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Bacteria able to grow in the absence of salt as well as in the presence of relatively high salt concentrations (e.g., 8% in the case of Staphylococcus aureus) are designated halotolerant (or extremely halotolerant if growth extends above 2.5 M). Although several classifications or categories have been proposed ( 274, 329, 351), the most widely used is that of Kushner, who defined moderate halophiles as organisms growing optimally between 0.5 and 2.5 M salt ( 168). To describe microorganisms according to their behavior toward salt, different classification schemes have been devised. To quote from Hof’s paper: “it may be concluded that most of the important groups of bacteria are able to live in concentrations up to about 15% salt and that many groups are physiologically active even at much higher salt concentrations.”
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This organism, designated Pseudomonas beijerinckii, grew from 3 to 18% salt but not at 0.5%, showing its obligate halophilic character. Using media containing between 12 and 18% salt, she isolated a Pseudomonas-type bacterium from salted beans preserved in brines varying in salt concentration from 6 to 29%. In addition to red archaeal types, different types of white colonies were isolated, including endospore-containing Bacillus species able to grow at 24% NaCl. An early classic study of halophilic bacteria is the work of Hof ( 127), who inoculated salt mud from a solar salt facility on Java onto a variety of media of different salinities. One of the bacterial groups isolated grew in 0 to 15% NaCl, whereas other bacteria studied exhibited growth over the range of 5 to 25% ( 178). The occurrence of nonpigmented halotolerant bacteria was probably first mentioned in 1919 by LeFevre and Round in their study of the microbiology of cucumber fermentation brines. This in itself is a feat that may be much more difficult to achieve than the rigid, salt-requiring metabolism of the halophilic archaea, which lyse the moment the salt concentration in their environment drops below 10 to 15%. Thus, species such as Salinivibrio costicola and Halomonas halodenitrificans are able to grow over a range of water activities between 0.98 (close to freshwater) to 0.86 (close to saturated NaCl) ( 168).
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If the last decade has been that of the extreme halophiles, we can hope that the next one will see their more modest, moderate cousins (in the spiritual sense only) take their proper place in the scientific canon.” The moderately halophilic bacteria pose specific questions to the scientist, many of them related to their adaptability to a wide range of salinities. Further work on these relatively little-studied microorganisms may be expected to bring dividends in the form of insight on the relation of internal and external solute concentrations, and on the state of cell-associated ions within the cytoplasm. However, Kushner ( 168) clearly states: “Though they are less exciting at first glance than the extreme halophiles the moderately halophilic bacteria, and solute-tolerant microorganisms in general, pose quite sufficiently interesting questions, especially those implied by their ability to grow over wide ranges of solute concentrations. During early research on the microbiology of hypersaline environments, the halophilic bacteria were often neglected, even though they inhabit a wide range of habitats such as saline lakes, saltern ponds, desert and hypersaline soils, and salted foods, a range much less restricted than the habitats in which the halophilic archaea thrive ( 276, 284, 285). Research on the halophilic and halotolerant bacteria often seems to be less glamorous than the study of the archaea, with their unique adaptations, including a highly saline cytoplasm, specialized salt-requiring proteins, and the unique light-driven proton and chloride pumps bacteriorhodopsin and halorhodopsin ( 171). Compared to the extensive literature on the physiology, biochemistry, and ecology of the aerobic red halophilic archaea (family Halobacteriaceae), the aerobic halophilic bacteria have been relatively little studied.