Emergence of Multidrug Resistant Salmonella enterica (MDR-S) Serovars in Hospitalized Horses Due to Spread of an Endemic Plasmid pSEIL-3

Ziv Dor ¹ Anat Shnaiderman-Torban ² Kira Kondratyeva ¹ Maya Davidovich-Cohen ³ Assaf Rokney ³ Amir Steinman ² Shiri Navon-Venezia ¹

¹Ariel University, Molecular Biology, Navon-Venezia Lab of Bacterial Pathogens & Antibiotic Resistance, Israel
²Koret School of Veterinary Medicine, the Robert H. Smith Faculty of Agriculture, Food and Environment, Hebrew University of Jerusalem, Israel
³Government Central Laboratories,, Ministry of Health, Israel

Salmonella enterica is a highly important zoonotic pathogen. We aimed to investigate MDR Salmonella (MDR-S) in hospitalized horses being in close human contact.

MDR-S isolates were collected in Koret-School-of-Veterinary-Medicine during a prospective MDR-Enterobacteriaceae survey (Dec2015-Dec2017). Salmonella was isolated from fecal and clinical samples, identified (VITEK2) and serotyped (Kaufmann-White-Le-Minor). MDR-phenotype was confirmed and antibiotic-resistance-genes were identified and sequenced. ESBL-encoding plasmids were purified and sequenced (long-read MinION) and WGS was performed for each serovar (Illumina-HiSeq). Conjugation experiments were performed to prove horizontal plasmid transferability.

Twelve MDR-S isolates were recovered from foals and adult horses. Isolates all produced the same extended-spectrum-beta-lactamase-CTX-M-3, and showed an identical MDR-profile, including resistance to three antibiotic classes: cephalosporins, trimethoprim/sulfamethoxazole and aminoglycosides. Three MDR-S serovars were identified- Cerro, Havana, Liverpool, found for the first time in horses. Molecular epidemiology analysis revealed clonal spread of serovar-Cerro between seven horses, together with an endemic spread of a single CTX-M-3-encoding plasmid (designated pSEIL-3), which horizontally and serovar-independently disseminated between 10 horses. pSEIL-3, a-86.4kb IncM2-plasmid proved to be broad-host-range by conjugation from Salmonella to E.coli-J53 and to K.pneumoniae. Moreover, using a specific multiplex-PCR we proved the presence of pSEIL-3 in other MDR-enteric bacteria isolated from three horses, proving in-situ transfer of the endemic plasmid in horses’ gut.

This is an alarming report on the emergence of MDR-S in hospitalized horses, associated with gut-colonization and foal morbidity. We demonstrate the zoonotic potential of horses to host and shed MDR-S and serve as a reservoir for highly transferrable broad-host-range MDR-plasmids posing a disturbing ‘one-health’ risk.