370. Characterization of Microbiota in 250-Day-Old Bovine Fetuses and Their Dams.

Abstract Emerging evidence suggests that microbial exposure of the bovine fetus may begin before birth. Here, we evaluated the microbiota in bovine fetuses at day 250 of gestation and their respective dams at the time of fetal harvest, and whether these microbial communities differed in response to maternal vitamin and mineral (VTM) supplementation that dam exposed during their gestation. Angus-crossbred heifers (F0; n = 72; 14–15 months) were assigned to either a basal diet (CON; n = 36) or a VTM-supplemented diet (n = 36; 113 g·heifer⁻¹·d⁻¹). F1 offspring (n = 11 CON; n = 17 VTM) were bred and maintained on a common diet until day 250 of gestation. At approximately day 250, a subset of F1 pregnant heifers (N = 15; n = 7 CON and n = 8 VTM) were euthanized, and maternal and fetal tissues and fluids were collected under aseptic conditions in a large-animal slaughter facility. Microbial communities in eleven sample types representing maternal (ruminal fluid and tissue, vagina, whole blood) and fetal (lung, ruminal fluid and tissue, amniotic and allantoic fluid, blood, cotyledon) compartments were characterized using 16S rRNA gene sequencing. Contamination controls were also included and used to identify and remove potential contaminant amplicon sequence variants (ASVs). Overall, microbial signatures were detected in each fetal compartment evaluated. Across the seven fetal-associated sample types, the mean number of observed ASVs ranged from 247 to 366 at a rarefaction level of 50,000 sequences. Fetal microbial community structure was similar across fetal sample types but was markedly distinct from maternal ruminal and vaginal microbiota (PERMANOVA, P < 0.0001). Maternal blood samples were distinct from both fetal and other maternal (ruminal and vaginal) communities. No effect of dam's maternal VTM supplementation on the microbial community structure or diversity was detected for any fetal or maternal sample type (P > 0.05). Fetal-associated microbial communities were consistently dominated by Lactobacillus, Allobaculum, Fusobacterium, Peptoclostridium, Blautia, and Ligilactobacillus, whereas the maternal microbiota varied considerably by sample type. Across fetal sample types, unknown sources accounted for 49.0% of estimated contributions, followed by maternal blood at 36.6% and the maternal vagina at 15.0%. Maternal rumen fluid and tissue were near zero, suggesting limited detectable contribution from rumen-associated maternal sources. Overall, our results demonstrate that diverse microbiota signatures are detectable across multiple fetal compartments and are distinct from maternal microbiota during late gestation in beef cattle. For image description, please refer to the figure legend and surrounding text. For image description, please refer to the figure legend and surrounding text. For image description, please refer to the figure legend and surrounding text. For image description, please refer to the figure legend and surrounding text. For image description, please refer to the figure legend and surrounding text. For image description, please refer to the figure legend and surrounding text. For image description, please refer to the figure legend and surrounding text.