The healthy equine uterus harbors diverse microorganisms, study finds

Photo by Soledad Lorieto

The healthy equine uterus is home to a rich and diverse community of microorganisms, researchers report.

Scientists in the United States and Australia identified a distinct core microbiome, plus a rich and diverse microbiome that varies with geographical location.

The findings open the way to the development of pre or probiotics to aid the natural balance of the uterine microbiome in mares, with the potential to boost overall reproductive health.

Reed Holyoak, Hasitha Premathilake and their fellow researchers said the mammalian uterus was long considered a sterile environment to sustain life. However, this was challenged via some important studies in humans. Since then, the presence of a rich and dynamic microbiome in the mammalian endometrium (uterine lining) has been established in several species, including the horse.

“The importance in understanding this potential shift in accepted dogma is underscored by the fact that many pregnancy-related complications in humans, and other mammals, especially those in important livestock species, are well known to be of bacterial origin.”

The study team, writing in the journal Scientific Reports, noted that equine endometritis occurs in 25 to 60% of breeding mares resulting in infertility, inflammation of uterus after birth, as well as septicemia and fatalities in newborn foals.

Historically, genera such as Staphylococcus, Escherichia, Pseudomonas, and Klebsiella were considered to be invasive pathogens indicating problems in mares.

“However, this and our previous studies indicate that these four genera are common members of the healthy mare uterine microbiome, underscoring the importance of understanding the structure and diversity of healthy uterine microbiome of the mare and to define a ‘core microbiome’.”

In their just-published study, the researchers analyzed the microbiomes of 35 healthy mares that are long-time residents of three farms in Oklahoma, Louisiana, and Australia, as well as that of 19 mares purchased from scattered owners in the Southern Mid-Western states of the US.

They found that the most abundant genera across the study animals were Pseudomonas (27%) followed by Lonsdalea (8%), Lactobacillus (7.5%), Escherichia/Shigella (4.5%), and Prevotella (3%). Oklahoma and Louisiana samples were dominated by Pseudomonas (75%).

Lonsdalea (28%) was the most abundant genus in the Australian samples, but was not found in any other region.

“Microbial diversity, richness, and evenness of the equine uterine microbiome is largely dependent on the geographical location of the animal,” they reported. “However, we observed a core uterine microbiome consisting of Lactobacillus, Escherichia/Shigella, Streptococcus, Blautia, Staphylococcus, Klebsiella, Acinetobacter, and Peptoanaerobacter.”

The core microbiome was defined as genera present in all samples at an abundance of at least 0.1%.

The authors noted that Pseudomonas is the most abundant genus in the canine endometrium as well. Pseudomonas aeruginosa is thought to be an opportunistic pathogen that is responsible for many instances of equine endometritis.

“Indirect evidence suggests that organisms belonging to the genus Pseudomonas in our study consists of several species,” they said. However, they were unable to reliably assign species with the database used for classification. “However, we are in the process of isolating and fully characterizing the Pseudomonas species identified in equine uteri.”

They said the species composition of the Australian samples, with the dominance of Lonsdalea, revealed an interesting trend. In five of the 14 samples, Lonsdalea represented more than 60% of the organisms identified. In another two samples, they exceeded 20%.

“This was an intriguing finding as Lonsdalea has not previously been reported in mammals. It is a relatively new bacterial genus that previously belonged to the genus Brenneria. Lonsdalea (as well as Brenneria) is a known phytopathogen that belongs to the Enterobacteriaceae family and is a pathogen of poplar and oak trees.

“The large contribution of this one genus to Australian samples, and its presence in a majority of the animals tested is intriguing and rules out a transient environmental infection.

“As our Australian samples came from one semi-closed herd in one location, further investigation is required before the significance of Lonsdalea in equine uteri could be determined. It is also possible that Lonsdalea found in Australia and Erwinia found in Oklahoma and Louisiana belong to a hitherto unclassified organism(s) belonging to the same superfamily.”

The authors acknowledged that differences in climate and feed could have contributed to some of the variations identified in the study. They further acknowledged that only a limited number of geographical locations were covered in this study.

However, their research bridged a research gap that existed in the commensal uterine microbiome of healthy mares.

The findings, they said, represent a platform for future research on elucidating the uterine microbiome at a species level.

“We hope that other researchers would build on this study and further refine the healthy equine uterine microbiome.

“Furthermore, this paves the way for developing microbes as pre or probiotics to enhance and maintain the natural balance of the uterine microbiome, thus improving the overall reproductive health of the mare.”

The study team comprised Holyoak and Premathilake, from Oklahoma State University, as were two other researchers involved in the study. Other members are with Auburn University in Alabama, Louisiana State University and Charles Sturt University in Australia.

Holyoak, G.R., Premathilake, H.U., Lyman, C.C. et al. The healthy equine uterus harbors a distinct core microbiome plus a rich and diverse microbiome that varies with geographical location. Sci Rep 12, 14790 (2022).

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