MRSA is Staph Aureus, a gram-positive organism, and colistin is only (to my knowledge) used in treating gram negative infections (such as e coli, which can carry mcr-1).

MRSA can be treated with a number of different antibiotics, including vancomycin, daptomycin, linezolid, Bactrim, and clindamycin to name a few. Teixobactin is not yet on the market or approved for use in humans.