The effect of NaCl on growth and volatile metabolites produced by antagonistic endophytic bacteria isolated from Prosopis cineraria

Previously, we reported that endophytic Pseudomonas aeruginosa PC5 isolated from Prosopis cineraria, abundantly produced antimicrobial volatile organic compounds (VOCs) and significantly suppressed the growth of Pythium aphanidermatum which causes damping-off in cucumber in in vitro assays. However, under pot culture conditions, no significant effect of this bacterium on cucumber damping-off was noticed under salt water irrigation (100 mM NaCl), while the antagonistic bacterium Acinetobacter johnsonii PC3 isolated from the same host which produced low levels of antimicrobial VOCs was effective in reducing the disease incidence under salt water irrigation. In this in vitro study, the effect of NaCl on the growth P. aphanidermatum and VOC composition of these two endophytic bacterial strains was evaluated. NaCl had inhibitory effects on the growth of both P. aeruginosa PC5 and A. johnsonii PC3. Gas chromatography-mass spectrometry analysis of the VOCs produced by these endophytes revealed that dimethyl disulfide (DMDS), a well-known antimicrobial compound, was the major compound (33.82%) released by P. aeruginosa PC5 in the absence of NaCl, whereas the production of DMDS was completely inhibited when it was cultured in a medium containing high concentrations of NaCl. Trimethylsilanol was released abundantly by A. johnsonii PC3 when grown in the salt-amended medium compared to control, suggesting possible involvement of this compound in the suppression of P. aphanidermatum by this bacterium under salt-stress conditions.