International Space Station, the 17-year-old airtight US space lab is actually hosting thousands of species other than Homo sapiens, said a new study. It has revealed that the 250-mile-high space station's internal surface harbors nearly 4,000 microbe species.

A research paper, published in PeerJ, the microbe samples for the study were gathered back in 2014 under Project MERCCURI, a citizen science program. The interesting initiative was conducted by a team of cheerleaders, from National Basketball Association and National Football League, who also happened to be engineers and scientists. Together they swabbed down a few professional sports stadiums, identified the microbes' samples and sent them to the International Space Station to study if they survive there.

In exchange, the scientists, from the University of California at Davis, who had taken part in the project, had asked astronauts in ISS to swab down the space lab and send the samples back to Earth. As per the report, Bacillus aryabhatti, which were gathered from a practice football field that the Oakland Raiders use, grew up fastest on ISS.

"Diversity is generally associated with a healthy ecosystem," David Coil, who is a microbiologist at the University of California in Davis, told the Washington Post. According to him, a diverse population of microscopic inhabitants is likely a sign of a healthy spacecraft. Since earthlings are planning for longer missions in space now, it is important for scientists to properly understand these microbes.

UC Davis scientists conducted sequencing on the genomes of the samples that were sent to Earth from ISS. In each of the samples, the researchers recognized 1,036 to 4,294 operational taxonomic units, a biological measure used for the classification of closely connected organisms that reflect the number of species approximately. Following this, the experts compared the result with what they found from surveying humans and their homes' microbiomes.

The study revealed that the skylab mainly had human-associated microbes, specifically the type that resides on human skin.

However, the sequencing method that the scientists used on Earth can only identify the known species. So, scientists couldn't rule out the chances of something alien residing up there.

"Honestly, I wasn't very surprised at all by the findings," said the first author of the study, microbiologist Jenna Lang, to the publication. Since all the instruments that make their way to the ISS undergo thorough sterilization process, it's only normal that any germ present in the space station gets there riding on the astronauts. "I fully expected the ISS surface to look like human skin and...upper airway, which is, for the most part, did," added Lang. She also mentioned that when the crew of the space station changes, chances are the lab's microbes will also change.

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Currently, scientists are trying to know what happens to the good microbes up there in the space station and how they interact with the microbiome from the space. "There's a lot of much bigger studies going on now asking these bigger questions. Ours is more of a preliminary piece of data," said Coil.