Molecular Epidemiology of <i>mcr-1</i> -Positive Polymyxin B-Resistant <i>Escherichia coli</i> Producing Extended-Spectrum β-Lactamase (ESBL) in a Tertiary Hospital in Shandong, China

<i>Escherichia coli</i>, a rod-shaped Gram-negative bacterium, is a significant causative agent of severe clinical bacterial infections. This study aimed to analyze the epidemiology of extended-spectrum β-lactamase (ESBL)-producing <i>mcr-1</i> -positive <i>E. coli</i> in Shandong, China. We collected 668 non-duplicate ESBL-producing <i>E. coli</i> strains from clinical samples at Shandong Provincial Hospital between January and December 2018, and estimated their minimum inhibitory concentrations (MICs) using a VITEK^® 2 compact system and broth microdilution. Next-generation sequencing and bioinformatic analyses identified the <i>mcr-1</i> gene and other resistance genes in the polymyxin B-resistant strains. The conjugation experiment assessed the horizontal transfer capacity of the <i>mcr-</i>1 gene. Of the strains collected, 24 polymyxin B-resistant strains were isolated with a positivity rate of 3.59% and among the 668 strains, 19 clinical strains carried the mobile colistin resistance gene <i>mcr-1</i>, with a positivity rate of approximately 2.8%. All 19 clinical strains were resistant to ampicillin, cefazolin, ceftriaxone, ciprofloxacin, levofloxacin, and polymyxin B. Seventeen strains successfully transferred the <i>mcr-1</i> gene into <i>E. coli</i> J53. All transconjugants were resistant to polymyxin B, and carried the drug resistance gene <i>mcr-1</i>. The 19 clinical strains had 14 sequence types (STs), with ST155 (n = 4) being the most common. The whole-genome sequencing results of pECO-POL-29_mcr1 revealed that no IS<i>Apl1</i> insertion sequences were found on either side of the <i>mcr-1</i> gene. Our study uncovered the molecular epidemiology of <i>mcr-1</i>-carrying ESBL-producing <i>E. coli</i> in the region and suggested horizontal transmission mediated by plasmids as the main mode of <i>mcr-1</i> transmission.