GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

6. MICROBIAL GROWTH

6.2. ADAPTIVE RESPONSES OF MICROORGANISMS TO STRESS FACTORS

In their natural habitats, microorganisms are exposed to numerous adverse environmental factors (such as UV and ionizing radiation, extreme temperatures, pH levels, and toxic compounds). Throughout evolution, microorganisms have developed a complex network of adaptive mechanisms that enable them to cope with specific environmental conditions and survive under stress.

There is a well-established general definition of adaptation. For instance, M.S. Hilyarov defines adaptation as a combination of morphological, physiological, behavioral, population, and other traits of a given species that ensure its specific lifestyle under particular environmental conditions.

Adaptation is also understood as the very process of developing adjustments to a given habitat. In microbiology, this latter interpretation of the term is traditional.

A distinction is made between general adaptations, partial adaptations, and preadaptations.

General adaptations represent adjustments to broad ecological niches, such as seawater, hypersaline brines of salt lakes, or hot springs. Partial adaptations refer to specialization for a specific lifestyle, such as the utilization of certain substrates or development within particular microzones. Preadaptation is an Organism's property that has adaptive value for future, yet-unrealized forms of environmental interaction. For example, the ability of bacterial endospores to withstand prolonged boiling, as well as the capacity of certain microorganisms to degrade xenobiotics—synthetic chemical compounds previously non-existent in nature—can likely be considered preadaptations, since such resistance proves lifesaving only upon anthropogenic exposure.

It is generally accepted that Cells exposed to adverse conditions experience a state of stress. Factors that induce this cellular stress state are defined as stressors (or stress factors).

Stress is classified into lethal and non-lethal. Non-lethal stress, in turn, is subdivided into programmed and unprogrammed. A fundamental difference between these Two Types of stress is that under programmed stress, the organism is fully competent to enter a dormant state (anabiosis). Under unprogrammed stress, fundamentally different anti-stress mechanisms are triggered because the organism is unprepared for dormancy. An example of programmed stress is the Temperature effect on actively growing microbial cells. Programmed stress also differs in that the unfavorable factor acts more gradually, such as through the progressive depletion of nutrients in the medium. An example is micromycetes undergoing all preliminary developmental stages to prepare for sporulation.

Of particular interest is the fact that anti-stress adaptation mechanisms are remarkably similar in eukaryotes and prokaryotes, regardless of their level of Cellular Organization. These include alterations in membrane Lipid Composition, synthesis of heat Shock Proteins and protective compounds, and anti-radical defense systems. Furthermore, All living organisms—from Bacteria to humans—respond to adverse factors by synthesizing so-called stress proteins, The amount of which is proportional to the stress intensity. It is hypothesized that stress proteins are produced by organisms to protect essential cellular proteins.

Let us examine the functioning of certain anti-stress adaptation mechanisms in microorganisms in greater detail.