Deadly bacteria could be wafting undetected through air conditioners. Microbiologists in Maryland have shown that current tests can miss airborne bacteria even when the cells are abundant. Many unexplained hospital disease outbreaks, they suggest, could be triggered by infected ventilation systems.
Bacteria that infect moist ventilation systems include species that cause pneumonia, meningitis and legionnaire's disease. So hospitals routinely screen air samples for potential pathogens, as do public-health scientists trying to track down the source of disease outbreaks linked to particular buildings.
The standard procedure is to take a sample of air, dissolve it, and spread the mixture on a nutrient-laden Petri dish. By counting the number of bacterial colonies that develop, microbiologists estimate the density of bacteria in the air sample.
This test is good at detecting actively dividing cells. "The feeling was that if the bacteria were not growing, they were dead," says Rita Colwell of the University of Maryland College Park, who led the new research. But over the past decade, microbiologists have discovered that many bacteria enter a dormant state and cannot be coaxed to multiply on standard nutrients.
Colwell reasoned that being tossed into the air with no food might drive bacteria into dormancy. So with her Maryland colleagues, and Environmental Protection Agency scientists based in Cincinnati, she tested how common this state is in airborne bacteria.
To a sealed chamber, the researchers connected air sampling tubes and an atomizer which could create a fine spray from cultures of bacteria in liquid.
Serratia marcescens, which has been known to cause meningitis; Klebsiella planticola, which can cause pneumonia; and Cytophaga allerginae, a soil bacterium, were used in the tests.
The results were striking. In a sample of S. marcescens taken immediately after spraying, the standard colony-forming assay produced a figure of just 1,740 cells per milliliter of air.
To detect dormant cells, the researchers exposed dissolved air samples to nutrients and an antibiotic that makes bacterial cell walls weaken so that cells balloon out and are easy to see under a microscope. This second test gave a figure of 24,850 bacteria per milliliter meaning nearly 93 percent of the cells were missed by the standard assay.
The disparity between the techniques grew over time. After four hours the microscope method found 11,900 cells per milliliter of air. The colony-counting method detected none, Colwell and her team report in the latest issue of Applied and Environmental Microbiology.
"It's almost like the days before people knew there were %J bacteria. When we think we're state of the art, it turns out that we're missing something important," says David Klein, director of the bacterial respiratory diseases research program at the National Institute of Allergy and Infectious Diseases near Washington.
Pub Date: 9/21/97