FIG. 6.

Effects of Mg²⁺ on several reference bacteria and rhizoplane bacteria. Test bacteria were inoculated into Mg²⁺-free HSG medium (open bars) or HSG medium containing 0.98 mM Mg²⁺ (shaded bars) and were incubated for 2 days. Among the test bacteria, those originally isolated from plant surfaces responded to Mg²⁺, but the others, from mammals, showed no growth in HSG medium containing 0.98 mM Mg²⁺. When the cell growth is nearly negative in the Mg²⁺-free medium while clearly positive in the Mg²⁺-containing medium, the threshold level of required Mg²⁺ for the bacterium is thought to be comparatively high (e.g., S. yanoikuyae EC-S001 or B. subtilis IFO12113). On the other hand, bacteria which grow well in the Mg²⁺-free medium are regarded as possessing a low threshold level of Mg²⁺ (e.g., B. cepacia EC-K014). Values are averages from triplicate experiments after 48 h of incubation. Error bars, standard deviations. All experiments were performed in triplicate (n = 3), except for S. yanoikuyae EC-S001 (n = 9) and B. subtilis IFO12113 (n = 6).