Page:NIOSH Manual of Analytical Methods - Chapter J.pdf/12

 must be exercised to ensure people do not tamper with the samplers and that you do not inadvertently aerosolize microorganisms on surfaces or in duct-work. c.

Flow Calibration Accurate airflow rates are very important in calculating the concentration of microorganisms in the air. All samplers should be calibrated before and after sampling to ensure the flow rate is within the manufacturer's specifications and does not change from the initial calibration. Calibration may be performed using a primary standard such as a spirometer or bubble calibrator. Where it is not possible to calibrate using a primary standard, a secondary standard such as a dry gas meter may be used. The calibration of such a secondary standard should be traceable to a primary standard. See Chapter D. General Considerations for Sampling Airborne Contaminants of this manual for further details on calibration of airflow rates.

d.

Culture Media General detection and enumeration media are normally used in the collection of fungi, bacteria, and thermophilic Actinomycetes. Plates can be replicated on differential or selective media for identification after the organisms have been collected. In addition, it may be advisable to concurrently use more than one type of culture medium to collect aerosolized microorganisms, because of inherent biases caused by media selection. The following are some general guidelines for media: (1)

Fungi Traditionally, malt extract agar (MEA) and rose bengal agar (RBA) have been recommended as broad spectrum media for the collection and enumeration of fungi [Morring et al. 1983; Burge et al. 1977; Smid et al. 1989]. MEA and RBA are generic terms and formulations vary from supplier to supplier and laboratory to laboratory. One MEA recipe is a less nutritious, unamended 2% MEA, which is reported to promote better sporulation than MEA amended with glucose and/or peptone [Hunter et al. 1988; Strachan et al. 1990]. With RBA, the colonies remain small; however, natural or artificial light may make the medium toxic to some fungi. In addition, pigmentation of the fungal growth on RBA complicates the identification process. Based on the work of researchers in Australia and Holland, dichloran glycerol 18 agar (DG-18) is recommended for identification and enumeration of fungi [Hocking and Pitt 1980; Pitt et al. 1983; Verhoeff et al. 1990]. This medium is adequate for most fungi, including xerophilic fungi. DG-18 does not have the disadvantages of RBA. To inhibit the growth of bacteria, antibiotics, such as streptomycin, may be added to the RBA medium [DIFCO 1984].

1/15/98

93

NIOSH Manual of Analytical Methods