Semen was once assumed to be naturally microbe-free; any bacteria found in the fluid was assumed to be a sign of infection. But research over the past decade has shown that semen has its own microbial community, just like microbiomes of the gut and vagina.
The semen microbiome can contain a melange of microbes. Most come from glands in the upper reproductive tract including the testes, seminal vesicles and prostate which contribute various components to semen. Drifter bacteria from urine and the urethra can also get swept up in the fluid during ejaculation, and microbes from a person's blood or from their sexual partners might also take up residence in semen. But how any of these individual species of bacteria might affect health has largely been a mystery.
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