These include filtration methods

(Hahn et al., 2004), density-gradient centrifugation or elutriation and extinction-dilution whereby samples are diluted, ideally down to single cells, before their culture in isolation (Watve et al., 2000; Connon & Giovannoni, 2002; Ben-Dov et al., 2009; Song et al., 2009; Wang et al., 2009). Many bacteria, particularly those that are oligotrophic in the environment, are very slow-growing. Extended incubation times are a prerequisite 5-Fluoracil mw for the cultivation of such bacteria, with the added benefit that faster-growing members within the mixed populations progressively die off over time, reducing the bacterial competition. The culture of soil bacteria for up to 12 weeks has revealed increasing colony counts and an increased recovery of rarely isolated strains with Selleck Navitoclax time (Davis et al., 2005). Similarly, long-term incubation for up to 24 weeks has been successful for the isolation of strains from the SAR11 clade (Song et al., 2009). Even members of the TM7 Division, which have yet to be cultivated in isolation, were

able to form colonies visible to the naked eye when incubation times of 50 days were used [unpublished observation reported in a review by Hugenholtz (2002)]. Many bacteria have specific nutrient or chemical requirements for growth (Graber & Breznak, 2005; Tripp et al., 2008). For example, members of the genera Abiotrophia and Granulicatella, previously known as the nutritionally variant streptococci, require pyridoxal or l-cysteine for growth (Ruoff, 1991), while Tannerella forsythia requires an exogenous source of N-acetyl muramic acid (Wyss, 1989). The characterization of phylogenetically related species may provide clues to the metabolic requirements of organisms that are so far resistant to culture. Cultivation

media may be modified or enriched with this in mind, resulting in the isolation of previously ‘unculturable’ organisms Ribonucleotide reductase (Sait et al., 2002; Davis et al., 2005). However, simply adding the required substrate to cultivation media may not, in all cases, enable culture of the target organism. For example, slow-growing acetotrophs of the genus Methanosaeta are often outcompeted by faster-growing Methanosarcina spp. in mixed culture. On the other hand, Janssen (2003) found that the incorporation of acetone and isopropanol as enrichments led to the production (by species that ferment these substrates) of a slow and steady source of acetate that allowed Methanosaeta spp. to flourish. Because of a reliance on beneficial bacterial interactions within the source environment, attempts to cultivate certain bacteria under laboratory conditions have sometimes been met with success only when these bacteria are cocultivated with helper strains (Ohno et al., 1999, 2000; Nichols et al., 2008).