Growth status of rhizobia in relation to their tolerance to low water activities and desiccation stresses

Two strains of Rhizobium meliloti and one of R. japonicum were grown in broth adjusted to selected water activities ( a w ) using KCl, NaCl, LiCl or glycerol. The optimum doubling time for R. meliloti RCR 2011 of 2.5 h (at a w 0.999) was not affected but the yield at the stationary phase was decreased by a reduction of the a w to 0.99. Regardless of the solute used, as the a w was reduced below 0.99 growth rates and yields decreased and lag times were lengthened. The extent to which these responses changed depended on the solute used to lower the a w . Lag times were affected most when LiCl, NaCl and KCl were used to lower a w below 0.9866. No growth occurred at a w 0.969 when NaCl or KCl were used but consistent growth did occur at this a w when glycerol or LiCl were employed. In contrast with these results, LiCl was more toxic than NaCl for the slow-growing R. japonicum 5. This strain was susceptible to a w 0.9840 (glycerol). Prior growth of R. meliloti RCR 2011 and 1.5 in media of lowered a w improved the tolerance of strain RCR 2011 but not of strain 1.5 to desiccation (0% relative humidity). The growth stage of R. meliloti RCR 2011 affected the survival of this strain during storage at different r.h. values. When subjected to drought stress, stationary phase cells survived better than cells collected at three different times during the exponential growth phase. The optimum r.h. value for storage of R. meliloti strain RCR 2011 was within the range 22–43%. Extracellular polysaccharides provided protection against storage at 0% r.h. but their presence was detrimental during storage in semi-dried states.