Process Control Minitoring by Stress Response Page: 4 of 15
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important for optimizing local physical/chemical environment, they do not provide protection
from rapid system-wide changes. Since running away is generally not an option, the microbial
alternative is rapid adaptation, often involving one or more systems of stress response. Although
microorganisms in the open environment are often stressed, and simultaneously exposed to
multiple stressors, the study of microbial stress response has been primarily restricted to
laboratory systems, considering the response of well-characterized "model" organisms to very
specific types of stress. This somewhat reductionist characterization of stress has resulted in a
somewhat unsatisfying set of definitions of microbial stress, generally including the following:
" Any deviation from optimal growth conditions that results in reduced growth rate
" An environmental situation that results in damage of cellular components in the absence
of a cellular response
" Any situation that stimulates expression of known stress-response genes
The laundry list of better-studied stressors and microbial response systems includes: starvation,
heat shock, cold shock, envelope stress, oxidative stress, oxygen deprivation, osmotic challenges,
acid stress, sodium stress, and SOS response to DNA damage. Stress is also relative -
temperature and pH that are stressful to one species may be optimal for the growth of another.
Thus, we suggest the need for a more environmentally relevant definition of stress, one that
incorporates the concept of microbial niche and the adaptive landscape. The most generally
accepted definition of niche is that proposed by Hutchinson  - "An N-dimensional
hypervolume of environmental conditions within which the organism can maintain a
population". There are two categories of environmental conditions: 1) physical/chemical (e.g.,
temperature, salinity, flow, pressure) and resources (e.g., nutrients, energy sources, space). The
adaptive landscape for any one species includes: 1) the range of conditions in which the species
functions competitively and can maintain a population, 2) an adaptive range in which the
population can function but is no longer competitive, and 3) an adaptive limit at which
individuals can not persist. For example, organisms transiently exposed to an adaptive limit of
temperature elicit the heat shock stress response that serves for their recovery. However, they
cannot persist at the elevated temperature. It is also evident that the adaptive limit for any one
stressor may change when the organism is exposed simultaneously to multiple stressors.
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Hazen, Terry C. & Stahl, David A. Process Control Minitoring by Stress Response, article, April 17, 2006; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc898669/m1/4/: accessed April 22, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.