Oxidative stress in microbial cells shares many similarities with other cell types but it has its specific features which may differ in prokaryotic and eukaryotic cells. We survey here the properties and actions of primary sources of oxidative stress, the role of transition metals in oxidative stress and cell protective machinery of microbial cells, and compare them with analogous features of other cell types. Other features to be compared are the action of Reactive Oxygen Species (ROS) on cell constituents, secondary lipid- or protein-based radicals and other stress products. Repair of oxidative injury by microorganisms and proteolytic removal of irreparable cell constituents are briefly described. Oxidative damage of aerobically growing microbial cells by endogenously formed ROS mostly does not induce changes similar to the aging of multiplying mammalian cells. Rapid growth of bacteria and yeast prevents accumulation of impaired macromolecules which are repaired, diluted or eliminated. During growth some simple fungi, such as yeast or Podospora spp., exhibit aging whose primary cause seems to be fragmentation of the nucleolus or impairment of mitochondrial DNA integrity. Yeast cell aging seems to be accelerated by endogenous oxidative stress. Unlike most growing microbial cells, stationary-phase cells gradually lose their viability because of a continuous oxidative stress, in spite of an increased synthesis of antioxidant enzymes. Unlike in most microorganisms, in plant and animal cells a severe oxidative stress induces a specific programmed death pathway--apoptosis. The scant data on the microbial death mechanisms induced by oxidative stress indicate that in bacteria cell death can result from activation of autolytic enzymes (similarly to the programmed mother-cell death at the end of bacillary sporulation). Yeast and other simple eukaryotes contain components of a proapoptotic pathway which are silent under normal conditions but can be activated by oxidative stress or by manifestation of mammalian death genes, such as bak or bax. Other aspects, such as regulation of oxidative-stress response, role of defense enzymes and their control, acquisition of stress tolerance, stress signaling and its role in stress response, as well as cross-talk between different stress factors, will be the subject of a subsequent review.
OSD definitions vary between European countries and are not directly comparable, which hampers comparisons between statistics collected in different countries. Awareness of this fact and further efforts for standardization are necessary.
The course of protein degradation during growth of a [14C]leucine-labelled population of BaciZZus megaterium with a surplus of the non-radioactive amino acid indicated the presence of a labile protein fraction decaying with a half-life of less than 1 h. The half-life of the remaining 'stable' fraction was much longer (40 h or more). A nutrient shift-down increased, and a shift-up decreased the relative size of the labile fraction and the rate of degradation of the 'stable' fraction. When bacteria were prelabelled in the presence of ethionine, both the size of the labile fraction and the rate of degradation of the 'stable' fraction were increased. A shift-up in temperature caused a large increase in the size of the labile fraction while the rate of degradation of 'stable' proteins increased only slightly. The rate of degradation of the labile fraction was not changed significantly by any treatment. The results suggest that the main target of regulation of protein turnover by environmental conditions is the relative size of the labile protein fraction. I N T R O D U C T I O NPartial or complete degradation of protein molecules in growing bacterial populations appears to play a role primarily in protein processing or the removal of abnormal proteins, such as the products of translational errors. The half-lives of such processes are probably no longer than several minutes (Pine, 1970; Goldberg, 1972; Bukhari & Zipser, 1973). In a special case, the germination of spores of bacilli, the labile protein fraction is formed by several kinds of proteins, synthesized during sporogenesis; their degradation supplies the germinated spore with amino acids making possible a rapid onset of protein synthesis (Setlow, 1975). However, most proteins in growing bacteria are relatively stable and their degradation rates do not usually exceed 1 % h-' (Goldberg & St John, 1976).The general rate of protein turnover in Escherichia coli is regulated by nutrition as well as by temperature (Pine, 1973). These conclusions were derived mostly from experiments in which the turnover rate was calculated from samples taken at one or two time intervals. Our study of protein turnover in Bacillus megaterium is based on the measurement of the kinetics of protein degradation, from which the proportions of the labile and 'stable' fractions and their degradation constants could be separately determined. We found that the size of the labile fraction, with a half-life of less than 1 h, and to a lesser extent the degradation rate of the 'stable' fraction, are influenced by cultivation conditions. M E T H O D SThe asporogenous strain of Bacillus megaterium KM, and the composition of mineral C medium were described previously (Chaloupka et al., 1975). The medium was supplemented with glucose ( 5 mg ml-I). When the influence of different media was studied, either NH,Cl was omitted and replaced with m-leucine (5 mM) as the sole nitrogen source (C -N + L), or casein hydrolysate (Oxoid; 10mgml-I) was added (C + AA). The media were supplemented with m-leucine (+L; ...
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