General Microbiology - Schlegel H. 1987

Prokaryotes: A Systematic Survey
Curved Rods: Spirilla and Vibrios (Groups 6 and 9)

Spirilla and vibrios are gram-negative aquatic Bacteria capable of locomotion via flagella.

Spirilla are distinguished by a spiral Cell Structure and bipolar flagellation; they obtain energy through Respiration. Several genera of spirilla are recognized. The genus Spirillum is represented by a single species, S. volutans. This is a giant spirillum that can consistently be found in liquid pig manure; it became famous for the discovery of “volutin” (polyphosphates). In pure culture, it grows only at a reduced O₂ concentration (approximately 5%) and should therefore be considered microaerotolerant. The majority of spirilla are grouped into the genus Aquaspirillum (A. itersonii, A. serpens). These organisms likewise typically do not tolerate normal partial pressures of oxygen.

Vibrios. Vibrios are facultative anaerobes with a METABOLISM similar to that of enterobacteria (Mixed Acid Fermentation). The best-known species is Vibrio cholerae, the CAUSATIVE AGENT OF cholera. It is waterborne (transmitted via sewage) and proliferates in the intestinal tract, where it secretes a toxin and induces enzymatic lysis of the mucosa, resulting in severe fluid loss.

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Fig. 3.9. The bacterium Bdellovibrio bacteriovorus, parasitic on other bacteria. A. Growth cycle within the host bacterium. B. Erwinia carotovora Cells infected with Bdellovibrio; 2,200×. C. Initial stage of infection of a pseudomonad; 20,000×. (Stolp H., Naturwissenschaften, 55 [1968], 57.)

Bdellovibrio bacteriovorus is an aerobic bacterium that parasitizes other bacteria. The cells are relatively small and highly motile thanks to a thick (50 nm) flagellum (Fig. 3.9); they can travel 100 µm per second, which corresponds to 70 times their own body length. Upon encountering a host bacterium, the parasite attaches to its Cell wall by its anterior end (opposite the pole bearing the flagellum) (Fig. 3.9). The parasite sometimes rotates around its longitudinal axis. The infected cell soon rounds up, taking on the appearance of a spheroplast. Bdellovibrio penetrates through The Cell wall into the periplasmic space. There, the parasite cell grows in length until the nutrients drawn from the progressively shrinking host protoplast are exhausted. This is followed by multiple transverse fission, yielding several cells of equal size. Eventually, the host cell dissolves, and the progeny of the parasitic bacterium are released to attack other bacteria anew. As the parasite multiplies and spreads, bacterial lysis becomes macroscopically noticeable: lytic zones, or “plaques,” appear on bacterial lawns, while light scattering decreases in Suspensions. Unlike Bacteriophages, which replicate only in growing bacteria, Bdellovibrio also infects and lyses non-growing cells. Strains of Bdellovibrio isolated from various soils differ in their host range. They lyse predominantly gram-negative bacteria, with a preference for pseudomonads and enterobacteria. While the wild type is an obligate parasite and always requires “prey bacteria,” certain mutants are capable of saprophytic growth on complex media. There is good reason to believe that the parasitism of Bdellovibrio serves as an adaptation to life in nutrient-poor environments.

Strictly anaerobic vibrios. A second group of vibrio-shaped bacteria comprises the strict anaerobes Desulfovibrio, Selenomonas, Butyrivibrio, and Succinivibrio. In terms of its metabolism, Desulfovibrio belongs to the sulfate-reducing bacteria (Sec. 9.2). Selenomonas sputigena occurs in the human Oral Cavity within dental plaque, whereas S. ruminantium is found in the rumen of ruminants. Both species are characterized by laterally positioned flagella and grow by fermenting CARBOHYDRATES to propionic and acetic acids.