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Animals : an open access journal from MDPI2026; 16(4); 618; doi: 10.3390/ani16040618

Characterization of the Endometrial Microbiota of Healthy Mares Across the Estrous Cycle.

Abstract: In the past, bacteria detected in the mare's uterus were generally interpreted as a sign of endometritis, since the uterus was considered a sterile environment. This assumption has been challenged by the introduction of culture-independent molecular techniques, particularly 16S rRNA amplicon sequencing, which have demonstrated that healthy mares harbor an endometrial microbiota. The aim of this study was to characterize the endometrial microbiota of healthy mares and to determine whether microbial composition differs between estrus and diestrus. Endometrial samples were collected from eleven healthy Standardbred mares during estrus and diestrus and analyzed by sequencing the V1-V2 region of the bacterial 16S rRNA gene. A total of 24 bacterial phyla and 599 genera were identified. At the phylum level, Firmicutes, Proteobacteria, Bacteroidota, and Actinobacteriota accounted for most of the relative abundance, while the most abundant genera were , , , , , , and . Alpha diversity was significantly higher during estrus, likely due to hormonally mediated changes in cervical opening and local immunity. Beta diversity analyses showed substantial overlap between estrus and diestrus samples. The phase of the cycle had a weak effect on microbiota structure, while inter-individual differences between mares explained a larger proportion of the observed variation. These findings suggest that the uterine microbiota of healthy mares is largely stable across the estrous cycle, with phase-dependent and mare-specific fluctuations in microbial composition.
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Publication Date: 2026-02-15 PubMed ID: 41751079PubMed Central: PMC12937484DOI: 10.3390/ani16040618Google Scholar: Lookup
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Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

Overview

  • This study investigated the types and diversity of bacteria living in the uterus of healthy mares and examined how these bacterial communities change during different phases of the estrous cycle (estrus and diestrus).
  • The research used advanced DNA sequencing techniques to characterize the uterine microbiota, revealing that healthy mares have a stable and complex bacterial community in their uterus, which varies slightly depending on hormonal cycle phases and individual differences between mares.

Background

  • Traditionally, bacteria found in the mare’s uterus were thought to indicate infection, specifically endometritis, because the uterus was believed to be sterile under healthy conditions.
  • This assumption has changed with the advent of culture-independent molecular methods, especially 16S rRNA gene sequencing, which allows detection of bacteria without needing to grow them in cultures.
  • Such methods have demonstrated that healthy mares do naturally harbor bacteria within their uterine environment, challenging previous beliefs about uterine sterility.

Aim of the Study

  • To characterize the bacterial populations (microbiota) present in the endometrium (uterine lining) of healthy mares.
  • To determine if the composition of these bacterial communities differs between two phases of the mare’s reproductive cycle: estrus (heat) and diestrus (non-heat phase).

Methods

  • Endometrial samples were taken from eleven healthy Standardbred mares during both estrus and diestrus phases.
  • The samples underwent sequencing of the V1-V2 region of the bacterial 16S rRNA gene, a common method to identify and classify bacteria based on genetic material.
  • Sequences were analyzed to identify bacterial phyla (broad classification groups) and genera (more specific groups).

Key Findings

  • A total of 24 bacterial phyla were identified across the samples, demonstrating a highly diverse microbial environment in the mare uterus.
  • 599 bacterial genera were detected, showing rich bacterial diversity.
  • Four major bacterial phyla accounted for most of the bacteria present:
    • Firmicutes
    • Proteobacteria
    • Bacteroidota
    • Actinobacteriota
  • The most abundant genera were not specified in the abstract, but they represent dominant bacterial groups within these phyla.
  • Alpha diversity, which measures how many different species are present and how evenly distributed they are, was significantly higher during estrus compared to diestrus. This might reflect hormonally-driven changes:
    • Widening of the cervix during estrus allowing more microbial movement
    • Changes in local immune responses affecting bacterial communities
  • Beta diversity analyses, which examine differences in species composition between samples, showed significant overlap between samples collected during estrus and diestrus, indicating stability in microbiota composition across these phases.
  • The reproductive cycle phase had only a minor effect on the overall microbiota structure.
  • Individual differences between mares explained more of the variability observed than the estrous cycle phase, highlighting mare-specific microbiota signatures.

Conclusion and Implications

  • The mare uterus contains a stable, diverse microbiota that persists across different phases of the estrous cycle.
  • While there are some fluctuations linked to hormonal changes and individual mare differences, these variations are relatively minor.
  • This research challenges older views of the sterile uterus and provides a baseline for understanding the endometrial microbiota in healthy mares.
  • Future studies could explore how this microbiota influences mare reproductive health and disease states like endometritis.

Cite This Article

APA
Donato GG, Necchi D, Gionechetti F, Ala U, Nebbia P, Robino P, Stella MC, Vandaele H, Pallavicini A, Nervo T. (2026). Characterization of the Endometrial Microbiota of Healthy Mares Across the Estrous Cycle. Animals (Basel), 16(4), 618. https://doi.org/10.3390/ani16040618

Publication

Animals : an open access journal from MDPI
ISSN: 2076-2615
NlmUniqueID: 101635614
Country: Switzerland
Language: English
Volume: 16
Issue: 4
PII: 618

Researcher Affiliations

Donato, Gian Guido
  • Department of Veterinary Sciences, University of Turin, 10095 Grugliasco, Italy.
Necchi, Denis
  • Keros Embryo Transfer Center, 8980 Passendale, Belgium.
Gionechetti, Fabrizia
  • Department of Life Sciences, University of Trieste, 34127 Trieste, Italy.
Ala, Ugo
  • Department of Veterinary Sciences, University of Turin, 10095 Grugliasco, Italy.
Nebbia, Patrizia
  • Department of Veterinary Sciences, University of Turin, 10095 Grugliasco, Italy.
Robino, Patrizia
  • Department of Veterinary Sciences, University of Turin, 10095 Grugliasco, Italy.
Stella, Maria Cristina
  • Department of Veterinary Sciences, University of Turin, 10095 Grugliasco, Italy.
Vandaele, Hilde
  • Keros Embryo Transfer Center, 8980 Passendale, Belgium.
Pallavicini, Alberto
  • Department of Life Sciences, University of Trieste, 34127 Trieste, Italy.
Nervo, Tiziana
  • Department of Veterinary Sciences, University of Turin, 10095 Grugliasco, Italy.

Conflict of Interest Statement

G.G.D., F.G., U.A., P.N., P.R., M.C.S., A.P., T.N. declare no conflicts of interest; D.N. and H.V. are employees of Keros.

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