Dear Marwa

Colistin resistance may be mediated by different mechanisms, including transferable genes such as the mcr genes (note that different variants have been described), but also by chromosomal alterations such as efflux pump overexpression (frequently leading to heteroresistance) or modifications in the LPS (such as modifications of lipid A with phosphoethanolamine and 4-amino-4-deoxy-L-arabinose ) mediated by chromosomal mutations (for instance mutations at pmrHFIJFKLM operon or in regulation systems such as pmrAB or mgrB).

For more in depth information can read Olaitan et al ., Front Microbiol. 2014;5:643.