The Air You Breathe is Loaded with Microbes
PhyloChip boasts a lot of analytical power in a small package.
Credit: Gary Andersen, Lawrence Berkeley National Laboratory

The air you breathe is teeming with more than 1,800 kinds of bacteria, including harmless relatives of microbes associated with bioterrorist attacks, according to a new study.

The finding, detailed online this week in the journal for the Proceedings of the National Academy of Sciences, will allow scientists to create a baseline against which future researchers can measure changes in bacterial populations due to factors such as climate change. Plus, by knowing what’s typically aloft in the air, scientists could distinguish between normal and suspicious fluctuations—a sign of a bioterrorist attack.

"Before this study, no one had a sense of the diversity of the microbes in the air," said study leader Gary Andersen of the Earth and Sciences Division of the Lawrence Berkeley National Laboratory in California.

The researchers sampled air from San Antonio and Austin, Texas, every day for 17 weeks and then sent the samples to Berkeley Lab for analysis. The two cities are comparable in many regards—including population densities, elevation and topography—and differ only in that they are about 60 miles apart. The results would thus show whether background levels of airborne bacteria differ between cities.

In the past, scientists relied on bacterial cultures to identify microbes in air samples. This approach, however, can't pick up organisms unable to survive in the culture, which can be up to 99 percent of the bacteria in a sample.

Instead, Andersen and his colleagues used a DNA micro-array to probe air samples for a gene involved in producing a protein, called 16S rRNA, that is found in all bacteria. Called the "PhyloChip," the quarter-size device [image] can detect up to 9,000 unique versions of this gene, each one belonging to a different type of bacteria.

The test found evidence of more than 1,800 types of airborne bacteria. It also showed that location is not as important as the time of year and weather for predicting microbial diversity than location. For example, during warmer and dryer conditions they found increased levels of spore-forming bacteria.

The findings will help scientists determine whether a pathogen's presence is weather-related or a sign of a bioterrorist attack. "A spike may not be due to a biological attack, but to normal weather fluctuations that draw bacteria up from their natural reservoir," said study team member Eoin Brodie, also of Berkeley Lab's Earth and Sciences Division.

The team detected relatives of Francisella tularensis, a naturally occurring bacterium that causes tularemia, also known as rabbit fever. Present in all U.S. states except Hawaii, the potent bacterium has been flagged as a potential bioterror weapon. If scientists could determine whether the amount of F. tularensis detected in the air is within normal levels, they could avoid triggering false alarms of a biological weapons attack.

A global census of airborne bacteria wouldalso allow scientists to better track the effects of climate change on microbial populations in the atmosphere. For example, scientists have recently linked wind-blown dust from Africa's Sahara desert that reaches North America [image] to increases in asthma in the Caribbean.