Hazard inherent in microbial tracers: reduction of risk by the use of Bacillus stearothermophilus spores in aerobiology.

The use of a "biological tracer" forms an essential part of many aerobiological experiments. Where release of such tracers is likely to result in deliberate or inadvertent human exposure, safety becomes a primary consideration in the selection of the tracer organism. Of the three most commonly used organisms, namely Bacillus subtilis, Escherichia coli, and Serratia marcescens, only the first comes near to satisfying the need for nonpathogenicity and even it has been incriminated as a cause of human infection, sometimes with a fatal outcome. The relevant characteristics of B. stearothermophilus were, therefore, investigated. Because it can grow only at elevated temperatures (minimum 41 C; optimum 56 C), it should not pose a threat to human health and this view is supported by experimental evidence to be presented. It is extremely easy to grow and maintain in the laboratory, and spore suspensions are easily prepared and stored. It withstands the stresses of aerosolization and sampling and its stability in the aerosol state compares favorably with that of B. subtilis var. niger.