Polymyxins are one of most important antibiotics available for multidrug-resistant Gram-negative infections. Diverse chromosomal resistance mechanisms have been described, but the polymyxin ...resistance phenotype is not yet completely understood. The objective of this study was to characterize colistin resistant
mcr-1
-producing strains isolated from human infections over one year in a hospital setting (Hospital das Clínicas, São Paulo, Brazil). We isolated 490 colistin-resistant Gram-negative rods, of which eight were
mcr-1.1
-positive
Escherichia coli
, the only species with this result, indicating a low incidence of the
mcr-1
production mechanism among colistin-resistant isolates. All
mcr-1.1
positive isolates showed similarly low MICs for colistin and were susceptible to most antibiotics tested. The isolates showed diversity of MLST classification. The eight
mcr-1.1
-positive
E. coli
genomes were sequenced. In seven of eight isolates the
mcr-1.1
gene is located in a contig that is presumed to be a part of an IncX4 plasmid; in one isolate, it is located in a contig that is presumed to be part of an IncHI2A plasmid. Three different genomic contexts for
mcr-1.1
were observed, including a genomic cassette
mcr-1.1-pap2
disrupting a DUF2806 domain-containing gene in six isolates. In addition, an IS1-family transposase was found inserted next to the
mcr-1.1
cassette in one isolate. An
mcr-1.1-pap2
genomic cassette not disrupting any gene was identified in another isolate. Our results suggest that plasmid dissemination of hospital-resident strains took place during the study period and highlight the need for continued genomic surveillance.
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