Members of the Deinococcaceae family are some of the most remarkable organisms known.  Their most notable trait is their extreme resistance to ionizing radiation.  These organisms can survive doses of gamma radiation thousands of times stronger than the lethal dose for a human.  D. radiodurans, the first member of this family identified in 1956, was found in radiation-sterilized cans of meat.(7) Other members of this family were isolated from irradiated food products, elephant and llama feces, hot springs, soil, sewage, airborne dust, and humans. (2)

Originally, our group planned on isolating “Micrococcus radiodurans” because our initial research seemed to indicate that of the radiation resistant microbes known, it would be the most easily found.  Using the method of enrichment culture, i.e. modifying the culture conditions to enhance the growth of the desired organism while eliminating the growth of other organisms, our group planned on isolating “M. radiodurans” from soil, air and skin samples.

Even though the natural habitat of M. radiodurans was not known, we were confident that it could be isolated because it had been found in so many different environments including clean rooms where extensive sterilization methods are used.  While Deinococcus is ubiquitous, they are certainly not numerous.  Estimates are that there is only one Deinococcus cell per several thousand colony-forming units (CFU) of other organisms. (7) In spite of their small number, our group planned to use their resistance to ionizing radiation to eliminate the competing flora with ultraviolet radiation and allow the few Deinococcus present in our samples to multiply without competition.

Micrococcus is not a fastidious organisms and does not require special media.  It grows on plain nutrient agar (NA).  Because desiccation causes the same type of DNA damage as ionizing radiation, we thought that our target organism would also be able to tolerate high salt concentrations.  Other investigations support this assumption. (2) We made NA plates with 7.5% NaCl and nutrient broth with 6% NaCl along with TSA plates to grow our samples.  The high salt levels would help to reduce the growth of unwanted organisms and we felt that the TSA plates would promote faster growth of our organism provided that the competition was eliminated.  We planned to use soil from the most arid regions possible, again selecting for desiccation and intense sunlight resistance, as the environmental source.  The media plates would be to expose to several different levels of UV radiation.  Any organism that grew would have to be radiation resistant.

Micrococcus can be distinguished by just a few physical traits so we were confident if we found Gram-positive cocci, appearing in pairs or tetrads, strictly aerobic, catylase positive and radiation resistant, we could confidently conclude that we had isolated “Micrococcus radiodurans.”