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Tag words: bacteria, archaea, procaryote, prokaryote, procaryotic, prokaryotic, microbiology, microbe, Euryarchaeota, Crenarchaeota, Korarchaeota, methanogen, Methanobacterium, Methanococcus, thermoacidophile, Sulfolobus, hyperthermophile, extreme halophile, Halococcus, Halobacterium, extremophile, Bergey's Manual, The Prokaryotes, Domains of Life, phylogenetic tree, Gram negative bacteria, Gram positive bacteria, green bacteria, Chlorobium, Chloroflexus, purple bacteria, Thiopedia, Chromatium, Rhodobacter, Rhodospirillum, Heliobacterium, Chloracidobacterium, cyanobacteria, Nostoc, Oscillatoria, Anabaena, Synechococcus, spirochete, Borrelia, Treponema, Leptospira, spirilla, vibrios, pyogenic cocci, myxobacteria, lithotrophic bacteria, nitrogen fixing bacteria, endospore forming bacteria, enteric bacteria, aerobic bacteria, anaerobic bacteria, proteobacteria, E. coli, Salmonella, Shigella, Erwinia, Yersinia, Pseudomonas, pseudomonad, Vibrio, Rhizobium, Rickettsia, Bordetella, Neisseria, Haemophilus, Legionella, Campylobacter, Helicobacter, Firmicutes, Staphylococcus, Streptococcus, Bacillus, Clostridium, Listeria, lactic acid bacteria, Enterococcus, Lactococcus, Lactobacillus, Actinomycete, Streptomyces, Mycobacterium, Corynebacterium, Rickettsia, Chlamydia, Xanthomonas, Burkholderia, Ralstonia.

Kenneth Todar currently teaches Microbiology 100 at the University of Wisconsin-Madison.  His main teaching interest include general microbiology, bacterial diversity, microbial ecology and pathogenic bacteriology.

Bacillus cereus bacteria.Print this Page

Important Groups of Procaryotes (page 7)

(This chapter has 10 pages)

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Endospore-forming bacteria produce a unique resting cell called an endospore. They are Gram-positive and usually rod-shaped, but there are exceptions. The two important genera are Bacillus, the members of which are aerobic endospore-forming bacteria in the soils, and Clostridium, whose species are anaerobic sporeformers of soils, sediments and the intestinal tracts of animals.

Figure 16. Endospore-forming bacilli (phase contrast illumination). Endospores are dehydrated, refractile cells appearing as points of bright light under phase microscopy. Endospore-forming bacteria are characterized by the location (position) of the endospore in the mother cell (sporangium) before its release. The spore may be central, terminal or subterminal, and the sporangium may or may not be swollen to accommodate the spore.

Figure 17. Anatomy of an endospore, cross section drawing by Viake Haas. Endospores differ from the vegetative cells that form them in a variety of ways. Several new surface layers develop outside the core (cell) wall, including the cortex and spore coat. The cytoplasm is dehydrated and contains only the cell genome and a few ribosomes and enzymes. The endospore is cryptobiotic (exhibits no signs of life) and is remarkably resistant to environmental stress such as heat (boiling), acid, irradiation, chemicals and disinfectants. Some endospores have remained dormant for 25 million years preserved in amber, only to be shaken back into life when extricated and introduced into a favorable environment.

Figure 18. The sequential steps in the process of endospore formation in Bacillus subtilis.

Some sporeformers are pathogens of animals, usually due to the production of powerful toxins. Bacillus anthracis causes  anthrax, a disease of domestic animals (cattle, sheep, etc.) which may be transmitted to humans.  Bacillus cereus is becoming increasingly recognized as an agent of food poisoning. Clostridium botulinum causes botulism a form of food-poisoning, and Clostridium tetani causes tetanus

Figure 19. Robert Koch's original photomicrographs of Bacillus anthracis. In 1876, Koch established by careful microscopy that the bacterium was always present in the blood of animals that died of anthrax. He took a small amount of blood from such an animal and injected it into a healthy mouse, which subsequently became diseased and died. He took blood from that mouse and injected it into a another healthy mouse. After repeating this several times he was able to recover the original anthrax organism from the dead mouse, demonstrating for the first time that a specific bacterium is the cause of a specific disease. In so doing, he established Koch's Postulates, which still today supply the microbiological standard to demonstrate that a specific microbe is the cause of a specific disease.

In association with the process of sporulation, some Bacillus species form a crystalline protein inclusion called parasporal crystals. The protein crystal and the spore (actually the spore coat) are toxic to lepidopteran insects (certain moths and caterpillars) if ingested. The crystals and spores of Bacillus thuringiensis are marketed as "Bt" a natural insecticide for use on garden or crop plants. Another species of Bacillus, B. cereus, produces an antibiotic that inhibits growth of Phytophthera, a fungus that attacks alfalfa seedling roots causing a "damping off" disease. The bacteria, growing in association with the roots of the seedlings, can protect the plant from disease.

Also, apparently in association with the sporulation process, some bacilli produce clinically-useful antibiotics. Bacillus and Paenibacillus antibiotics, such as bacitracin and polymyxin are usually polypeptide molecules that contain unusual amino acids.

Actinomycetes and related bacteria are a large group of Gram-positive bacteria that usually grow by filament formation, or at least show a tendency towards branching and filament formation. Many of the organisms can form resting structures called spores, but they are not the same as endospores. Branched forms superficially resemble molds and are a striking example of convergent evolution of a procaryote and a eucaryote together in the soil habitat. Actinomycetes such as Streptomyces have a world-wide distribution in soils. They are important in aerobic decomposition of organic compounds and have an important role in biodegradation and the carbon cycle. Products of their metabolism, called geosmins, impart a characteristic earthy odor to soils. Actinomycetes are the main producers of antibiotics in industrial settings, being the source of most tetracyclines, macrolides (e.g. erythromycin), and aminoglycosides (e.g. streptomycin, gentamicin, etc.). Two bacteria in this diverse group are important pathogens of humans: Mycobacterium tuberculosis is the cause of  tuberculosis; Corynebacterium diphtheriae is the cause of diphtheria. Also, many nonpathogenic mycobacteria and corynebacteria live in associations with animals.

Figure 20. Schematic diagrams illustrating mycelial growth and spore formation in several genera of actinomycetes.

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Kenneth Todar has taught microbiology to undergraduate students at The University of Texas, University of Alaska and University of Wisconsin since 1969.

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