Microbial aerosol filtration: Growth and release of a bacteria–fungi consortium collected by fibrous filters in different operating conditions

Quantitative (CFU counting, ergosterol rate) and qualitative (SEM observations) analyses and a dedicated experimental set-up were combined in this study to examine the growth and release of a bacteria-fungi consortium collected by fibrous filters used in HVAC systems. The influence of three parameters was examined: air relative humidity (60%, 80% or close to 100%), the presence of air flow after microbial contamination of filters, and the nature of the fibrous media (glass or polypropylene fibers).
First, the initial filtration efficiency of the microbial consortium, composed of bacterial cells of Staphylococcus epidermidis and fungal spores of Penicillium oxalicum, was analyzed and compared to the initial filtration with a polyvinyl-acetate aerosol. Then, microbial behavior was evaluated in terms of microbial growth on to filters (in the situation of a simulated air flow stop) and particles released from filters (after a simulated restart of the ventilation). The results demonstrated that, whatever the operating conditions, S.epidermidis did not grow onto filters. On the contrary, P. oxalicum demonstrated significant growth multiplying by more than 100 times its number of colonies (CFU) and by more than 10 times its ergosterol rate in saturated moist air conditions. SEM images provided information about microbial behavior. A significant mycelia development was observed after 48 h of filters conditioning at 100% RH, while after a longer period of conditioning (168h), P. oxalicum spores were mostly observed. Regarding particle release, spores were detected downstream of the filters conditioned during 168 h when ventilation was restarted.