Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 14

This textbook comes at a very exciting time in our understanding of the interrelationship of all living things on the planet. After the landmark work of Carl Woese, all organisms on Earth were assigned to three major groups called domains: the bacteria (formerly eubacteria), the archaea (formerly archaebacteria), and the eukaryotes (see Woese and Fox, Suggested Reading). However, it is now clear that two major divisions account for these three groups. Bacteria form one of these divisions, while eukaryotes are now believed to have diverged out of the archaea. Figure 1 shows the microbiologists’ view of the living world, where microbes provide most of the diversity and eukaryotes occupy a relatively small niche. This is not a far-fetched concept. Sequence data show that we differ from chimpanzees by only 2% of our DNA sequence, while 25 to 50% of the genes in a typical bacterium are unique to the species. Furthermore, while mammals diverged from each other on the order of millions of years ago, the main bacterial lineages diverged billions of years ago.

Figure 1 A molecular tree of life capturing diversity using ribosomal proteins from sequenced genomes (see Hug et al., Suggested Reading). Selected major linages within bacteria are indicated, including the Proteobacteria and the subgroups Alpha, Beta, Delta, Epsilon, Gamma, and Zeta, the Firmicutes, and the Candidate Phyla Radiation, which is almost completely devoid of cultured representatives. For the Archaea, two superphyla, TACK and DPANN, are indicated and described in the text. The position of the archaeal Lokiarchaeota lineage is indicated. Genome sequences from members of the Lokiarchaeota lineage indicate that they possess molecular systems previously believed to be found only in Eukaryotes. Red dots indicate lineages that have no cultured representatives. Adapted with permission from Hug L, et al, Nat Microbiol 1:16048 (2016), https://doi.org/10.1038/nmicrobiol.2016.48.

Bacteria can differ greatly in their physical appearance under the microscope. Although most are single celled and rod shaped or spherical, some are multicellular and undergo complicated developmental cycles. The cyanobacteria (formerly called blue-green algae) are bacteria, but they have chlorophyll and can be filamentous, which is why they were originally mistaken for algae. The antibiotic-producing actinomycetes, which include Streptomyces spp., are also bacteria, but they form hyphae and stalks of spores, making them resemble fungi. Another bacterial group, the Caulobacter spp., have both free-swimming and sessile forms that attach to surfaces through a holdfast structure. Some of the most dramaticappearing bacteria of all belong to the genus Myxococcus, members of which can exist as free-living single-celled organisms but can also aggregate to form fruiting bodies, much like slime molds. As mentioned above, bacterial cells are usually much smaller than the cells of higher organisms, but one very large bacterium, Epulopiscium, can be over half a millimeter long, longer than even most eukaryotic cells (see Angert, Suggested Reading). In addition, unlike most bacteria that multiply by simple division, Epulopiscium gives birth to multiple live progeny. Despite the fact that some bacteria are found in dramatically different shapes and sizes, they cannot be distinguished simply by their physical appearance; instead, it is necessary to use biochemical criteria, such as the sequences of their ribosomal proteins or RNAs (rRNAs), whose sequences are characteristic of the three domains of life.