[An important mission for microbiologists in the new century-cultivation of the unculturable microorganisms]

The present review summarizes the recent advances in cultivating the so-called recalcitrant microorganisms in various natural environments. Firstly, the use of new electron donors and acceptors reveals a list of unique and previously unrecognized physiotypes (Table 1), and the growth supporting reactions include oxidation of phosphite, arsenite, soil humic substances, benzene, Fe (II) and aryl-halides. The novel pure cultures obtained include: Desulfatogenum phosphitoxidans, lithochemotrophic arsenite oxidizing bacteria, Dechloromonas agitata, Dechalosoma suillum, and various Proteobacteria capable of decomposing soil humic substances anaerobically. Secondly, the adaption of nutrient-poor media (down to 1/100 of the traditional concentration), including sea water, soil extract and extension of natural habitats (insect hind-gut, submarine hot vent) are extremely rewarding. For example, cultivation of the ubiquitous SAR11 marine bacterioplankton clade reveals a phylum, small in cell size and genome, but dominating in ocean surface, thus constituting a huge biomass on earth. In addition, the nanosized hyperthermophilic Archaeon from a submarine hot vent, represents an unknown phylum, and because of its symbiotic living with Ignicocous cells, it is named Nanoarchaeum equitans. Its genome size is only 500kb, the smallest for prokaryote. Considering its hyperthermic living environment and small genome, N. equitans may possibly still a primitive form of microbial life and affords an advantageous culture for the study of life origin and evolution on earth. Thirdly, several novel isolation techniques have been developed such as gel microdrop encapsulation and diffusion chamber, both with the advantages of high-throughout operation and simulating to certain extent of the natural environment, thus allowing growth synergy based on cross-feeding to occur. Finally, it should be noted that the cultivation of unculturable microorganisms mentioned in this review does not require expensive and sophisticated equipments and most of the techniques can be adapted in our laboratory at home.