ST-410

Dear Editor, The worldwide dissemination of New Delhi metallo-β-lactamase 1 (NDM-1), an Ambler class B metallo-β-lactamase (MBL) conferring resistance to all β-lactams except monobactams, is of great concern for public health.NDM-4, which differs from NDM-1 by a single aminoacid substitution (Met154Leu), was demonstrated to possess increased carbapenemase activity.As an infrequent bla_NDM-4 allele, only several sporadic cases of infections due to NDM-4-producing Escherichia coli have been described in India, Austria, and Europe.At present, only one NDM-4 plasmid (pJEG027; GenBank accession NO KM400601) has been characterized.⁴ Several recent studies revealed that the bla_NDM-4 gene was carried by plasmids belonging to different replicon types, including F, FII, L/M and X3 in E. coli and K. pneumoniae; however, only two partial sequences of IncFII plasmids (accession NO KP826711 and KP826707) and one partial sequence of an IncX3 plasmid (accession NO KP826709) except pJEG027 are available for the genetic context of bla_NDM-4.A comparative plasmid anal. between pEC55-NDM-4 and pJEG027, which is the only completely sequenced NDM-4 plasmid of a K. pneumoniae strain isolated in Austria, provided support for Espedido et al′s hypothesis that pJEG027 might have arisen from a pNDM-HN380-like plasmid ancestor (for example, pEC55-NDM-4) through the events of a different IS5 insertion and an IS26-mediated flanking deletion of cutA1-groL.The second completely sequenced bla_NDM-4 carrying plasmid pEC55-NDM-4, in our study, is nearly identical to the first characterized bla_NDM-1-harboring IncX3 plasmid pNDM-HN380, further suggesting that bla_NDM-4 might have emerged on IncX3 plasmids via a point mutation in bla_NDM-1.The occurrence of a pNDM-HN380-like plasmid carrying bla_NDM-4 in this study, together with recent observations of IncX3-type plasmids carrying different bla_NDM alleles, including bla_NDM-1 (pNDM-HN380), bla_NDM-4 (pJEG027), bla_NDM-5 (pNDM_MGR194, pNDM5_0215 and pEc1929), and bla_NDM-7 (pEC50-NDM7), in different countries (Austria, India, China and Canada) and different species (K. pneumoniae, E. coli, Serratia marcescens, and Enterobacter hormaechei), strongly indicates that IncX3 plasmids, which have a narrow host range in Enterobacteriaceae, might have played a major role in the rapid global dissemination of NDM-type MBLs among Enterobacteriaceae.In summary, our study represents the first report of a NDM-4 producing E. coli isolate recovered from a blood culture of a patient without a history of foreign travel.The FQ-resistant E. coli 14-55 with multiple β-lactamase encoding genes, including bla_NDM-4, bla_CTX-M-15, bla_SHV-12, bla_TEM-1 and bla_CMY-61, belonged to ST410, a potential international clone for the dissemination of CTX-M-15.Further surveillance is thus warranted to monitor the future dissemination of potentially endemic clones of ST410 that harbor bla_NDM.The bla_NDM-4 carrying IncX3 plasmid characterized in this study is nearly identical to pNDM-HN380 (with bla_NDM-1), which was reported in Hong Kong.This finding, together with recent observations of IncX3 plasmids carrying different bla_NDM alleles in different countries, further suggests that IncX3 plasmids might have become a common vehicle for the dissemination of different NDM alleles among Enterobacteriaceae worldwide.To understand the epidemiol. and control the spread of antibiotic-resistant pathogens, we have been investigating carbapenem-resistant Enterobacteriaceae from human patients in China.Previously, we reported a high incidence (33.3%) and endemic spread of NDM-1-pos. Enterobacteriaceae in Henan Province.Here we describe the first NDM-4-producing E. coli ST410 isolate, which, to our knowledge, is the first case in China, and report the characterization of the bla_NDM-4 harboring IncX3 plasmid.

The global rise of multidrug-resistant Escherichia coli ST410 presents a growing clinical threat. We isolated a highly lytic phage targeting ST410, but rapid resistance emerged both in vitro and in vivo . Beyond receptor mutations, we identified a hypermucoid mutant that evades phage infection primarily through colanic acid overproduction, providing broad phage resistance. This colanic acid overproduction also led to diminished macrophage phagocytosis in vitro and accelerated mortality in mice. Transcriptomic and genetic analyses linked this phenotype to activation of the Rcs pathway. Notably, naturally occurring ST410 isolates with colanic acid-overproducing phenotypes were also observed, suggesting that capsule-mediated immune evasion may arise in clinical populations. To overcome resistance, we designed a rational four-phage cocktail with diverse receptors, achieving robust therapeutic efficacy in infection models. Our findings highlight that phage resistance can be accompanied by enhanced virulence potential through capsule-mediated immune evasion, with colanic acid playing a central role, emphasizing important considerations for phage therapy design.