Bacterial bad guys: ranking the threat of six common microbes. (Science Selections).

Studies have shown that occupants of moldy buildings can suffer from adverse health effects ranging from eye irritation to severe respiratory problems to (rarely) death. Only a fraction of the microorganisms found in living spaces have been characterized to any extent, but now a research team from Finland's National Public Health Institute reports that among the bacteria and fungi occurring in buildings, bacteria may pose the greater health risk to occupants [EHP 111:85-92]. The team's work provides the first hierarchical assessment of some of the dangers posed by selected noninfectious indoor microorganisms, especially those associated with water damage.

To rank the hazards of indoor microorganisms, the team selected three species each of bacteria and fungi that are typically (but not exclusively) found in water-damaged buildings, or that grow especially well in such buildings. The gram-positive bacteria Streptomyces californicus and Bacillus cereus are known inhabitants of water-damaged buildings, and the gram-negative bacterium Pseudomonas fluorescens is found in many indoor and outdoor environments. The fungus Penicillium spinulosum is regularly found in both normal and water-damaged indoor environments, whereas the fungi Stachybotrys chartarum and Aspergillus versicolor tend to accompany only heavy indoor water damage. Of the strains of bacteria and fungi studied, four were isolated in buildings with moisture problems, but two that the team found to be the most potent--P. fluorescens and B. cereus--were isolated in residences without proven water damage.

The team assessed the health impacts of these organisms, grown on an ordinary laboratory medium, by exposing them in vitro to mouse and human cells. The team evaluated the effects of the organisms by measuring the cells' production of interleukin 1[beta], interleukin 6, tumor necrosis factor-[alpha], and nitric oxide (all of which are involved in inflammation response) and by measuring cell viability. The team tested a range of concentrations of the organisms as well as a range of exposure times of up to 48 hours.

When comparing equivalent numbers of each organism, the team found the three bacteria to be the more potent inflammation stimulators, and ranked them in the order P. fluorescens, then S. californicus, then B. cereus. The fungi followed the bacteria in potency, in the order Sta. chartarum, then A. versicolor, then Pe. spinulosum. The team found the results regarding cell viability to be less definitive, but ranked the organisms to reflect more pronounced effects from some fungi and reduced effects from some bacteria: P. fluorescens (most potent), Sta. chartarum, S. californicus, A. versicolor, B. cereus, and finally Pe. spinulosum (least potent).

Although the team's findings provide some of the first guidance on the degree to which the microorganisms of the indoor environment can affect human health, much research remains to be done, acknowledges first author Kati Huttunen. For instance, additional studies should evaluate lower organism concentrations and longer exposure times that more closely approximate those found in indoor environments. Other research should assess the importance of surface and building materials as growth media on the toxin production of fungi and bacteria, and investigate synergistic or antagonistic effects that could occur with other organisms or substances in the indoor environment. Finally, further study should focus on other microorganisms, as well as other strains of these six organisms, and explore health criteria other than inflammation and cell viability, both in vitro and in vivo.