Effect of osmotic stress on Astragalus cicer microsymbiont growth and survival

• The osmoprotective effect of glycine betaine on milkvetch microsymbionts growing under osmotic stress conditions was observed.
• The entry of bacterial cells into the viable but nonculturable state (VBNC) during higher concentration of sucrose and PEG.
• Astragalus cicer microsymbionts were able to develop biofilm under stress conditions (PEG, KCl, sucrose).

In this study, the effect of various concentrations of stress inductors such as KCl, sucrose, and polyethylene glycol (PEG) on the growth and survival of Astragalus cicer microsymbionts in the presence and absence of an osmoprotectant, glycine betaine (GB), was determined. It was found that the higher concentrations of the stress factors lengthened the generation time and, additionally, in the case of sucrose and PEG, caused entry of the cells into the viable but nonculturable state (VBNC). The presence of the osmoprotectant, glycine betaine (GB), significantly decreased the effects of stress induced by KCl, sucrose, and PEG, as indicated by reduction of the generation time of A. cicer isolates, an increase in the proportion of living to dead cells in comparison to bacteria under stress conditions but in the absence of GB. We also investigated the ability of A. cicer symbionts to form biofilm in the different stress conditions. It was demonstrated that PEG enhanced biofilm formation, whereas KCl and sucrose at higher concentrations negatively affected this process.