Isolation and Genotypic Characterization of Oligotrophic Bacteria From Soils.

The distribution and concentration of nutrients in soil vary from niche to niche. Oligotrophs, a group of bacteria which are adapted to grow under conditions of C/nutrient starvation present in a wide range of microhabitat. But owing to the selective isolation of rapidly growing bacteria by conventional cultivation method, the exploration of oligotrophs seems limited. This motivates us to perform a preliminary isolation and genotypic characterization of oligotrophic bacteria from soils. In the present study a variety of sandy soils, wetland soils and red soils were collected, serially diluted and plated on thousand fold diluted nutrient agar to isolate oligotrophic bacteria. Genotypic characterization of these isolates was performed using 16S rDNA sequence analysis. Besides, growths of these oligotrophs were tested on non-diluted nutrient agar and methanol containing agar. A total of 44 strains with different colony morphology were selected and purified. Based on 16S rDNA sequence some of these isolates showed higher sequence similarities to their most closely related neighbors, which were affiliated with a wide range of genera including Aurantimonas, Gordonia, Methylobacterium, Methylophilus, Mycobacterium, Paracoccus, Rhizobium and Sinomonas. Others share sequence similarities less than 99% with their most closely related species are assigned to the genera Altererythrobacter, Bacillus, Brevundimonas, Erythrobacter, Nocardioides, Roseomonas and Sphingopyxis. All the tested isolates can grow on nutrient agar, and most of them also showed methanol-utilizing abilities. The present work provided a preliminary study on the diversity of oligotrophs derived from the sampling sites. The nutrient requirement of these oligotrophs will be study in more detail to clarify their oligotrophic behavior, and the multiple traits regarding plant growth promotion of these oligotrophs will be examined to explore the diverse oligotrophs used in agriculture.