# Genomic Characteristics of a Multidrug-Resistant Extraintestinal Pathogenic Escherichia coli RZ-13 Isolates from Diarrheic Calves with High Mortality in China

## TL;DR

A highly virulent and drug-resistant Escherichia coli strain from sick calves in China was studied to understand its genome and resistance mechanisms.

## Contribution

Discovery of two highly transmissible plasmids in a multidrug-resistant ExPEC strain with unique resistance gene arrangements.

## Key findings

- The RZ-13 strain contains four plasmids, two of which (pRZ13-1 and pRZ13-3) carry numerous resistance genes and are highly transmissible.
- Plasmid pRZ13-3 has an exceptionally high transfer frequency, exceeding that of previously reported IncFII plasmids.
- The MDR region of pRZ13-1 is structurally complex and widely present in both animal and human isolates.

## Abstract

Extraintestinal pathogenic Escherichia coli (ExPEC) poses escalating threats to human and veterinary health amid rising antimicrobial resistance. We isolated a highly virulent ExPEC strain RZ-13 (ST345, O134:H21) from diarrheic calves at a large beef cattle farm in Rizhao City, and conducted whole genome sequencing, conjugation experiments, and antimicrobial susceptibility testing to elucidate its genomic architecture and resistance mechanisms. The RZ-13 genome comprises one chromosome and four plasmids. The chromosome harbors virulence factors for adhesion, invasion, biofilm formation, and iron acquisition. Notably, plasmids pRZ13-1 (265,777 bp, IncHI2-IncHI2A) and pRZ13-3 (74,304 bp, IncFII) carry the majority of resistance genes. Plasmid pRZ13-1 carries 25 resistance genes, including blaCTX-M-55, floR, qnrS1, sul3, and tet(A), as well as a complete tellurite resistance gene cluster, terABCDEFZY1. Its multidrug resistance (MDR) region features an IS26-mediated tandem amplification and an approximately 29 kb inverted structure. Comparative analysis indicated that the MDR region carried by this plasmid is highly prevalent in both animal-derived and human-derived isolates. Plasmid pRZ13-3 harbors an IS91-mediated mobile region that integrates both antimicrobial resistance and stress adaptation genes, which have been repeatedly identified in plasmids from diverse sources, including animals and humans. Conjugation experiments confirmed both pRZ13-1 and pRZ13-3 plasmids are self-transmissible and confer multidrug-resistant phenotypes to recipient strains, with pRZ13-3 exhibiting an exceptionally high transfer frequency of 8.9 × 10−2, substantially exceeding that of previously reported IncFII plasmids. These findings demonstrate that pRZ13-1 and pRZ13-3 serve as critical vehicles for resistance dissemination through complex mobile genetic element structures and efficient horizontal transfer, highlighting the urgent need for surveillance of livestock-reservoir ExPEC to mitigate public health risks.