placental microbiome placental microbiome the placental microbiome is the nonpathogenic, commensal bacteria claimed to be present in a healthy human placenta and is distinct from bacteria that cause infection and preterm birth in chorioamnionitis. until recently, the healthy placenta was considered to be a sterile organ but now genera and species have been identified that reside in the basal layer. placenta and its tissue layers it should be stressed that the evidence for a placental microbiome is controversial. most studies supporting the existence of a placental microbiome lack the appropriate experimental controls, and it has been found that contamination is most likely responsible for reports of a placental microbiome. the placental microbiome more closely resembles that of the oral microbiome than either the vaginal or rectal microbiome. bacterial species and genera culturable and non-culturable bacterial species in the placenta obtained following normal term pregnancy have been identified. binomial namecommensaltransientpotentialpathogen