The effects of growth phase, reductions in the water activity (a(w)) of the growth medium and mild desiccation on the composition and the degree of unsaturation of cellular fatty acids (CFA) of Sinorhizobium meliloti, Bradyrhizobium elkanii and Bradyrhizobium japonicum were studied. During the course of growth, an interchange of cis-vaccenic with lactobacillic acid and a slight increase in palmitic acid were observed while other fatty acids remained constant. The degree of unsaturation was significantly higher in the exponential phase of growth. Reductions in the a(w) of the medium led to an increase in lag phase, a reduction in growth rate and maximal optical densities (OD) in stationary phase cells. A decrease in the degree of unsaturation of CFA was also observed as the a(w) was reduced from 0.999 to 0.969 and after desiccation to 83.5% relative humidity (R.H.). The changes in the degree of unsaturation of CFA observed after growth at reduced a(w) may be one of the pre-adaptation steps to endure more severe desiccation.
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