Fig. 4

Effect of exogenous Fe²⁺ and Fe³⁺ on SS2 growth. The SS2 strain was inoculated on an iron-restricted medium (containing 100 mM iron chelator 2,2′-bipyridine), and added with different concentrations of iron ions, namely Fe²⁺ (0.1, 0.2 and 0.3 mmol / L) and Fe³⁺ (0.1, 0.2 and 0.3) mmol /L), and the growth of each strain was measured at 6 h and 12 h. The addition of exogenous Fe²⁺ and Fe³⁺ exerted a significant growth reducing effect on the mutant strain compared with wild-type, complemented and overexpression strains. The columns express the means and standard deviations of three or more experiments. *, significantly different at p < 0.05; **, significantly different at p < 0.01; ***, significantly different at p < 0.001