Lack of O-Polysaccharide Renders Bradyrhizobium japonkum More Resistant to Organic Acid Stress

摘要

In previous studies, we isolated an isogenic LPS~-mutant of Bradyrhizobium japonicum 61A101C, which was completely devoid of O-polysaccharide and had altered cell surface characteristics. Subsequently, the mutated gene was identified, cloned, and used to complement the LPS~- mutant strain JS314 to restore the phenotype. Since it has been reported that in Escherichia coli LPS O-polysaccharide is involved in resistance to an organic acid such as acetic acid under low pH (Barua et al., Molecular Microbiology 43: 629- 640, 2002), we compared the organic acid resistance of the three B. japonicum strains; wild-type 61A101C, the LPS~-mutant JS314, and the complemented strain to determine whether the role of O-polysaccharide in the resistance to organic acid could be generalized. Growth of all three strains was inhibited by the presence of 3 mM acetic acid under acidic condition (pH 5.5). To our surprise, however, in the presence of 2 mM acetic acid, wild-type and the complemented strains did not grow while the LPS~- mutant showed a significant growth. Therefore, unlike in E. coli, the lack of O-polysaccharide of LPS appears to render B. japonicum more resistant to organic acid.