DESCRIPTION OF THE PHYLUM ACTINOBACTERIA (MARGULIS 1974)
| EUBACTERIA> FIRMICUTAE> ACTINOBACTERIA |
PHYLUM ACTINOBACTERIA LINKS
| Actinobacteria (pronounced: ak-TIN-o-bak-TE-re-uh) is derived from two Greek roots that mean “rayed” (aktina -αχτίνα) and “little stick” (bakterion -βακτήριον). The reference is to the ray-like mycelium made by some of the taxa. |
| INTRODUCTION TO THE ACTINOBACTERIA This is a large and very diverse group of microorganisms that range in form from from cocci to rods to filamentous fungus-like masses (mycelia). In vegetative form they are non-motile, but some of the complex taxa do form motile swarmer cells. They can be free-living, commensals, and parasites. This phylum was defined by Stackenbrandt et al. (1997), primarily by similarities in 16S rRNA sequences (see Figure 1A). The All Species Living Tree Project (Yarza et al. 2008 and 2010; Munoz et al. 2011) suggests that the Actinobacteria is not within the clade that includes the rest of the Firmicutes (see Figure 1B). The ecology of this phylum is remarkably varied. They can be found in forest soil (Niva et al. 2006), arid soil (Holmes et al. 2000), freshwater (Niva et al. 2006, Warnecke et al. 2004), marine waters, digestive tracts (Lawson et al. 2005), hot springs (Chen et al. 2004), and acid mine drainage (Clark and Norris 1996). Many are parasites and among the most important human health concerns. Conversely, some taxa are sources for the most effective antibiotics. |
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| FIGURE 1A. The result of 16S rRNA comparisons for taxa in the Actinobacteria from Stackebrandt et al. (1997). Since then Hugenholtz and Stackebrandt (2004) removed Sphaerobacterales from this phylum. See descriptions of the Actinomycetales, Bifidobacteriales, Acidimicrobiales, Coriobacteriales, and Rubrobacteriales below. |
FIGURE 1B. A simplified summary tree for the Eubacteria adapted from the All Species Living Tree Project (Yarza et al. 2008 and 2010; Munoz et al. 2011). Note the sister relationship between the Cyanobacteria and the Firmicutes. Also, note that Actinobacteria (as well as the Deinococci) are far removed from the Firmicutes.
| Actinobacteriae (Actinomycetales + Bifidobacteriales) This class includes two orders: Actinomycetales and Bifidobacteriales. Of the two, Actinomycetales is very speciose and remarkably diverse with many taxa that are of economic importance. They are among the dominant taxa in soil. For example, Streptomyces is a slow-growing bacterium that evolved a chemical means of combating competing soil bacteria. The first to be isolated was streptomycin by Albert Schatz in the lab of Selman A. Waksman in 1943. Waksman then went on to discover many of the early mycin antibiotics and received a Nobel Prize for the discovery of Streptomycin, which was found to be able to cure tuberculosis, was challenged in court for many years by Schatz (Auerbacher 2006). Streptomycin grows as a branching filament that expands in the soil as a mycelium (text with tooltip) A mycelium (mycelia, pl.) is a mass of hyphae or fungus-like filaments. , like a microbial fungus. Streptomyces also produces fungus-like actinospores within defined sporangia (Figure 2). The complexity of Streptomyces is borne out by the complexity of its genome in that it has the largest number of genes found so far in a bacterium (Bentley et al. 2002). Actinomyces (Figure 3), the source of actinomycin, also is a soil microbe that has a mycelial growth form (Figure 3). All members of this group are not structurally complex, but of great importance to humanity. The corynebacteria include Mycobacterium (Figure 4), one of the great scourges of humankind. Within that genus are the causative agents of tuberculosis and leprosy. Gosh et al. (2009) speculate that one-third of all humans might be infected with Mycobacterium but are symptom-less carriers. Furthermore, they suggest that the success of the bacterium to combat the body’s defenses and the onslaught of antibiotics