IntroductionBeing the essential part of the terrestrial microbiota and their involvement in chemical, physical and biological transformations, bacteria are considered a very important group of microorganisms. Bacteria that cannot grow easily on artificial nutrient media are considered unculturable. Evidence for the presence of yet-to-be-cultured bacteria came from molecular data. The ability to obtain DNA sequence information from an environmental sample through PCR manipulations and direct sequencing allowed the identification of these phylogenetically important groups. When a sample is collected from the environment, the total number of bacterial cells in the sample is extremely high, which is not suitable for the isolation of uncharacterized bacteria. Few methods can be applied to reduce the number of microorganisms in mixed samples before culture. The majority of growing media are rich in nutrients. It is now thought that these conditions may favor the growth of faster-growing bacteria at the expense of slower-growing species, some of which grow in nutrient-poor environments and may be inhibited by conventional substrate-rich media (Deming and Baross, 2000). are some reasons that explain the uncultivability of microorganisms. It could be that the organism has low prevalence or is particularly slow growing and has been overlooked in cultural analyses. Many genetically distinct phenotypes are phenotypically indistinguishable; for example, few bacteria are resistant to cultivation on conventional media. Some bacteria have demanding growth requirements, including the need for specific physical conditions such as pH conditions, incubation temperatures, or atmospheric oxygen levels. There may be competition for nutrients in the middle of paper......iotechnology, 3, 301–308.Courtois S, Cappellano CM, Ball M, Francou FX and Normand P, (2003).Recombining environmental libraries provide access to microbial diversity for drug discovery from natural products. Applied and Environmental Microbiology, 69, 49-55. Venter JC, Remington K, Heidelberg JF, Halpern AL and Rusch, D, (2004). Random sequencing of the Sargasso Sea environmental genome. Science, 304, 66-74. Curtis TP, Sloan WT, Scannell JW, (2002). Estimation of prokaryotic diversity and its limits. Proceedings of the National Academy of Sciences, USA, 99, 10494-10499. Tyson GW, (2004). Community structure and metabolism through reconstruction of environmental microbial genomes. Nature, 428, 37-43. Venter JC, Remington K, Heidelberg JF, Halpern AL and Rusch D, (2004). Random sequencing of the Sargasso Sea environmental genome. Science, 304, 66–74.