γ-Polyglutamic acid (γ-PGA) produced by Bacillus amyloliquefaciens C06 promoting its colonization on fruit surface

Bacillus amyloliquefaciens C06, an effective biological agent in controlling brown rot of stone fruit caused by Monilinia fructicola, was also found to produce extra-cellular mucilage and form mucoid colonies on semi-solid surfaces. This study aimed to characterize the extra-cellular mucilage produced by B. amyloliquefaciens C06 using transposon mutagenesis and biochemical and physical analyses. The mucilage production in B. amyloliquefaciens C06 was demonstrated to be associated with ywsC gene expression and characterized to be of high molecular weight, consisted of only glutamic acid and linked with non-peptide bonds, thus identified as γ-polyglutamic acid (γ-PGA). Compared with wild type B. amyloliquefaciens C06, its mutants deficient in producing γ-PGA, e.g. M106 and C06ΔywsC showed less efficiency in biofilm formation, surface adhesion and swarming ability. It was also demonstrated that γ-PGA was not essential for C06 to form colony on semi-solid surfaces, but was able to improve its colony structure. In vivo evaluation showed that disruption of γ-PGA production in C06ΔywsC impaired its efficiency of colonizing apple surfaces.