Background: Polymyxins have re-entered use against problem Gram-negative bacteria. Resistance rates are uncertain, with estimates confounded by selective testing. Methods: The BSAC Resistance Surveillance Programme has routinely tested colistin since 2010; we reviewed data up to 2017 for relevant Enterobacterales (n=10,914). Unexpectedly frequent resistance was seen among the Enterobacter cloacae complex isolates (n=1749); for these, we investigated relationships to species, genome, carbon source utilisation and LPS structure. Results: Annual colistin resistance rates among E. cloacae complex isolates were 4.4% to 20%, with a rising trend among bloodstream organisms; in contrast, annual rates for Escherichia coli, Klebsiella spp. and E. aerogenes generally remained <2%. WGS split the E. cloacae complex isolates into 7 Genogroup clusters, designated A-G. Among isolates assigned to Genogroups A-D, 47/50 sequenced were colistin resistant, and many, from Genogroups A-C identified as E. asburiae. Isolates belonging to Genogroups E-G consistently identified as E. cloacae and were rarely (only 3/45 representatives sequenced) were colistin resistant. Genogroups F and G – the predominant colistin-susceptible clusters – were metabolically distinct from other clusters, notably regarding utilisation or not of L-fucose, formic acid, D-serine, adonitol, myo-inositol, L-lyxose and polysorbates. LPS from resistant organisms grown without colistin pressure lacked substitutions with 4-amino-arabinose or ethanolamine but was more structurally complex, with more molecular species present. Conclusions: Colistin resistance is frequent in E. cloacae the complex and increasing among bloodstream isolates. It is associated with: (i) particular genomic and metabolic clusters, (ii) identification as E. asburiae and (iii) with more complex LPS architectures.