The existence of a placental microbiota and in utero colonization of the fetus has been the subject of recent debate. The objective of this study was to determine whether the placental and fetal tissues of mice harbor bacterial communities. Bacterial profiles of the placenta and fetal brain, lung, liver, and intestine were characterized through culture, qPCR, and 16S rRNA gene sequencing. These profiles were compared to those of the maternal mouth, lung, liver, uterus, cervix, vagina, and

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... ine, as well as to background technical controls. Positive bacterial cultures from placental and fetal tissues were rare; of the 165 total bacterial cultures of placental tissues from the 11 mice included in this study, only nine yielded at least a single colony, and five of those nine positive cultures came from a single mouse. Cultures of fetal intestinal tissues yielded just a single bacterial isolate: Staphylococcus hominis, a common skin bacterium. Bacterial loads of placental and fetal brain, lung, liver, and intestinal tissues were not higher than those of DNA contamination controls and did not yield substantive 16S rRNA gene sequencing libraries. From all placental or fetal tissues (N=49), there was only a single bacterial isolate that came from a fetal brain sample having a bacterial load higher than that of contamination controls and that was identified in sequence-based surveys of at least one of its corresponding maternal samples. Therefore, using multiple modes of microbiologic inquiry, there was not consistent evidence of bacterial communities in the placental and fetal tissues of mice.