditions from the
optimum inflicts a stress on an organism.
• The extent of the change will determine whether the
organism is killed, ceases growth, or has an increased lag
time and reduced growth rate.
• Most bacteria are able to tolerate small changes in an
environmental parameter and can adapt over the time scale
of minutes, hours, or days (Hill and others 1995).
• Microorganisms do this by both yielding to the stress
conditions and making suitable provisions for survival or
attempting to resist the stress (Herbert 1989). For most
organisms, this tolerance can be pushed to maximum limits
if the cell is provided with sufficient opportunity to sense
and adapt to the deteriorating environment.
4. Microorganisms will produce proteins and enzymes to
adapt to various environments
• Entire groups of microorganisms such as psychrophiles,
acidophiles, and halophiles have adapted their lifestyles to
prefer these extreme environments. Psychrophiles are
microorganisms that can grow at 0 °C and have an optimum
growth temperature of 15 °C or less and a temperature
maximum of around 20 °C.
• Acidophiles are microorganisms that have their growth
optimum between about pH 1.0 and 5.5, and halophiles are
microorganisms that require high levels of sodium chloride
for growth such as 2.8 molal and up to 6.2 molal for
• Changes in environmental conditions away from the
optimal value can cause the induction of many elaborate
• These strategies are generally directed at survival rather
5. Extreme Environments
• Microorganisms are found in a wide range of environments that differ in
pH, temperature, atmospheric pressure, salinity, water availability, and
• In some environments, these conditions can be at either end of a continuum
(e.g., very alkaline or acidic; extremely hot or cold). Such environments are
called extreme environments.
• The microorganisms that survive in such environments are described as
• Although extreme environments are usually considered to have decreased
microbial diversity, as judged by the microorganisms that can be cultured,
with the increased use of molecular detection techniques, it appears that
many contain a surprising diversity of microorganisms.
• Work to establish relationships between the microorganisms that can be
observed and detected by molecular techniques and culturable
microorganisms is ongoing.
• Many microbial genera have specific requirements for survival in extreme
environments. For example, a high sodium ion concentration is required to
maintain membrane integrity in many halophilic procaryotes, including
members of the genus Halobacterium. Halobacteria require a sodium ion
concentration of at least 1.5 M, and about 3 to 4 M for optimum growth.
6. • Microbial life at high salt concentrations is phylogenetically
very diverse. Hypersaline environments with salt
concentrations up to NaCl saturation are inhabited by
halophilic and highly halotolerant representatives of all
three domains of life: Archaea, Bacteria and Eukarya.
• The mechanisms used by these salt-requiring or highly salt-
tolerant microorganisms to withstand the high salt
concentrations, and in many cases also to adapt their
physiology to changes in the salt concentrations in their
environments, are diverse as well.
• Biological membranes are permeable to water. Therefore,
water moves into and out of cells driven by differences in
water activity between the cytoplasm and the outside
8. • To maintain a high osmotic pressure inside the
cells, different strategies can be used, which are
• The ‘salt-in’ strategy where osmotic balance is
achieved by accumulating high concentrations of
inorganic salts in the medium. As Na+ ions are
excluded as much as possible from cells in all
three domains of life, the ‘salt-in’ strategy is
based on KCl rather than on NaCl as the main