Zoonotic Microorganisms and Spread of Acquired Polymyxin Resistance Determinants

The usefulness of colistin (polymyxin E) as an antibiotic of last resort in human medicine is challenged by the rapid spread of plasmid-mediated resistance (mcr) genes. Previously, most Gram-negative bacteria were susceptible to colistin, and since resistance-conferring mutations also reduced fitness in microorganisms, colistin was used in farming, particularly in poultry and swine, to prevent the occurrence of intestinal infections in early stages of development. Currently different mcr types (1 to 10) and subtypes have been described, although the first reported, mcr-1, is still the most prevalent worldwide. Various types of plasmids have contributed to the dispersal of mcr genes in Gram-negative microorganisms, such as Escherichia coli, Salmonella enterica and Shigella spp. Coming from carrier animals and through derived-food, these bacteria are potentially transmitted to humans, where they are able to mobilize their resistance determinants to other enterobacteria like Klebsiella pneumoniae, which is especially compromising after the dispersion of clones with low susceptibility to carbapenems, the only other last resort antimicrobial with polymyxins.

Functional expression of colistin resistance determinants is a subject of maximal interest. Among the 10 mcr genes, not all are equally expressed to confer colistin resistance, nor are all plasmids carrying mcr genes easily mobilized. In addition, it is not well understood whether there is an additive role of mutations in PmrAB, the two-component system that regulates LPS decoration and that can also confer colistin resistance. Contributions technically consistent and providing novelties to the field, increasing our knowledge about functioning of these molecular mechanisms including their transmission, would be highly appreciated.

This Topic will welcome manuscripts focused in the transmission and expression of colistin resistance determinants in Gram-negative bacteria, mainly by original research article type, in one or several of the following subjects:

• Describing new isolates and/or polymyxin resistance determinants from animals, foods or human and their related environments

• Communicating polymyxin resistance determinants (plasmids, genes and/or mutations)

• Analyzing phenotype expression: antimicrobial resistance and fitness

• Determining mobilization potential, by vertical and/or horizontal transmission of clones or resistance determinants.