New species of Corynebacterium identified in the eyes of horses

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Better knowledge of the normal equine eye microbiota may help us understand its role in the development and progression of eye infections.
Photo by Jon Pinder

Researchers have identified a new species of Corynebacterium that appears to naturally inhabit the eyes of healthy horses.

The study team named it Corynebacterium conjunctivae. They describe it as a commensal organism, meaning it normally does no harm and may even benefit the eye.

Eye surfaces provide a habitat for different bacterial and fungal microorganisms. They contribute to the eye defence mechanism by producing antibacterial substances that prevent colonization by pathogens.

Despite this, eye infections are common in horses. Often, they are caused by usually harmless members of the conjunctiva microbiota that act as opportunistic pathogens after damage to the eye surface or surrounding areas.

Therefore, better knowledge of the normal equine conjunctiva microbiota may reveal more about its role in the development and progression of eye infections. It may also help clinicians to prevent complications arising from the normal conjunctiva microbiota and possibly identify better treatment measures.

José Fernández-Garayzábal and his fellow researchers, writing in the journal Animals, noted that various studies have focused on identifying the normal eye microbiota in horses. Most confirm that Gram-positive bacteria are predominant, with the genus Corynebacterium among the most frequently identified.

However, Corynebacteria isolated from normal ocular flora are commonly identified only at the genus level.

“This drawback limits the availability of information on the diversity of Corynebacterium species present in the normal flora of the eye surface,” they said.

The study team set out to characterize some commensal Corynebacterium-like organisms isolated from the conjunctival sac of healthy adult horses by applying several techniques.

Samples were taken from eight healthy animals from the John Thomas Vaughan Large Animal Teaching Hospital at Auburn University in Alabama.

Twenty-two unidentified Gram-positive, rod-shaped organisms were recovered. Based on cellular shape and biochemical criteria, the isolates were tentatively assigned to the genus Corynebacterium.

Molecular-based testing showed the isolates shared 99.4 to 100% 16S rRNA gene sequence similarity among the strains, demonstrating close genealogical links. They had 96.5% similarity with Corynebacterium tapiri 2385/12T, which was the closest related species, but less than 96.0% with all other Corynebacterium species.

Further analysis showed that the isolates formed a distinct lineage and exhibited only weak associations with C. tapiri and other validated Corynebacterium species.

“The isolates were distinguished from related Corynebacterium species by a number of phenotypic properties,” they said. “On the basis of phenotypic and phylogenetic evidence, it is proposed that the unknown isolates from horses be classified in the genus Corynebacterium as Corynebacterium conjunctivae sp. nov.”

The study team comprised Fernández-Garayzábal, Stacey LaFrentz, Almudena Casamayor, Eva Abarca, Haitham Mohammed, Rosemary Cuming, Cova Arias, Lucas Domínguez and Ana Vela, variously affiliated with Complutense University and the Ophthalmology Service at ARS Veterinaria, both in Spain; Auburn University; Assiut University in Egypt; and Scone Equine Hospital in Australia.

Fernández-Garayzábal, J.F.; LaFrentz, S.; Casamayor, A.; Abarca, E.; Mohammed, H.H.; Cuming, R.S.; Arias, C.R.; Domínguez, L.; Vela, A.I. Corynebacterium conjunctivae: A New Corynebacterium Species Isolated from the Ocular Surface of Healthy Horses. Animals 2022, 12, 1827. https://doi.org/10.3390/ani12141827

The study, published under a Creative Commons License, can be read here

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