Hence, the endeavour to devise novel cultivation methods for microorganisms that appear to be inherently resistant to artificial culture is a most important one. This minireview discusses the possible reasons for ‘unculturability’ and evaluates advances in the cultivation of previously unculturable bacteria

from complex bacterial communities. Methods include the use of dilute nutrient media particularly suited for the growth of bacteria adapted to oligotrophic conditions, and the provision of simulated natural environmental conditions for bacterial culture. This has led to the recovery of ‘unculturables’ from soil and aquatic environments, likely to be due to the inclusion of essential nutrients and/or signalling molecules from the native environment. For the purpose of this minireview, the terms ‘unculturable’ and ‘as yet uncultivated’ are used to describe organisms NVP-BKM120 clinical trial that have yet to be grown on ABT-737 molecular weight artificial media in vitro. It is well established that only approximately 1% of bacteria on Earth can be readily

cultivated in vitro– the so-called ‘great plate count anomaly’, based on the observation that microscopic counts are considerably larger than the equivalent total viable counts (Staley & Konopka, 1985; Amann et al., 1995; Hugenholtz et al., 1998). There are currently estimated to be 61 distinct bacterial phyla, of which 31 have no cultivable representatives (Hugenholtz et al., 2009). The topology of the archaeal phylogenetic tree remains uncertain, but it is clear that the 54 species of Archaea cultured to date represent PIK3C2G only a fraction of the total diversity, with 49 lineages mostly uncultured (Auguet et al., 2010). Because the majority of bacteria and archaea remain unculturable, the diversity of complex bacterial communities is inevitably underestimated using standard cultivation methods. Furthermore, organisms of key importance to the community and the entire ecosystem in the environment or pathogens of plants and animals may be overlooked if they are

unculturable. Consequently, with the development of molecular culture-independent techniques, there has been a move towards the characterization of mixed bacterial populations within biomass from the environment and in samples from animals (including humans) using PCR amplification of housekeeping genes particularly that encoding 16S rRNA gene, cloning for purification and sequencing for identification (Giovannoni et al., 1990; Pace, 1997). As a result, numerous novel phylotypes have been identified among bacterial communities from a wide range of habitats: from seawater and soil to the health- and disease-associated microbiota of humans (Munson et al., 2002; Rappe & Giovannoni, 2003; Zhou et al., 2004; Aas et al., 2005). Despite the availability of varied molecular methods for the evaluation of bacterial communities, cultural analyses are far from redundant.