ANTIBACTERIAL MECHANISM OF LONG-CHAIN POLYPHOSPHATES IN STAPHYLOCOCCUS-AUREUS

The results of previous studies indicated that the antibacterial effec ts of long-chain polyphosphates (sodium polyphosphate glassy [SPG] and sodium ultraphosphate [UP]) to Staphylococcus aureus ISP40 8325 could be attributed to damage to the cell envelope (cell wall or cell membr ane). Also, Ca2+ (0.01 M) or Mg2+ (0.01 M) reversed the bactericidal a nd bacteriolytic effects of polyphosphates in S. aureus. In the presen t study, 0.4 M sodium chloride (NaCl) protected the cells from leakage caused by SPG and 0.6 M NaCl protected the cells from leakage by UP. Polymyxin, a peptide antibiotic that causes cell membrane damage, indu ced leakage even in the presence of 0.6 M NaCl. In the presence of 0.4 M NaCl, bacterial leakage was significantly reduced by disodium ethyl enediamine tetraacetate (EDTA), a metal chelator that causes cell wall damage. Bacterial leakage by polyphosphates was significantly greater at pH 8 than at pH 6, which suggested that metal-ion chelation was in volved in the antibacterial mechanism. A dialysis membrane (MWCO 100) was used to separate free metal and polyphosphate-bound metal. Levels of free Ca2+ and Mg2+ in polyphosphate-treated cells were significantl y lower than those of the cells without polyphosphate. This free-metal dialysis study provided chemical evidence to show that long-chain pol yphosphates interacted with S. aureus cell walls by a metal-ion chelat ion mechanism. In addition, long-chain polyphosphates were shown to bi nd to the cell wall, chelate metals, and remain bound without releasin g metal ions from the cell wall into the suspending medium. A hypothes is is proposed in which the antibacterial mechanism of long-chain poly phosphates is caused by binding of long-chain polyphosphates to the ce ll wall of early-exponential phase cells of S. aureus ISP40 8325. The polyphosphates chelate structurally essential metals (Ca2+ and Mg2+) o f the cell wall, resulting in bactericidal and bacteriolytic effects. The structurally essential metals probably form cross bridges between the teichoic acid chains in the cell walls of gram-positive bacteria.