Metal Ions Reverse the Inhibitory Effects of Selected Food-Grade Phosphates in Staphylococcus aureus

Previous studies showed that levels as low as 0.1% sodium ultraphosphate (UP), 0.1% sodium polyphosphate glassy (SPG) and 0.5% tetrasodium pyrophosphate (TSPP) were bactericidal and bacteriolytic to early-exponential phase cells of Staphylococcus aureus ISP40 8325. In the present study, Ca²⁺ (0.01 M) or Mg²⁺ (0.01 M) reversed the bacteriolytic effects of UP (0.1%) and SPG (0.1%) to S. aureus . In addition, Ca²⁺ (0.01 M) or Mg²⁺ (0.01 M), when added to the culture medium before inoculation, protected cells from growth inhibition by UP and SPG. Moreover, the bactericidal effects of UP or SPG were reversed by Ca²⁺ or Mg²⁺ in metal-rescue experiments in which the metals were added to polyphosphate-containing medium after 1 h of incubation. No additive effect existed between Mg²⁺ and Ca²⁺. Growth inhibition of TSPP was not reversed by Mg²⁺ or Ca²⁺, but it was reversed by Fe³⁺ when Fe³⁺ was added to protect cells 1 h before the addition of TSPP. These studies show that the antibacterial effects of phosphates can be altered substantially by the metal-ion content of the environment.