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persist for days to months in soil (4). A time-course study of pathogens in soil can be 207 performed using RNA (or messenger RNA) analysis or by culture (if culturable). 208 If nucleic acid preservation is desired, a commercial preservation solution with a 209 validated expiration date is recommended. The preservative should be evaluated before 210 use for compatibility with the method or system under evaluation. Likewise, commercial 211 nucleic acid extraction kits intended for use with soil are recommended. Appropriate 212 bacterial controls should be used. 213 Most naturally occurring microorganisms do not grow on common culture media (5). 214 However, culturing microbes does provide an indication of the diversity and types of 215 microbes occurring in the soil. Culturing can also be used to indicate whether a soil was 216 adequately sterilized. A general-purpose solid culture medium for soil samples is R2A 217 Agar, which is a low-nutrient agar formulated to promote growth of stressed or slow- 218 growing bacterial cells. To culture soil bacteria using the R2A method: 219 • A 10-fold dilution is created by suspending 3 g of soil sample in 27 mL of 220 0.1% (by mass) peptone solution. 221 • Vortex the soil suspension for 30 sec, shake 25 times in a wide arc, and allow 222 the tube to sit for about 5 min until the coarse particles have settled. 223 • Spread-plate 0.1 mL of the supernatant solution onto an R2A agar plate. 224 It is recommended that serial 10-fold dilutions in 0.1% peptone be prepared and plated in 225 order to obtain isolated bacterial colonies. Dilutions of 10 -6 through 10 -8 could produce a 226 few plates with 20-200 isolated colonies for most soils. Incubate the plates at 20 – 28 °C 227 for 10 days. Fast growing bacteria will appear in about 2 days while slow growing 228 bacterial and other microbial colonies do not appear until 8-10 days of incubation (6). 229

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