Abstract
Background: Many emergent pathogenic agents are cross-transmitted from animals to humans. Horses are considered as potential reservoirs of commensal, zoonotic and multidrug-resistant bacteria. The equine bites could be a way of developing infections caused by these agents, considering equine species as a public health concern. The more we know about the equine oral microbiota the best we can control secondary problems created by their commensal flora.
There are very few reports of Serratia rubidae, a zoonotic and opportunistic bacterium, both in human and veterinary medicine.

Aim: This study aimed to evaluate the Gram-negative microbiota of healthy equine oral cavity and their antimicrobial susceptibility.

Methods: During an equine routine oral procedure, eight healthy horses were selected to this study, after discard any abnormal dental condition. Samples were collected from the subgingival space and gingival margin from the tooth 406 and both the identification and antimicrobial susceptibility test of Gram-negative bacteria were performed.

Results: This study reports the isolation of 32 Gram-negative agents, 27 of which were multidrug-resistant to the antimicrobial classes tested. High resistance rates were obtained to commonly used antimicrobial, particularly macrolides and aminoglycosides as to carbapenems that are specific to human medicine. Two multi-drug resistance strains of Serratia rubidaea were found in the mouth of two healthy horses.

Conclusion: Most Gram-negative isolates found in healthy horses were zoonotic and multi-drug resistant. This is a strong reason to consider the horse as an animal with a major place in the “One Health” concept. Equine clinicians should have precautions when working with horse’s mouth and during decision of an antimicrobial therapy protocol, giving relevance to the importance of the antimicrobial sensitivity tests. According to our knowledge, this is the first report about isolation of Serratia rubidaea from the mouth of the equine species.

Key words: Antimicrobial Resistance; Horse; Oral microbiota; Serratia