The team also determined that location was not as strong a source of microbial variation as time and weather. Specifically, the time of the year during the 17-week testing period was the most significant source of variation, followed by atmospheric conditions. For example, warmer and dryer conditions led to increased amounts of spore-forming bacteria.
"This information may help explain temporal spikes, which is important in bioterror surveillance," adds Eoin Brodie, also with Berkeley Lab's Earth Sciences Division. "A spike may not be due to a biological attack, but to normal weather fluctuations that draw bacteria up from their natural reservoir."
In this way, bacterial censuses can help explain whether a pathogen's presence is natural or indicative of a biological attack. In one example, the team detected relatives of Francisella tularensis, a naturally occurring bacterium that causes tularemia, also known as rabbit fever. This especially potent bacterium is a possible candidate as a bioterror weapon. But it's also very common. Tularemia has been reported in all U.S. states except Hawaii. This natural background can confound the detection of a terrorist attack and trigger false alarms. The trick is to determine whether the amount of F. tularensis detected in an air sample is in synch with normal levels, or if it's suspicious.
"Almost all of the bacterial bioterror pathogens are in the environment and in the air naturally, so we need to find their natural backgrounds," says Andersen.
An airborne bacterial census will also enable scientists to track how climate change impacts the microbial composition of the atmosphere. This process is already occurring. Wind-blown dust and biomass from Africa's expanding Sahara desert are reaching North America in significant quantities. Recent research links this dust to an increase in asthma cases in the Carib
Contact: Dan Krotz
DOE/Lawrence Berkeley National Laboratory