| Summary: | Background: Actually, no data on the prevalence of plasmid colistin resistance in Tunisia are available among clinical bacteria. Objectives: This study aimed to investigate the current epidemiology of colistin resistance and the spread of the <i>mcr</i> gene in clinical Gram-negative bacteria (GNB) isolated from six Tunisian university hospitals. Methods: A total of 836 GNB strains were inoculated on COL-R agar plates with selective screening agar for the isolation of GNB resistant to colistin. For the selected isolates, <i>mcr</i> genes, beta-lactamases associated-resistance genes and molecular characterisation were screened by PCRs and sequencing. Results: Colistin-resistance was detected in 5.02% (42/836) of the isolates and colistin-resistant isolates harboured an ESBL (<i>bla</i><sub>CTX-M-15</sub>) and/or a carbapenemase (<i>bla</i><sub>OXA-48</sub>, <i>bla</i><sub>VIM</sub>) encoding gene in 45.2% of the cases. The <i>mcr</i>-1 gene was detected in four <i>E. coli</i> isolates (0.59%) causing urinary tract infections and all these isolates also contained the <i>bla</i><sub>TEM-1</sub> gene. The <i>bla</i><sub>CTX-M-15</sub> gene was detected in three isolates that also carried the IncY and IncFIB replicons. The genetic environment surrounding the <i>mcr</i>-carrying plasmid indicated the presence of <i>pap-2</i> gene upstream <i>mcr</i>-1 resistance marker with unusual missing of ISApl1 insertion sequence. The Conclusions: This study reports the first description of the <i>mcr</i>-1 gene among clinical <i>E. coli</i> isolates in Tunisia and provides an incentive to conduct routine colistin susceptibility testing in GNB clinical isolates.
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