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 Wind-blown dust from the expanding Sahara Desert reaches far out into the Atlantic Ocean, and eventually North America. Scientists hope to learn how this process, which is linked to climate change, alters the microbial population of the air. (Photo NASA)
“Before
this study, no one had a sense of the diversity of the microbes in the
air,” says lead author Gary Andersen of Berkeley Lab’s Earth Sciences
Division.
The research, which will be published this week in the online early edition of the Proceedings of the National Academy of Sciences,
serves two purposes. It paves the way for regional bacterial censuses
that will help a Department of Homeland Security bioterrorism
surveillance program differentiate between normal and suspicious
fluctuations in airborne pathogens. It will also help scientists
establish a baseline of airborne microbes, which they can use to track
how climate change affects bacterial populations.
“We want to determine the background levels of airborne pathogens
and other microbes because only very limited work has been conducted on
cataloging organisms in the air,” says Andersen. “This work underscores
how much we don’t know about airborne bacterial populations, or where
the bacteria come from.”
In the past, scientists relied
on bacterial cultures to determine what microbes are present in an air
sample. In this method, the culture media is exposed to the sample, and
whatever grows is counted. Unfortunately, this approach leaves out all
of the organisms that can’t survive in the culture, which in some cases
is as much as 99 percent of the bacteria in a sample.
In this census, however, Andersen and colleagues used a vastly more
comprehensive test developed by Todd DeSantis, who is also with
Berkeley Lab’s Earth Sciences Division. Their DNA microarray probes air
samples for a gene involved in making proteins, called 16S rRNA, which
is found in all bacteria. The square-shaped microarray, which is called
PhyloChip and is roughly the size of a quarter, can detect up to 9,000
different types of this gene, each unique to a different type of
bacteria. The microarray is sensitive enough to differentiate among
these thousands of gene sequences, meaning it can analyze an air sample
and list every type of organism present.
To conduct
the study, daily air samples were taken at several locations in San
Antonio and Austin over a 17-week period. The samples were sent to
Berkeley Lab where they were analyzed by the microarray. It found 1,800
types of bacteria, including some pathogens, wafting in the air over
the two cities. This diverse population matches the complexity of soil
populations, which is considered to be one of the richest habitats for
microbes.
 PhyloChip boasts a lot of analytical power in a small package.
“This gives us hope that we can
eventually develop a regional airborne microbial census, perhaps even a
nationwide or global census,” says Andersen. “This will also help us
determine the sources of airborne bacteria. Does it come from nearby
farms and water treatment plants, or is it imported by the wind from
another state or country?”
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
Caribbean.
“We need to determine what’s in the air, so
we can determine how climate change affects microbial diversity,” says
Andersen. “We found that there are a lot of airborne bacteria,
including pathogens, which we did not know are out there.”
“Urban aerosols harbor a diverse and dynamic bacterial population” is
published in the Dec. 18 to 22, 2006, online edition of the Proceedings of the National Academy of Sciences.
In addition to Andersen, Brodie and DeSantis, fellow Earth Sciences
Division scientists Jordan Moberg, Ingrid Zubietta, and Yvette Piceno
contributed to the research. The research was funded by the Department
of Homeland Security and the Department of Energy.
Source: Berkeley Lab
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