Colistin (polymyxin E), the last line of defense against multidrug resistant Gram-negative bacteria, was increasingly being used in the treatment of serious clinical infections nowadays (Lucidi et al., 2024). Mobile colistin resistance (MCR) enzymes are plasmid-encoded proteins that confer resistance to colistin. Nonmobile colistin resistance (NMCR) enzymes, which refer to chromosome-encoded colistin resistance proteins, serve as progenitors of MCR enzymes (Zhang et al., 2019a, Ullah et al., 2021). However, the rapid and global dissemination of the plasmid-mediated resistance to colistin, mcr-1 gene, which encodes a mobile phosphoethanolamine transferase (PET), was first identified in late 2015 (Wu et al., 2023). Nine additional alleles of mcr genes (mcr-2~10) have been reported worldwide (Sun et al., 2018, El-Sayed et al., 2020, Liu et al., 2023). In order to enhance the prevention and control of colistin antibiotic resistance, it was crucial to unravel the progenitors and reservoirs of the MCR family enzymes. As of now, only four progenitors and reservoirs of MCR families (MCR-1/2/4/8) were elucidated (Wei et al., 2018, Zhang et al., 2019a, Ullah et al., 2021). However, the ancestors and natural reservoirs associated with the remaining MCR members remain mysteries. Several recent studies have reported the widespread presence of MCR enzymes in Aeromonas, among which the main one is MCR-3 (Shen et al., 2018, Zhang et al., 2019b). In the past few years, Aeromonas hydrophila ZYAH75 (GCF_003491245.1) which grew on MHA plates with up to 128 µg/mL of colistin from a patient in 2015 was isolated by our lab (Fig. S1), and two novel NMCR enzymes were discovered. However, it was not clear which NMCR enzymes were from Aeromonas and what were the reservoirs of MCR families these progenitors (NMCR) responsible for.

In this study, attempts to close the missing gap of knowledge between MCR families of Aeromonas and their progenitors have been made. Meanwhile, the presence of mobile genetic elements (MGEs) was thoroughly considered to analyze the evolutionary pathway of MCR, MCR-like and NMCR in Aeromonas.