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 ultraphospha te (UP), 0.1% sodium polyphosphate glassy (SPG) and 0.5% tetrasodium p yrophosphate (TSPP) were bactericidal and bacteriolytic to early-expon ential phase cells of Staphylococcus aureus ISP40 8325. In the present study, Ca2+ (0.01 M) or Mg2+ (0.01 M) reversed the bacteriolytic effe cts 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 inoculati on, protected cells from growth inhibition by UP and SPG. Moreover, th e bactericidal effects of UP or SPG were reversed by Ca2+ or Mg2+ in m etal-rescue experiments in which the metals were added to polyphosphat e-containing medium after 1 h of incubation. No additive effect existe d 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 prot ect cells 1 h before the addition of TSPP. These studies show that the antibacterial effects of phosphates can be altered substantially by t he metal-ion content of the environment.