Resuscitation of microorganisms after gamma irradiation.

Microbiological analysis of rock exposed to gamma-radiation doses between 0 and 9.34 kGy indicated that some microorganisms became viable but nonculturable (VBNC) and lost metabolic capacity as measured by BIOLOG microtiter plates. To investigate this phenomenon, portions of irradiated rock were placed at 4 degrees C for 2 months in an attempt to resuscitate the microbes to a culturable state. Culturable heterotrophs were enumerated and BIOLOG plates were used to determine the metabolic capability of the microbial community. Culturable bacteria that had previously been nonculturable were found at all doses. The number of colony types decreased from 26 in the nonirradiated control rock to between 9 and 10 in rock irradiated at doses ranging from 2.34 to 9.34 kGy. BIOLOG plates indicated partial recovery of metabolic capacity in all the samples tested. Fatty acid methyl ester analysis of the recovered isolates using the MIDI system (Microbial ID, Inc.) yielded three distinct groups of related bacteria. All resuscitated isolates clustered with the original nonirradiated isolates at the genus level, and 92% of them clustered at the species level. These results indicate that microbes were likely resuscitated from a VBNC state.