Possible Mechanism of Action of β‐Lactam‐Enhancing Factor on Methicillin‐Resistant Staphylococcus aureus

Abstract

We have recently found a factor (Factor T) in aged mixtures of tungstate and phosphate which greatly enhances the antibacterial effects of β‐lactams on methicillin‐resistant strains of staphylococcal species such as methicillin‐resistant Staphylococcus aureus (MRSA), but shows only weak effects on methicillin‐susceptible S. aureus and bacterial strains other than staphylococci. Factor T alone did not strongly inhibit cell metabolism and bacterial growth unless an excess amount was added. When Factor T was added to the culture medium beforehand, the growth of MRSA cells was rapidly suppressed just after addition of oxacillin (MPIPC). However, the growth of the cells was inhibited gradually when these two reagents were added in reverse order. For full expression of the enhancing effect, it seemed necessary for cells of MRSA strains to be incubated with Factor T for at least 2–3 hr. When the cells were washed after being sensitized by incubating them for 5 hr with Factor T, it took approximately 1 hr for the cells to recover their resistance to MPIPC. Factor T reduced the amount of penicillin‐binding protein‐2′ (PBP 2′), and thus sensitized the MRSA strains to β‐lactams.

Lacunary-Substituted Undecatungstosilicates Sensitize Methicillin-Resistant Staphylococcus aureus to .BETA.-Lactams.

“…2b). When SiW 11 Co was used instead of SiW 11 , TPP/(CPADAP) 2 Co still enhanced the sensitizing effect, but hydrophilic cations such as ethanolamine showed no enhancement effect (data not shown).…”

“…However, the result was opposite, which suggests that the hole is not essential to produce the sensitizing effect on MRSA strains, although polyoxotungstates with a lacunary hole are generally more effective than other molecular species. 9,10) When small and lipophilic cations, such as TPP and (CPADAP) 2 Co, were mixed with the culture medium containing SiW 11 , the sensitizing effect of SiW 11 was further enhanced, whereas a bulky polycation (such as protamine) inhibited it (Fig. 2b).…”

“…18,19) Therefore, the author considers that a charge interaction between these polyoxotungstates and some unknown (perhaps positively charged) materials in the culture medium or bacterial cells plays an important role on the sensitizing effect (e.g., amino acids including oligopeptides and aliphatic amines such as phosphatidylethanolamine are candidates for making ion-pairs with SiW 11 ). It is possible that metal cations in the lacunary hole may only neutralize the high negative charge of SiW 11 , like TPP/(CPADAP) 2 Co did, thereby further enhancing the sensitizing effect.…”

“…It has been reported that such a lipophilic cation functions as a phase-transfer agent to many polyanions 16,17) ; i.e., SiW 11 makes an ion-pair with TPP/(CPADAP) 2 Co and the complex becomes extractable to organic solvents. Some investigators a) The atomic valency of each substituents is shown as a roman number in parentheses after the element.…”

“…The other chemicals, culture media, established bacterial strains used in this study, and the method of preparing PW 11 are described in our previous reports. 1,2,11) Medium pH was kept at pH 7.0Ϯ0.1 to avoid decomposition of the polyoxotungstates. Although all the tungsten compounds used in the experiments are heat-stable, WO 4 2Ϫ and/or SiO 3 2Ϫ were mixed with the agar medium below about 45°C.…”