in a Petri dish to allow the microorganisms to colonize. The Petri dishes are incubated and the colonies are identified and enumerated [Muilenberg et al. 1992]. This method of serial dilution allows flexibility in dealing with unpredictable levels of spores by permitting a count of the spores collected on the filter either directly or by serial dilution of the wash solution. An inherent weakness in this procedure is that high analytical dilutions can statistically exclude taxa present in the air sample at low concentrations. This dilution technique favors the predominant fungi populations at the expense of minor populations. d.
Impingement The liquid impingers are a special type of impactor. Impingers are useful for the collection of culturable aerosols [White et al. 1975; Lembke et al. 1981; Henningson et al. 1988]. Impingers such as the Greenberg-Smith impinger or the AGI-30 use a liquid (e.g., a simple salt solution such as 0.3 mM phosphate-buffered dilution water) as the collection medium. Additives to the collection medium such as proteins, antifoam, or antifreeze aid in resuscitation of bacterial cells, prevent foaming and loss of the collection fluid, and minimize injury to the cells. The jet is positioned a set distance above the impinger base and consists of a short piece of capillary tube designed to reduce cell injury when the air is dispersed through the liquid and the particles are entrapped. The Greenberg-Smith and AGI-30 samplers operate by drawing aerosols at nominal flow rates of 28.3 and 12.5 L/min, respectively, through an inlet tube [Macher et al. 1995]. The d50 of these samplers is approximately 0.3 µm [Wolf et al. 1959; Cown et al. 1957]. The AGI-30 inlet tube is curved to simulate particle collection in the nasal passage [Cox 1987]. This makes it especially useful for studying infectious airborne microorganisms by separating respirable (collection fluid) and nonrespirable (inlet tube) microorganisms. When the AGI-30 is used to recover total airborne organisms from the environment, the curved inlet tube is washed with a known amount of collecting fluid after sampling because larger particles (i.e., over 15 µm) are collected on the tube wall by inertial force. After sampling for the appropriate amount of time, 10 mL of the full-strength collection fluid is filtered through a 0.45-µm pore size membrane filter. In addition, serial dilutions of the remaining collection fluid are handled similarly [Greenberg et al. 1992]. The membrane filter is placed in a 100-mm by 15-mm sterile plastic petri plate filled with the appropriate medium and incubated for later identification and enumeration.
e.
Characteristics of Several Bioaerosol Samplers Once the purpose or the goal of bioaerosol sampling is determined, the appropriate sampling method(s) may be chosen. The selected bioaerosol sampler(s) must be capable of high efficiency particle collection within the physical and biological conditions required by the microorganism(s) to be sampled. Experimental, theoretical, and physical characteristics of several commonly used bioaerosol samplers are shown in Table I. The physical characteristics (flow rate, diameter of hole or width of slit, area of nozzle, and velocity of air through the nozzle) were used to calculate the theoretical cut-diameters of the listed samplers. The theoretical characteristics were discussed in the preceding subsections. The particle size distribution of the bioaerosol is very important in the evaluation of the data obtained using the selected sampler. If the selected sampler does not provide particle size
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