might be its ability to go into a dormant spore phase within the body of the host. Broussard et al. (2008) studied a species that infects fish, most of which are symptom-less but do tend to have a higher incidence of tumors. Some species of Mycobacterium are free-living and grow in low pH environments (Niva et al. 2006). Corynebacteria (Figure 5) also harbor the causative agent of diptheria, a disease of the upper respiratory tract. The USA had 100,000-200,000 cases per year through the 1920’s (Atkinson et al. 2007). However, only three cases were reported in 2006 in the USA through the aggressive use of anti-diptherial booster shots for young children. Aside from human health issues (positive and negative), the order contains taxa that fix nitrogen (Frankia) and enter into symbiotic relationships with flowering plants like alder (Alnus). Propionobacteria (some species of which may cause acne) is used in cheese making where it produces the holes in Swiss cheese by the generation of gas. Bifidobacteria occur in the human digestive tract, especially the lower bowel. Schell et al. (2002) described them as “key commensals” in supporting gastrointestinal health. The authors sequenced the Bifidobacterium longum genome and suggested ways in which the microbial genome had adapted itself to the human intestinal environment (e.g. the identification of enzymes of Bifidobacterium species that allow them to be well-adapted to scavenging otherwise indigestible plant and animal matter). |
| Acidomicrobiales Unlike the Actinobacteriae, Acidimicrobiae (Figure 6) are moderately thermophilic, acidophilic, and iron oxidizers (Clark and Norris 1996, Clum et al. 2009). They fix CO2 by acquiring energy from ferrous iron, particularly pyrite (Montalvo et al. 2004). Thus, they are well-adapted to conditions in many abandoned mine waters and certain acidic, high sulfur and iron hot springs. Typically, they grow as non motile rods, but do become motile when grown heterotrophically on yeast (Clark and Norris 1996). One genus, Xestospongia, is associated with sponges (Montalvo et al. 2004). |
| Coriobacteriales This order contains many taxa that occur in the human gastrointestinal tract (Krogius-Kurikka et al. 2009) and other vertebrates (Lawson et al. 2005). They are small rods and strict anaerobes. One species, Cryptobacterium curtum, is associated with peridontitis in humans (Nakazawa et al. 1999). |
| Rubrobacteriales This is a group of thermophilic to mesophilic short rods. They also have a form of pigmentation that makes them pink. Rubrobacter (Figure 7) was first isolated by Yoshinaka et al. (1973) from hot springs by irradiating sediment with gamma radiation (Initially they were misidentified as Arthrobacter). Holmes et al. (2000) and Singletin et al. (2003) isolated new taxa from soil in the same way. Chen et al. (2004) exposed isolates to 1.8X106 rads with measurable survival rates. |
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| FIGURE 2. A mycelium of Streptomyces showing the actinospores (SEM micrograph). Image from http://www.bio.miami.edu/dana/106/mycobacterium.gif | FIGURE 3. The hyphal structure of Actinomyces as seen with an SEM. Image from http://www.sanger.ac.uk/Projects/S_coelicolor/gfx/h4tkm1m.gif |
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| FIGURE 5. Mycobacterium (SEM false color micrograph), the causative agent of tuberculosis. Image from NIH, in the Public Domain | FIGURE 6. Bifidobacterium, a common commensal in the human interstinal tract. Image by Y. Tambe, Wikimedia Commons |
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| FIGURE 6. Acidomicrobium, a moderately thermophilic, acidophilic, iron oxidizer. Image from Clum et al. 2009 | FIGURE 7. Rubrobacter, one of the most radiation tolerant taxa on earth in a false color SEM micrograph. Image from http://genome.jgi-psf.org/rubxy/rubxy.home.html |
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Chem. 37:2269-2275. |
| By Jack R. Holt. Last revised: 02/11/2013 |
