General Microbiology - Schlegel, H. 1987

Prokaryotes: A Systematic Survey
Endospore-Forming Rods and Cocci (Group 15)

The ability to form more or less heat-resistant spores is, with few exceptions, restricted to a group of rod-shaped Bacteria, the majority of which possess peritrichous flagella. These are Gram-positive bacteria. Aerobic and facultatively anaerobic rods are grouped into the genera Bacillus, Sporolactobacillus, and Sporosarcina, whereas anaerobic rods belong to the genera Clostridium and Desulfotomaculum. Many spore-forming species are well known for their unique biochemical properties. Only a few Representatives of the large genera Bacillus and Clostridium will be discussed here.

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Fig. 3.6. Schematic representation of typical spore-forming Cell morphologies. 1 - centrally located spore that does not swell the mother cell (Bacillus megaterium); 2 - terminal spore that does not swell the mother cell (Bacillus thuringiensis), containing protein inclusions; 3 - terminal spore causing club-shaped Swelling of the mother cell (Bacillus macerans); 4 - central spore giving the mother cell a spindle-shaped (clostridial) appearance (Bacillus polymyxa); 5 - round terminal spore giving the mother cell a drumstick (plectridial) shape (Bacillus sphaericus); 6 - lateral spore; the mother cell has enlarged, assuming a spindle shape (Bacillus laterosporus).

Aerobic spore-forming bacteria. Bacteria of this type inhabit the soil. Many bacilli form cellular chains or filaments. Based on the Morphology of spores and mother Cells (Fig. 3.6), bacilli can be divided into three groups:

(I) Spores are oval or cylindrical, no wider than the mother cell. This group includes most bacilli (B. megaterium, B. cereus, B. subtilis, B. licheniformis, B. anthracis, B. thuringiensis).

(II) Oval spores are wider than the mother cell; they "swell" The Cell from within during sporulation (B. polymyxa, B. macerans, B. stearothermophilus, B. circulans).

(III) Nearly round spore in the swollen end of the mother cell (B. pasteurii).

I. Bacillus megaterium, with cells measuring 2x5 µm, is a giant among eubacteria. B. cereus is somewhat smaller; this species now also includes a variant named "mycoides" due to its fungus-like growth pattern on Agar surfaces (B. cereus var. mycoides). Both "right- and left-handed" strains exist, but the overall colony appearance is highly characteristic (Fig. 3.7). Closely related to B. cereus are the anthrax pathogen B. anthracis, which lacks flagella and is surrounded by a glutamic acid capsule, and the insect-pathogenic bacterium B. thuringiensis. Bacillus subtilis, commonly known as the "hay bacillus" (since enrichment cultures are obtained from hay extract), as well as B. licheniformis, produce Polypeptide Antibiotics. The latter species is capable of growth using not only aerobic Respiration, but also Fermentation and anaerobic nitrate respiration.

II. The bacterium B. polymyxa (formerly designated B. asterosporus) earned its current name due to its abundant slime production (as well as barrel-shaped spores that appear star-shaped in cross-section). Like B. licheniformis, it produces 2,3-butanediol. B. stearothermophilus is an obligate thermophile (with an optimal growth Temperature of 50-65 C).

III. The species B. pasteurii is known as a classic example of a urea-degrading bacillus; it constitutively synthesizes urease, hydrolyzes urea into CO₂ and ammonia, and is adapted to high pH values. Physiologically similar is Sporosarcina ureae, which morphologically belongs to the sarcinae, but based on its key physiological traits should be classified among the bacilli (it is aerobic and forms heat-resistant spores containing dipicolinic acid).

Fig. 3.7. Filamentous chains of Bacillus cereus var. mycoides cells. On agar media, they grow into colonies resembling fungal growth. (Photo by D. Claus.)

Anaerobic spore-forming bacteria. These bacteria do not require oxygen for growth. Species grouped within the genus Clostridium generally lack Cytochromes and catalase. Due to a high content of flavin Enzymes, many clostridia produce hydrogen peroxide upon exposure to atmospheric oxygen, which exerts a toxic effect on their cells. Because spore-forming sulfate-reducing bacteria contain protoheme pigments, they have been separated from the genus Clostridium into a new genus, Desulfotomaculum (D. nitrificans, D. orientis, D. ruminis). In anaerobic spore-forming bacteria, the spore is typically significantly wider than the vegetative cell. Depending on THE POSITION OF the spore, the mother cell may adopt various shapes.

Clostridia ferment A wide variety of substrates, including Polysaccharides, Proteins, Amino Acids, and Purines (Section 8.5). Based on their preferred substrates, one can distinguish saccharolytic clostridia (e.g., Clostridium butyricum, C. acetobutylicum, C. cellulosae-dissolvens), peptolytic clostridia (C. histolyticum, C. sporogenes, C. tetani, C. botulinum), and uric acid-degrading clostridia (C. acidi-urici). Fermentation products include butyrate, butanol, acetone, 2-propanol, and, in many cases, large amounts of gas (H₂, CO₂). C. pasteurianum and many other saccharolytic clostridia fix molecular nitrogen. C. aceticum converts fructose or a mixture of CO₂ and molecular hydrogen into acetate.

As an addendum, mention should be made of Oscillospira guilliermondii, an unusually large (5 x 100 µm) anaerobic bacterium that divides by Cell Division and forms spores. It is frequently found in the cecum of guinea pigs and the rumen of ruminants.