The O-polysaccharide of Lipopolysaccharide Renders Bradyrhizobium japonicum More Sensitive to Organic Acid

초록

Lipopolysaccharide (LPS) is a major component of Gram-negative bacteria, which is composed of three domains: lipid A; the core which is an oligosaccharide consisting of an inner and outer region; and a distal repeating polysaccharide unit known as O-antigenic chain. In our previous study, we isolated an isogenic LPS- mutant of Bradyrhizobium japonicum 61A101C, which was devoid of O-polysaccharide and altered cell surface characteristics compared with the wild type strain. Subsequently, the mutated gene was identified, cloned and used to complement the LPS- mutant strain JS314 to restore the phenotype. Since it has been known 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 three B. japonicum strains, wild-type strain and the LPS- mutant JS314, along with the complemented strain. All three strains were inhibited in the presence of 3 mM of acetic acid under acidic condition (pH 5.5). However, to our surprise, in the presence of 2 mM of acetic acid the wild type and the complemented strains did not grow whereas the LPS- mutant showed considerable growth. Therefore, unlike in E. coli, it appears that the lack of O-polysaccharide of LPS renders B. japonicum more resistance to organic acid.