Effects of microwave irradiation on concentration, diversity and gene mutation of culturable airborne microorganisms of inhalable sizes in different environments

Previously, we have investigated the inactivation of total environmental bioaerosols using microwave irradiation at 2450 MHz. Here, the effects of microwave irradiation on the concentration, diversity and possible gene mutations of airborne culturable microorganisms of different size ranges were investigated. Air samples were collected in different environments (office, hotel and outdoor) using a six-stage Andersen sampler operated at its standard flow rate of 28.3 L/min without and with the microwave irradiation at three different power levels(119, 385 and 700 W) corresponding to energy doses of 7.1, 22 and 31.5 kJ, respectively. For each experimental condition, three to five independent replicates were conducted. Air samples collected onto different stages of the Andersen sampler were incubated at 26 °C, and colony forming units (CFUs) were manually counted and statistically corrected. The bacterial CFUs were further washed off from agar plates using deionized water, and subjected to polymerase chain reaction (PCR)-denaturing gradient gel electrophoresis (DGGE) for diversity and possible gene mutation analysis.Results revealed that for the outdoor environment, larger size (>3.3 μm) bacterial species dominated, while for the indoor environments those of smaller size (<2.1 μm) did. Use of the high power level (700 W) in general resulted in lower culturable bacterial counts regardless of the sampler stage and the sampling environment. The inactivation efficiencies of the microwave treatment were shown to strongly depend on the environments and the microwave energy dose under the experimental conditions tested. PCR-DGGE analysis indicated that use of the high power microwave irradiation (31.5 kJ) generally resulted in less culturable bacterial aerosol richness, while for medium (22 kJ) and low (7.1 kJ) energy doses their culturable species richness appeared to be similar to those of the control experiments. In contrast, smaller (1.1–2.2 μm) culturable fungal species were found to dominate regardless of sampling environments. In addition, culturable bacterial aerosol gene mutations with a high frequency after the microwave treatment were detected in the colonies developed. The results from this study provide further information in developing microwave-based bioaerosol inactivation technology.

► The inactivation efficiencies varied with environments, species and microwave energy doses.
► Bacterial aerosol gene mutations after the microwave treatment were detected in the colonies.
► The results are useful in developing microwave-based bioaerosol inactivation technology.