Charge levels and Gram (±) fractions of environmental bacterial aerosols

• Gram+ and Gram− bacterial aerosols are equally abundant in natural environments.
• Outdoor viable bioaerosols have smaller average particle size than indoor environment.
• Indoor and outdoor bioaerosol charges have skewed and normal distributions, respectively.
• A significant fraction of indoor bioaerosol particles are viable-but-not-culturable (VBNC).

Here, we investigated charge levels and Gram (+/–) fractions of environmental bacterial aerosols. The bioaerosols with +/– charges were collected separately over different time periods using an electrostatic sampler. Air samples were cultivated using selective media for G+ and G–, and further confirmed by Gram stain method. In addition, the viable bioaerosol size distributions over a 24 h time period were also obtained using an ultraviolet aerodynamic particle sizer (UV-APS). Finally, the elementary charge units carried by environmental G+ and G– bacterial aerosols were derived based on distribution-weighted particle aerodynamic diameters.Results showed that G+ had equal abundances with G– regardless of the charge polarity and environments. The outdoor viable bioaerosols were observed to have slightly smaller distribution-weighted particle size (1.64 µm) than that (1.79 µm) of the indoor environment. In general, the outdoor culturable bacterial aerosol charge levels turned to be normally-distributed with a peak around 21–29 elementary charge units, while the indoor ones seemed to be skewed toward 46–92 elementary charge units. Results here suggest that a significant fraction of viable-but-not-culturable (VBNC) bioaerosol particles might be present for indoor environments. Results here can not only help design better electrostatic sampling device, but also assist in bioaerosol exposure assessment.