n 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.
The scientists also sought to determine whether background levels of airborne bacteria change from city to city, or are generally the same throughout a region. To explore this question, they chose Austin and San Antonio because the two cities have similar population densities, elevation and topography, and they are only about 100 kilometers apart. After taking into account these similarities, they determined that the two cities shared a similar microbial composition.
"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 importe
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Source:DOE/Lawrence Berkeley National Laboratory
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