The effect of phosphate buffer on aerosol size distribution of nebulized Bacillus subtilis and Pseudomonas fluorescens bacteria

During bioaerosol research microorganisms are frequently aerosolized from p hosphate-buffered suspension via air-jet nebulization. Buffer will therefor e be present in organism-free droplets ("empties") as solutes in hydrous su rface coatings of airborne organisms or as dry surface coatings on organism s. It might be expected that the buffer would influence the resulting aeros ol size distribution and that the influence might vary with relative humidi ty. In this work we examined the effect of various buffer concentrations on the aerosol size distribution of Bacillus subtilis (B. subtilis) and Pseud omonas-fluorescens (EI fluorescens) at various relative humidities. Microor ganism suspensions in distilled deionized water and phosphate-buffered wate r were prepared and nebulized from a 3-jet Collison nebulizer into a sampli ng chamber maintained at 10% relative humidity. The aerosol was then conduc ted to a second chamber to which moist air was added to achieve the target relative humidity. Particle aerodynamic size distributions were measured at 10, 20, 40, 60, 80, and 90% relative humidity. Buffer concentrations used were 0, 2.5, 5, 7.5, and 10%. Phosphate buffer substantially affected the i ndicated aerodynamic size distribution of both bioaerosols, with greater ef fect seen for the Gram-positive B. subtilis than for the Gram-negative P. f luorescens, The effects were most pronounced above 80% relative humidity. F urther, aerosol size distributions of B. subtilis, but not P. fluorescens, were affected by relative humidity even in the absence of buffer, suggestin g a species-related difference in hygroscopic growth. Growth predicted from theory was generally consistent with observed behavior, although uncertain ties remain concerning organism-and instrument-related influences on measur ed behavior.