SUMMARY: The effect of sodium chloride on the multiplication of four freshwater and three salt-water strains of sulphate-reducing bacteria was examined by determining the numbers of organisms in a known population which were viable in media of unfamiliar NaCl concentrations. The salt-water strains all produced variants viable at high NaCl concentrations; a population able to multiply in 1 %-10 % NaCl was obtained by ‘training’ one salt-water strain. They differed, however, in the frequency with which non-exacting variants appeared, ranging from a strain in which the whole population grew with 0·25 % NaCl to one which would not grow with less than 0·45 % NaCl even after repeated attempts at acclimatization. Replacement experiments indicated that the exigent strain required chloride ion; a less-exacting strain required sodium ion. The fresh-water strains produced few variants viable at NaCl concentrations above 3 %, but one strain produced variants of the salt-water type after ‘training’ to grow with 4% NaCl; sodium ion was mainly responsible for the inhibition of this strain by high NaCl concentrations. We conclude that the differences among the salt- and fresh-water strains of do not at present justify their separation into the two species and .


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