The spores of the microorganism Bacillus subtilis var niger have been widely used as a bioaerosol tracer. Dead spores were used containing a small amount of radioactive tracer. Gradually, live spores were used despite problems with the effect of aerosolisation on the culturable fraction. B.subtilis var niger spores are now appearing in standard test methods without defining the test conditions and expecting the spores to be totally aerostable (i.e. 100% survive aerosolisation to form colonies on nutrient agar).
The procedures developed in the previous VAM Programme have been used to examine the effects of aerosolisation on B.subtilis var niger. Spores were aerosolised under controlled environmental conditions and captured in a glass cyclone sampler. The effects of three variables were investigated; spore spray suspension age, cyclone collection fluid age and test chamber relative humidity. Collection fluid age had no significant effect while relative humidity had a small effect. The most significant effect was caused by the length of time that the spores were stored in the spray suspension. Storing the spores for 3 hours in the spray suspension appeared to 'harden' them to the effects of aerosolisation.
The spores of B.subtilis var niger were used to compare the performance of four bioaerosol samplers. Only one sampler gave different results when the sampler inlet was facing or at right angles to the air flow. The performance of the samplers was also compared using three different particle size distributions of B.subtilis var niger spores in water with different concentrations of potassium iodide solutions. The results showed that only one sampler (a single sieve plate device) gave ratios of experimental to expected airborne concentration which depended on the particle size distribution of the bioaerosol challenge.