Differences in detected fluorescence among several bacterial species measured with a direct-reading particle sizer and fluorescence detector

Naturally-contained fluorescing chemicals (such as riboflavin or NADPH) can be used to detect the presence of biological organisms. A new instrument from TSI Incorporated measures fluorescence of particles using an ultraviolet laser operating at an excitation wavelength of 355 nm. We have employed this instrument (Model 3312 Ultraviolet Aerodynamic Particle Sizer (tm) Spectrometer) to assess the degree of fluoresence associated with a variety of biological aerosols. Nonfluorescent and fluorescent latex sphere and sodium chloride aerosols were first used to assure proper operation of the instrument and to obtain correct instrument settings. Biological aerosols were then generated by combining organisms with double distilled and filtered water in a Collison nebulizer operated at low pressure. After passage through a charge neutralizer and dilution with humidified air (45% RH), the aerosol was measured downstream for both particle size and fluorescence distributions. Bacterial aerosols generated include Bacillus subtilis subsp. niger (spores and vegetative cells), Staphylococcus epidermidis, Escherichia coli, and Mycobacterium abscessus (a surrogate for M. tuberculosis). Cladosporium spp. fungal spores were also evaluated, and the effect of heat treatment on fluorescence was tested using B, subtilis spores. For each test the percentage of organisms that produced a fluorescence signal above a threshold was recorded. The organisms demonstrated considerable differences in percent fluorescence, ranging from means of 11% for S. epidermidis to 44% for B. subtilis spores. Vegetative cells of B. subtilis were generally less fluorescent (mean of 33%) than the spores, while the highest level of fluorescence was associated with heat-treated spores (averaging about 75%). This instrument has some potential for use in settings where immediate detection of biological organisms is important. Work remains to be done on understanding the effect on fluorescence of organism viability, presence of nonbiological particles, and interferences from mixtures.