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7.1.2 Microbiological Characterization 211 content over time. Although free RNA degrades rapidly, encapsulated RNA (e.g., RNA 212 viruses) can be very stable. Additionally, many vegetative forms of bacterial pathogens 213 persist for days to months in soil (4). A time-course study of pathogens in soil can be 214 performed using RNA (or messenger RNA) analysis or by culture (if culturable). 215 If nucleic acid preservation is desired, a commercial preservation solution with a 216 validated expiration date is recommended. The preservative should be evaluated before 217 use for compatibility with the method or system under evaluation. Likewise, commercial 218 nucleic acid extraction kits intended for use with soil are recommended. Appropriate 219 commercial bacterial standards should be used. 220 Most naturally occurring microorganisms do not grow on common culture media (5). 221 However, culturing microbes does provide an indication of the diversity and types of 222 microbes occurring in the soil (6). Culturing can also be used to indicate whether a soil 223 was adequately sterilized (7). A general-purpose solid culture medium for soil samples is 224 R2A Agar, which is a low-nutrient agar formulated to promote growth of stressed or 225 slow-growing bacterial cells. To culture soil bacteria using the R2A method: 226  A 10-fold dilution is created by suspending 3 g of soil sample in 27 mL of 227 0.1% (by mass) peptone solution. 228  Vortex the soil suspension for 30 sec, shake 25 times in a wide arc, and allow 229 the tube to sit for about 5 min until the coarse particles have settled. 230  Spread-plate 0.1 mL of the supernatant solution onto an R2A agar plate. 231 The microbiological stability of soil samples can be gauged by measuring RNA

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