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 methic illin-resistant Staphylococcus aureus (MRSA) to beta -lactams. In this stud y, the effects of lacunary-substituted derivatives of SiW11 were tested to determine if the lacunary hole is required for the sensitizing effect. Init ially, it was supposed that the hole functioned as a binding site that inte racts with some bacterial components. However, most of the lacunary-substit uted species had stronger effects than SiW11, indicating that the hole is n ot essential for the sensitizing effect. Undecatungsto(ferri)ferrosilicate ([SiW11O39Fe](6/5-)) was the most potent compound tested, but undecatungsto cobaltosilicate ([SiW11O39CO](6-)) gave better results according to the fra ctional inhibitory concentration index. Small lipophilic cations enhanced t he sensitizing effect, but bulky cations inhibited it. This suggests that a charge interaction between the tungsten compounds and some unknown materia ls in the culture medium or bacterial cells plays an important role in the sensitizing effect.