The survival and potential growth of two different kinds of bacteria were investigated on NIOSH-certified polypropylene respirator filters. Bacillus subtilis var niger (B. subtilis) spores represented environmentally resistant bacteria, whereas Pseudomonas fluorescens (P. fluorescens) vegetative cells represented stress-sensitive bacteria. The bacteria were aerosolized and loaded on the respirator filters under three nutritional conditions: water, saliva, and tryptic soy broth (TSB). The loaded filters were incubated for 0-13 days and analyzed for culturable and total bacteria count. The analysis was optimized through the evaluation of three methods for eluting bacteria from respirator filter samples: low frequency shaking, vortexing, and ultrasonic vibrating. Vortexing was found to be the most effective with the highest total and culturable count and was therefore used for the analysis of bacterial survival and growth. Neither of the test bacteria was able to grow on the respirator filters even under optimal nutrition and incubation conditions. It therefore appears that reuse of a polypropylene respirator poses minimal risk of bacterial growth, provided the respirator has been carefully handled and stored. The data on respirator survival show that sensitive P. fluorescens cells lost their viability in less than three days, whereas resistant B. subtilis spores remained viable on the filter for over thirteen days of testing. It appears that resistant bacteria may pose risk of infection to the people near the respirator wearer if the bacteria are reaerosolized from the respirator. However, reaerosolization of a very small fraction of collected bacteria can occur only under extreme conditions of violent sneezing or coughing.
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