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May 29, 2026Microbial Genomics0 citationsOpen Access

Integrative and conjugative elements carrying high-level gentamicin resistance genes in Streptococcus dysgalactiae subsp. equisimilis from horses

TRTony Rochegüe SHSandrine Hughes ESEstelle Saras

Key Points

This research aims to investigate the presence of high-level gentamicin resistance genes in Streptococcus dysgalactiae subsp. equisimilis from horses and their potential transfer to human pathogens.
Isolated HLGR-Streptococcus dysgalactiae subsp. equisimilis from mares' uteri and lymph nodes during routine screenings (n=27).
Conducted phylogenetic analysis to characterize different sequence types and examine gene transfer capabilities.
Performed in vitro transfer experiments of gentamicin resistance genes from HLGR strains to erythromycin-resistant Streptococcus agalactiae.
Identified HLGR mediated by aph (2″)- Ib and aac (6’)- Iam, located on class 5 integrative mobilizable elements.
Determined that HLGR can transfer from Streptococcus dysgalactiae to Streptococcus agalactiae in vitro, indicating potential health risks.
Established that strains assigned to ST196 can disseminate among mares over a two-year period, highlighting the need for hygiene measures.

Abstract

In streptococci, the acquisition of high-level gentamicin resistance (HLGR) abolishes the synergy with beta-lactams, constituting a medical concern for humans and animals. This synergy between gentamicin and beta-lactams is useful to treat severe infections like endocarditis, among others. HLGR has been characterized mostly in roup B Streptococci mediated by the bi-functional gene aac (6) - aph (2). HLGR– Streptococcus dysgalactiae subsp. equisimilis (SDSE) (n =27) were isolated from the uterus of mares during a routine screening before in vitro fecundation. One HLGR– SDSE was isolated from a lymph node of a mare. The mares resided in six different studs located in four departments of France. HLGR –SDSE belonged to three sequence types (STs): ST196 (n =26), ST223 (n =1) and a single locus variant (SLV) of ST200 (n =1). For the first time, HLGR was found mediated by the two genes aph (2″) - Ib and aac (6’) - Iam located on a class 5 integrative mobilizable element (IME), here named HLGRIME. In HLGR– SDSE assigned to ST196 and ST223, the HLGRIME was inserted in a Tn 5252 -type transposon. In the isolate assigned to an ST200-SLV, the HLGRIME was inserted in an ICE Er0106 -type element. The Tn 5252 -type transposon was able to transfer in vitro from HLGR –SDSE to an erythromycin-resistant Streptococcus agalactiae strain. The transferability of HLGR from SDSE to human pathogens, such as S. agalactiae, is a major concern for health. The phylogenetic analysis displayed that SDSE from horses constituted a distinct phylogenetic clade from SDSE isolated from humans or other animals. Furthermore, we demonstrated that HLGR– SDSE belonging to ST196 were able to disseminate among mares within a single stud during 2 years. This clone has also been reported in the USA. Hygiene measures could prevent further dissemination of HLGR streptococci in veterinary settings, preserving horses’ health, and the transfer of HLGR to pathogens important in veterinary and human settings.

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