Polymyxin causes cell envelope remodelling and stress responses in mcr-1-harbouring Escherichia coli

Area of Science:

• Microbiology
• Genetics
• Molecular Biology

Background:

• Polymyxins are crucial last-resort antibiotics for treating multidrug-resistant Gram-negative bacterial infections.
• Emerging polymyxin resistance, particularly the mobile mcr gene in Escherichia coli, poses a significant public health threat.
• Understanding the molecular mechanisms of polymyxin resistance is vital for developing effective treatment strategies.

Purpose of the Study:

• To investigate the concentration-dependent transcriptomic responses of mcr-harbouring Escherichia coli (E. coli) exposed to polymyxin B.
• To elucidate the specific gene expression changes that confer polymyxin resistance in E. coli.

Main Methods:

• Treatment of an mcr-1-carrying E. coli clinical isolate (LH30) with polymyxin B at 2 mg/L and 8 mg/L.
• Viable counting and transcriptomic analysis of bacterial cultures before and 1 hour after treatment.
• Analysis of gene expression profiles to identify differentially expressed genes.

Main Results:

• Polymyxin B at 8 mg/L, but not 2 mg/L, significantly reduced E. coli viability.
• Upregulation of all four phosphoethanolamine (pEtN) transferase genes (mcr-1, eptA, eptB, eptC) was observed at 8 mg/L polymyxin B, indicating pEtN modification as a primary resistance mechanism.
• Significant alterations in genes related to cell envelope, fatty acid, lipopolysaccharide, lipid A, phospholipid biosynthesis, iron homeostasis, and oxidative stress pathways were detected.

Conclusions:

• Cell envelope remodeling, pEtN modification, iron acquisition, and oxidative stress response are critical for the survival of mcr-carrying E. coli under polymyxin treatment.
• This study provides novel mechanistic insights into polymyxin resistance at the gene expression level, aiding in the development of strategies to combat resistance.