Microbiol. Rev., Mar 1995, 143-169, Vol 59, No. 1
RI Amann, W Ludwig and KH Schleifer
The frequent discrepancy between direct microscopic counts and numbers of
culturable bacteria from environmental samples is just one of several
indications that we currently know only a minor part of the diversity of
microorganisms in nature. A combination of direct retrieval of rRNA
sequences and whole-cell oligonucleotide probing can be used to detect
specific rRNA sequences of uncultured bacteria in natural samples and to
microscopically identify individual cells. Studies have been performed with
microbial assemblages of various complexities ranging from simple
two-component bacterial endosymbiotic associations to multispecies
enrichments containing magnetotactic bacteria to highly complex marine and
soil communities. Phylogenetic analysis of the retrieved rRNA sequence of
an uncultured microorganism reveals its closest culturable relatives and
may, together with information on the physicochemical conditions of its
natural habitat, facilitate more directed cultivation attempts. For the
analysis of complex communities such as multispecies biofilms and
activated-sludge flocs, a different approach has proven advantageous. Sets
of probes specific to different taxonomic levels are applied consecutively
beginning with the more general and ending with the more specific (a
hierarchical top-to-bottom approach), thereby generating increasingly
precise information on the structure of the community. Not only do rRNA-
targeted whole-cell hybridizations yield data on cell morphology, specific
cell counts, and in situ distributions of defined phylogenetic groups, but
also the strength of the hybridization signal reflects the cellular rRNA
content of individual cells. From the signal strength conferred by a
specific probe, in situ growth rates and activities of individual cells
might be estimated for known species. In many ecosystems, low cellular rRNA
content and/or limited cell permeability, combined with background
fluorescence, hinders in situ identification of autochthonous populations.
Approaches to circumvent these problems are discussed in detail.
Copyright © 1995, American Society for Microbiology
Phylogenetic identification and in situ detection of individual microbial cells without cultivation
Lehrstuhl fur Mikrobiologie, Technische Universitat Munchen, Germany.
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