Extensive diversity of ionizing-radiation-resistant bacteria recovered from Sonoran Desert soil and description of nine new species of the genus Deinococcus obtained from a single soil sample

Abstract

The ionizing-radiation-resistant fractions of two soil bacterial communities were investigated by exposing an arid soil from the Sonoran Desert and a nonarid soil from a Louisiana forest to various doses of ionizing radiation using a (60)Co source. The numbers of surviving bacteria decreased as the dose of gamma radiation to which the soils were exposed increased. Bacterial isolates surviving doses of 30 kGy were recovered from the Sonoran Desert soil, while no isolates were recovered from the nonarid forest soil after exposure to doses greater than 13 kGy. The phylogenetic diversities of the surviving culturable bacteria were compared for the two soils using 16S rRNA gene sequence analysis. In addition to a bacterial population that was more resistant to higher doses of ionizing radiation, the diversity of the isolates was greater in the arid soil. The taxonomic diversity of the isolates recovered was found to decrease as the level of ionizing-radiation exposure increased. Bacterial isolates of the genera Deinococcus, Geodermatophilus, and Hymenobacter were still recovered from the arid soil after exposure to doses of 17 to 30 kGy. The recovery of large numbers of extremely ionizing-radiation-resistant bacteria from an arid soil and not from a nonarid soil provides further ecological support for the hypothesis that the ionizing-radiation resistance phenotype is a consequence of the evolution of other DNA repair systems that protect cells against commonly encountered environmental stressors, such as desiccation. The diverse group of bacterial strains isolated from the arid soil sample included 60 Deinococcus strains, the characterization of which revealed nine novel species of this genus.

FIG. 1.

Effect of radiation dose on the survival of culturable heterotrophic bacteria in the arid soil sample (S97-3).

FIG. 2.

16S rRNA gene sequence-based phylogeny indicating the relationship of the type strains of the nine new species of the genus Deinococcus to previously described species, strains, and environmental sequences falling in this lineage. Scale bar = 10 inferred nucleotide substitutions per 100 nucleotides.

FIG. 3.

Differential interference contrast photomicrographs showing the range of morphological types within the genus Deinococcus. (A) Strain KR-241^T. (B) Strain KR-87^T. (C) Strain KR-140^T.

FIG. 4.

Representative survival curve for D. radiodurans R1 and strain LB-34^T. The values are the means ± standard deviations of three independent experiments (n = 9 for R1 cultures and n = 12 for strain LB-34^T cultures).