Colistin-resistant mutants in S. Typhimurium were also obtained by direct plating on agar containing colistin at a concentration 10 or 50 times the MIC of wild type.Spontaneous resistant mutants displayed 2- to 35-fold increased colistin resistance with little to no fitness cost in a mouse model of sepsis. 44 independent mutants were characterized and 27 different missense mutations were identified in pmrA or pmrB, the two-component regulatory system. The colistin-resistant mutants displayed 2- to 15-fold increases in expression of pmrH indicating a dysregulation of pmrAB-regulated genes. Stable colistin resistance following direct plating was also observed in A.baumannii following the complete loss of LPS due to the inactivation of one of three genes involved in lipid A biosynthesis (lpxA, lpxD, or lpxC)