# Study on Antimicrobial Resistance and Molecular Characteristics of Riemerella anatipestifer

## TL;DR

This study examines antimicrobial resistance and genetic traits of Riemerella anatipestifer in China, aiming to improve disease management in poultry.

## Contribution

The study provides new insights into the multidrug resistance and genomic diversity of Riemerella anatipestifer strains in China.

## Key findings

- 88 Riemerella anatipestifer strains were collected from multiple regions in China, showing high multidrug resistance.
- Five resistance genes and seven virulence genes were identified in the strains.
- In vivo tests confirmed the effectiveness of antimicrobial agents against resistant strains.

## Abstract

At present, the antimicrobial resistance of Riemerella anatipestifer has become increasingly severe in China, causing substantial economic losses to the poultry breeding industry. Relevant research reports on this bacterium remain relatively scarce. The aim of this study was to investigate the antimicrobial resistance and genomic characteristics of R. anatipestifer strains isolated from several regions of China from 2023 to 2024, and conduct virulence protection tests to verify the in vivo protective effect of effective antimicrobial agents, thereby providing references for the prevalence status and clinical management of Riemerella anatipestifer disease.

Riemerella anatipestifer (R. anatipestifer) is a pathogenic bacterium belonging to the genus Riemerella within the family Flavobacteriaceae, which has multidrug resistance (MDR) and can cause high pathogenicity in waterfowl. The aim of this study was to investigate the antimicrobial resistance and genomic characteristics of R. anatipestifer strains isolated from several regions of China from 2023 to 2024. Two strains were selected for challenge tests, and virulence protection tests were conducted on florfenicol-resistant and florfenicol-sensitive strains. A total of 88 strains of R. anatipestifer were collected from the Shandong, Jiangsu, Guangdong, Hebei and Henan regions. The results showed that the 88 strains included serotypes 1, 2, 5, 6, 7 and 10. Serotype 5 was the most prevalent in the Shandong region. All strains were multidrug-resistant, with the hexaresistance accounting for the highest proportion (42.1%). A total of five resistance genes (tet(X), floR, ermF, qnrS, rmtB) and seven virulence genes were found (ompA, camp, AS87_04050, SIP, Fur, TbdR1, luxE). The challenge test showed that the LD50 of RA12 was 2.75 × 107 CFU/mL, and that of RA26 was 2.57 × 107 CFU/mL. Phylogenetic tree analysis revealed that strain RA26 was closely related to strain 20190403E1-1, and strain RA16 was closely related to strain JW1. In addition, serotypes 2 and 7 identified in this study have been undergoing clonal transmission in China. Virulence protection tests indicated that the results of in vitro drug susceptibility tests were consistent with the therapeutic effects after in vivo treatment, and no R. anatipestifer was found in the visceral tissues of surviving ducklings. This study provides a reference for the rational use of antibiotics.