While it is no surprise that people emit microbes from their body surfaces and even produce faint microbial clouds, it may come as a revelation that people’s microbial clouds are highly distinct. In fact, these clouds can be sampled and tested to determine who has occupied a particular space. Yes, our microbial clouds might even serve as airborne fingerprints. If this possibility were realized, forensic specialists might proclaim, “You can bathe, but you can’t hide!”

Just as our microbiomes are distinct, consisting of unique mixtures of bacterial strains, our microbial clouds are, well, individualized. To test the individualized nature of the personal microbial cloud, University of Oregon researchers sequenced microbes from the air surrounding 11 different people in a sanitized experimental chamber. The study found that most of the occupants sitting alone in the chamber could be identified within 4 hours just by the unique combinations of bacteria in the surrounding air.

These results appeared September 22 in PeerJ, in an article entitled, “Humans differ in their personal microbial cloud.” The article noted that bioaerosols (airborne biological particles including bacteria and bacteria-laden particles) have often been the focus of infection-control studies, but their role in seeding the built environment microbiome remains poorly understood. Bioaerosols are less frequently studied than human-associated bacteria that spread via direct contact with surfaces.

A new wind is blowing, however, with ever-wider deployment of sequencing technologies. In the current study, high-throughput sequencing of 16S rRNA genes was used to characterize the airborne bacterial contributions of individual study participants. Analyses accounted for several groups of bacteria that are ubiquitous on and in humans, such as Streptococcus, which is commonly found in the mouth, and Propionibacterium and Corynebacterium, both common skin residents.

Because the study focused on categorizing whole microbial communities rather than merely identifying pathogens, the University of Oregon researchers were able to distinguish unique combinations of common bacteria emitted by individual subjects.

“Our data make clear that an occupied space is microbially distinct from an unoccupied one, and reveal for the first time that individuals occupying a space can emit their own distinct personal microbial cloud,” wrote the authors. “[We] now have a clearer picture of individual shedding rates, personal identifiability, and the residual fate of the personal microbial cloud in the built environment.”

These findings emerged from two different experiments—examinations of suspended and settled bacteria—and more than 14 million sequences representing thousands of different types of bacteria found in 312 samples that were collected from an experimental chamber’s air and dust. “We expected that we would be able to detect the human microbiome in the air around a person,” said the study’s lead author, James F. Meadow, Ph.D. “But we were surprised to find that we could identify most of the occupants just by sampling their microbial cloud.”

The potential identifiability reported by the authors suggests a forensic application for indoor bioaerosols. The authors cautioned, however, that such applications will certainly require further research. “The patterns we found,” the authors noted, “are likely to be more nuanced in a crowd of occupants, in a larger indoor space, or in the presence of resuspended dust.”

Whatever the forensic possibilities, the study indicates that bacterial emissions from a relatively inactive person, sitting at a desk for instance, have a strong influence on the bacteria circulating in an enclosed space and on surrounding surfaces. “As humans we spend a substantial portion of our lives indoors, up to 90% in industrialized nations, and human density in urban areas is expected to increase,” the authors explained. “It is now apparent, given the results presented here, that the microbes we encounter include those actively emitted by other humans, including our families, coworkers, and perfect strangers.”

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