We studied a collection of 103 strains including 24 mcr-1 CR strains (19 Escherichia coli, 4 K. pneumoniae, 1 Enterobacter cloacae), 6 with a known mechanism of resistance (PmrA, PmrB, mgrB) for which 4 associated with mcr-1, 46 with unknown mechanism (Enterobacter spp., E. coli, Klebsiella spp., Salmonella enterica) and 27 intrinsically resistant Enterobacteriaceae (Serratia marcescens, Proteus spp., Providencia spp.