EFFECT OF AEROSOLISATION, GROWTH-PHASE AND RESIDENCE TIME IN SPRAY AND COLLECTION FLUIDS ON THE CULTURABILITY OF CELLS AND SPORES

Work has been carried out in an initiative to establish a U.K.-based m easurement infrastructure for aerosol and bioaerosols. The initiative is supported by the National Measurement System Policy Unit (NMSPU) of the U.K. Government Department of Trade and Industry (DTI) under the Valid Analytical Measurement System (VAM) scheme. The overall aim of t he work is to examine the appropriateness and practicality of validati ng and certifying microbiological reference materials for bioaerosol s tandards. One of the prime uses of such bioaerosol standards is the ch aracterisation of samplers and assay methods. Candidate microbiologica l materials were selected from a survey of mainly U.K. industrial (and others) user needs. The microorganisms selected are Escherichia coli (National Collection of Industrial and Marine Bacteria, NCIMB 86), Sac charomyces cerevisiae (National Collection of Yeast Cultures, NCYC 133 7) and Penicillium expansum spores (Rothamsted isolate no. C2636). Cul turing techniques are often used in conventional aerobiology to enumer ate the numbers of microorganisms. This publication reports on the pro gramme of work to examine the effects that aerosolisation parameters a nd residence time in the spray and collection liquids can have on bioa erosol culturability. Data showed that suspensions of S. cerevisiae (N CYC 1337) cells can be maintained at an average culturable fraction of 0.7 for a period of 3 h in a suitable buffer solution maintained at t he ambient room conditions prevailing in the studies (20 degrees C and 30% RH). This suspension can be aerosolised to produce a bioaerosol w ith a culturable fraction that depends on spray suspension age and col lected aerosol age. The culturable fraction of this bioaerosol was fou nd to decrease with spray suspension age and with collected aerosol ag e. This strain of the species may not be suitable as a microbiological reference material. The results of the work with E. coli (NCIMB 86) s howed that aerosolisation reduced the culturable fraction in the bioae rosol to close to zero, although the culturable fraction could be main tained at approximately 0.24 (stationary phase) and 0.33 (log-phase) f or cells maintained in the spray suspension for up to 3 h. It is concl uded that this strain of E. coli (although defined to be a fairly robu st strain of this species)is not suitable as a standard bioaerosol or microbiological reference material. The results of the studies with P. expansum (Rothamsted isolate No. C2636) spores showed that aerosolisa tion had little general effect on the culturability of the bioaerosol (average culturable fraction of 0.25). This observation is to be expec ted with fairly robust microorganisms such as fungal spores. In view o f their robustness, P. expansum, spores are recommended as a microbiol ogical reference material, and that a standardised aerosol of this tes t microorganism should be prepared according to the methodology descri bed in this paper for culturing, preparing the spray suspension, aeros olisation under the correct ambient conditions, collection and assay. The results of the tests indicate that traditional culturing technique s underestimate cell/spore numbers in the collected bioaerosols of the microorganisms used. Total count techniques are therefore recommended for the determination of cell/spore numbers in collected aerosols of the microorganisms used in the tests. The culturability of P. expansum spores is unaffected by aerosolisation, and total number may be deter mined either by total counts or by culture counting taking into accoun t the initial culturable fraction in the spray suspension. It should b e noted that different methods for counting total cell numbers may giv e different results. Copyright (C) 1996 Elsevier Science Ltd.