Anionic properties of beta-lactam-enhancing factor on methicillin-resistant Staphylococcus aureus

Does speciation matter for tungsten ecotoxicology?

Tungsten is a widely used transition metal that has not been thoroughly investigated with regards to its ecotoxicological effects. Tungsten anions polymerize in environmental systems as well as under physiological conditions in living organisms. These polymerization/condensation reactions result in the development of several types of stable polyoxoanions. Certain chemical properties (in particular redox and acidic properties) differentiate these polyanions from monotungstates. However, our current state of knowledge on tungsten toxicology, biological and environmental effects is based entirely on experiments where monotungstates were used and assumed by the authors to be the form of tungsten that was present and that produced the observed effect. Recent discoveries indicate that tungsten speciation may be important to ecotoxicology. New results obtained by different research groups demonstrate that polytungstates develop and persist in environmental systems, and that polyoxotungstates are much more toxic than monotungstates. This paper reviews the available toxicological information from the standpoint of tungsten speciation and identifies knowledge gaps and pertinent future research directions.

Effects of tungstosilicate on strains of methicillin-resistant Staphylococcus aureus with unique resistant mechanisms

In a previous study, it was found that polyoxotungstates such as undecatungstosilicate (SiW11) greatly sensitized strains of methicillin-resistant Staphylococcus aureus (MRSA) to beta-lactams. In this report, the effects of SiW11 on several MRSA strains with unique resistant mechanisms were studied. SiW11 was still effective to MRSA mutants with higher beta-lactam resistance due to reduced cell-lytic activity. Since the antimicrobial effect of TOC-39 (a cephem antibiotic with strong affinity to penicillin-binding protein (PBP) 2') was not strongly enhanced in any case, it was confirmed that the sensitizing effect of SiW11 is due to reduced expression of PBP2'. However, the sensitizing effect of SiW11 was relatively weak in MRSA strains with lowered susceptibility to glycopeptide antibiotics. A certain resistant mechanism other than the mecA-PBP2' system worked in such a strain. Interestingly, an MRSA mutant with the Eagle-type resistance was dramatically sensitized. This result suggests that SiW11 has another site of action besides reducing the expression of PBP2'.

The effects of tungstophosphate and tungstosilicate on various stress promoters transformed in Escherichia coli

Although tungsten is an important material in some industrial and chemical processes, the biological and biochemical effects, including the toxicity, of tungsten compounds are not known well. In this study, a reporter gene assay using special strains of Escherichia coli was performed to investigate the mode of action of two polyoxotungstates, i.e. undecatungstophosphate (PW(11)) and undecatungstosilicate (SiW(11)). When the bacterial cells were cultured with PW(11), osmY (a stress promoter gene sensitive to osmotic signals) was induced to some extent, while other stress promoters were expressed only slightly. SiW(11) gave similar results, but clpB (an analogue of human heat shock protein) was more strongly induced. It is possible that PW(11) and SiW(11) can produce an osmotic signal at lower concentrations without increasing ionic strength. Since the constituents of PW(11)/SiW(11) (i.e. HPO(4)(2-), SiO(3)(2-), WO(4)(2-)) showed almost no effect, a chemical feature unique to PW(11)/SiW(11) and originating from neither of their constituents, i.e. a polyanionic characteristic, may play an important role in their biological effects.

Lacunary-substituted undecatungstosilicates sensitize methicillin-resistant Staphylococcus aureus to beta-lactams

Some Keggin-structural (hetero)polyoxotungstates with a lacunary hole, such as undecatungstosilicate ([SiW11O39]8-, SiW11), greatly sensitize methicillin-resistant Staphylococcus aureus (MRSA) to beta-lactams. In this study, the effects of lacunary-substituted derivatives of SiW11 were tested to determine if the lacunary hole is required for the sensitizing effect. Initially, it was supposed that the hole functioned as a binding site that interacts with some bacterial components. However, most of the lacunary-substituted species had stronger effects than SiW11, indicating that the hole is not essential for the sensitizing effect. Undecatungsto(ferri)ferrosilicate ([SiW11O39Fe](6/5-)) was the most potent compound tested, but undecatungstocobaltosilicate ([SiW11O39Co]6-) gave better results according to the fractional inhibitory concentration index. Small lipophilic cations enhanced the sensitizing effect, but bulky cations inhibited it. This suggests that a charge interaction between the tungsten compounds and some unknown materials in the culture medium or bacterial cells plays an important role in the sensitizing effect.

Purification of a factor that enhances the antibacterial activity of beta-lactams against methicillin-resistant Staphylococcus aureus: its identification as undecaphosphotungstate

Previously, a factor (Factor T) was found in aged mixtures of tungstate and phosphate, which greatly enhances the antibacterial effects of beta-lactam antibiotics on methicillin-resistant Staphylococcus aureus. Factor T was purified and identified as undecaphosphotungstate ([PW11O39]7-). Undecasilicotungstate ([SiW11O39]8-), a compound closely related to undecaphosphotungstate, showed a similar enhancing effect.