Could you be identified by your unique bacterial "cloud"?
It is well known that we play host to an extraordinarily large amount of bacteria - in fact people have more bacterial cells in and on our bodies than we have human cells1. And no man is an island: these bacteria are constantly being transferred to our environment, either by direct human contact or by emission from our breath, skin, hair and clothes. Humans shed around one million particles per hour, many of these containing bacteria. Previous studies have shown that people can leave behind bacterial signatures on surfaces that they have touched and in some cases, these bacterial signatures can even be traced back to an individual person2.
A group of researchers from the University of Oregon in the United States have shown, for the first time, the existence of a "microbial cloud" emanating from a person. The aim of the first part of their study, published the open access and peer-reviewed journal PeerJ, was to characterise the airborne bacterial cloud of a person sitting in a sanitised experimental climate chamber. Previous studies have detected human-associated airborne bacteria after human activity indoors, which would have also included a combination of resuspended dust and emission from clothing. In the current study, the test chambers were thoroughly cleaned before and in-between experiments and participants remained relatively inactive by moving around on rolling chairs to eliminate the potential for resuspended dust. By sequencing bacterial 16S rRNA genes, the researchers found a difference between the airborne bacteria from an occupied chamber to that of an identical unoccupied chamber. The participants in the study emitted an airborne signal which, after just two hours, could be detected above background airborne bacterial communities that were already present in the air in the chamber. This airborne signal consisted of human-associated bacterial populations that are all part of a healthy human microbiome3 such as Streptococcus, which is usually found in the mouth, and Propionibacterium and Corynebacterium, which are often found on the skin. Different combinations of the microbial families allowed the researchers to distinguish between the occupants. Additionally, some of the occupants could be identified by distinct bacteria: for example, the only woman participating in the study was easy to discern by her emission of Lactobacillus crispatus, a bacterium that is commonly found dominating healthy vaginal samples.
"We expected that we would be able to detect the human microbiome in the air around a person, but we were surprised to find that we could identify most of the occupants just by sampling their microbial cloud," said lead author and postdoctoral researcher James F. Meadow.
These results lead the researchers to design a second study to further explore the detectability and personalized nature of a given individual's microbial cloud. Using the same chambers as in the first part of study, the researchers sampled eight people, for 90 minute intervals with either one or three air changes per hour. They found that each of the eight occupants emitted their own unique concentration of airborne particles. Microbial emissions were compared among individuals and to exhaust air they found that it would be possible to detect people within the ventilation system of a building.
The results of this study have potentially important applications in regulating air ventilation in health-care facilities especially during disease outbreaks. It seems that a relatively inactive person has a strong influence on bacteria circulating in an enclosed space. It is well known that ventilation plays a key role in the transmission of disease indoors; the results of this study show that increasing air flow rates from one to three air changes per hour nearly eliminates the detectible human microbial cloud.
Perhaps the most exciting finding of the study was that participants could be identified by their airborne bacterial emissions as well as their contributions to settled particles, which could have applications forensically. The authors acknowledge that a person occupying a room for a short amount of time could go undetected as they would not shed enough bacterial particles to overcome background airborne bacterial concentrations. They also recognize that individuals may not be distinguishable in a crowd of people or in the presence of large amounts of resuspended dust.
The authors state that "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"
It is becoming increasingly apparent that we come into contact with a staggering amount of microbes daily, from our family, our pets, our co-workers and even perfect strangers. With over sevem billion people in the world, each microbially unique, it is plausible that we would each have our own personalized microbial 'fingerprint'.