Phloxine B is a red dye, the USP grade of which is used as a color additive for food, drugs, and cosmetics. It has also been used as an alternative to Gram staining, to differentiate between Gram-negative and Gram-positive bacteria. At 100 µg/ml, it can inhibit methicillin-resistant S. aureus growth in vitro. Phloxine B has also been shown to modulate channel activity of wild-type cystic fibrosis transmembrane conductance regulator chloride channels (CFTR; Ki = 38 µM) and mutant delta F508 CFTR (Ki = 33 µM).