Beyond nocardioform: Transcriptionally active microbes and host responses in equine mucoid placentitis

Nocardioform placentitis (NP) is an understudied form of equine placentitis historically attributed to nocardioform bacteria, yet it remains uncertain whether these organisms are the sole pathogens involved.

To elucidate the pathophysiology of NP and the host-pathogen interaction.

In vivo clinical multi-omics study.

Dual RNA sequencing was performed to profile transcriptionally active microbial communities and concurrent placental transcriptome responses in samples from 31 placentas with and without NP. Untargeted metabolomics was performed to study the associated metabolites in the placenta.

The most abundant microbial transcripts belonged to Amycolatopsis, Crossiella, Lentzea, Enterococcus, and Mycobacterium. Bacterial gene expression in NP-affected placentas was enriched in pathways related to ribosomal activity and metabolic processes involving amino acid, carbohydrate, and glycosphingolipid metabolism. Concurrently, placental transcripts demonstrated significant upregulation of inflammatory pathways and downregulation of pathways associated with blood vessel formation. Untargeted metabolomics highlighted an elevated abundance of metabolites such as beta-D-fucose, nervonic acid, and zymostenol in the placentitis samples. Significant correlations were found between microbial genes (mraW, rlmB, amy, afuA, and cysC) and host inflammation genes (CXCL14, IL15RA, TASL, and IFIH1). Additionally, elevated beta-D-fucose, a microbe-specific metabolite, showed a strong correlation with microbial genes involved in stress-adaptive metabolism and DNA repair (ydhP, ybgC, serC, puuE, and radA). The bacterial enzymes involved in beta-D-fucose were notably upregulated and predominantly expressed by Amycolatopsis and Lentzea.

Classification based on RNA abundance limited the number of Crossiella cases (n = 3).

Both nocardioform and non-nocardioform bacteria are involved in NP-diagnosed cases, challenging the current generalisation of the term 'nocardioform placentitis' and supporting the need to broaden diagnostic protocols for mucoid placentitis. Multi-omics profiling revealed potential host-microbe interactions mediated by microbial metabolites, offering mechanistic insights and opportunities for improved diagnostic strategies.