Retained Activity of an O25b-Specific Monoclonal Antibody against an Mcr-1-Producing Escherichia coli Sequence Type 131 Strain.

Plasmid-encoded colistin resistance is emerging among extraintestinal pathogenic Escherichia coli strains, including those of the epidemic clone sequence type 131 (ST131)-H30. Mcr-1 transfers a phosphoethanolamine to the lipid A portion of lipopolysaccharide (LPS), conferring resistance to polymyxins. We investigated whether this modification changed the activity of the monoclonal antibody ASN-4, specific to the O25b side chain of ST131 LPS. We confirmed that, unlike colistin, ASN-4 retained its bactericidal and endotoxin-neutralizing activities and therefore offers a treatment option against extremely drug-resistant ST131 isolates.