In vitro antibacterial activities of phloxine B and other halogenated fluoresceins against methicillin-resistant Staphylococcus aureus.

Fluorescein dyes in which the benzoic acid moiety has been tetrachlorinated (50 to 100 micro g/ml) inhibit in vitro Staphylococcus aureus growth (MIC, 25 micro g/ml). Specifically, under standard room illumination, phloxine B at a concentration of 100 micro g/ml killed 99% of the cultures (mid-log phase). It also reduced S. aureus CFU by 10(4). Structure-activity analysis revealed that the activity against S. aureus increases with the increase in the number of the substituting halogens in the hydroxyxanthene moiety.