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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.

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