Feasibility of the Aerosol-to-Liquid Particle Extraction System (ALPES) for Collection of Viable Francisella sp

摘要

Several Biowatch monitoring sites in the Houston area have tested positive for Francisella tularensis and there is a need to determine whether natural occurring Francisella-related microorganism(s) may be responsible for these observed positive reactions. The collection, culturing and characterization of Francisella-related natural microorganisms will provide the knowledge base to improve the future selectivity of Biowatch monitoring for Francisella. The aerosol-to-liquid particle extraction system (ALPES) is a high-efficiency, dual mechanism collection system that utilizes a liquid collection medium for capture of airborne microorganisms. Since the viability of microorganisms is preserved better in liquid medium than on air filters, this project was undertaken to determine whether Francisella philomiragia and Francisella tularensis LVS maintain acceptable viability in the continuous liquid recirculation, high direct current voltage and residual ozone concentrations which occur during ALPES operation. Throughout a series of preliminary trial runs with representative gram-negative and gram-positive microorganisms, several design modifications and improvements to the ALPES optimized liquid handling, electrical stability, sampling and overall performance for biological sampling. Initial testing with Francisella philomiragia showed viability was preserved better in PBS buffer than HBSS buffer. Trial runs at starting cell concentrations of 1.8 x 10(sup 6) and 2.5 x 10(sup 4) CFU/L showed less than a 1-log decrease in viability for F. philomiragia after 24 h in the ALPES. Francisella tularensis LVS (live vaccine strain) was used as a surrogate for virulent F. tularensis in ALPES trial runs conducted at starting cell concentrations of 10(sup 4), 10(sup 5) and 10(sup 6) CFU/L. F. tularensis LVS was slow-growing and required highly selective growth media to prevent overgrowth by collected airborne microorganisms. In addition, one ALPES unit intake was HEPA filtered during the final trial runs with F. tularensis LVS to further reduce the levels of microbial background. Results from trials with F. tularensis LVS showed about a 1-log loss decrease in CFUs after 24 h, but maintained final cell concentrations in the range of 10(sup 3)-10(sup 4) CFU/L. These results indicate that the ALPES maintains acceptable viability of Francisella sp. in PBS buffer for up to 24 h and is a promising technology for the collection of viable airborne Francisella or Francisella-related cultures which may be observed at Biowatch monitoring sites in the Houston area and elsewhere.