Escherichia coli combats polymyxin exposure by modifying lipopolysaccharide in the outer membrane via the arn operon. Surrounding this operon is Insertion Sequence 1 (IS1), which codes for a transposase that is only expressed when a -1 ribosomal frameshift occurs (in which the arn operon is duplicated with IS1). After exposing E. coli cells to certain aminoglycosides, then Polymyxin B, these cells exhibited greater resistance to Polymyxin B than cells not exposed to aminoglycosides.