From 60 belly buttons, the team found 2,368 bacterial species...More at National Geographic and Wikipedia. [The most prevalent organisms were Staphylococci, Corynebacteria, Actinobacteria, Clostridia, and Bacilli - pretty much like the skin in general].
Some belly buttons harbored as few as 29 species and some as many as 107, although most had around 67. Ninety-two percent of the bacteria types showed up on fewer than 10 percent of subjects—in fact, most of the time, they appeared in only a single subject.
One science writer, for instance, apparently harbored a bacterium that had previously been found only in soil from Japan—where he has never been.
Another, more fragrant individual, who hadn't washed in several years, hosted two species of so-called extremophile bacteria that typically thrive in ice caps and thermal vents...
Despite the diversity, themes emerged. Even though not a single strain showed up in each subject, eight species were present on more than 70 percent of the subjects. And whenever these species appeared, they did so in huge numbers.
"That makes the belly button a lot like rain forests," Dunn said. In any given forest, he explained, the spectrum of flora might vary, but an ecologist can count on a certain few dominant tree types.
See also omphaloskepsis, navel fetishism, and alvinolagnia.