General Microbiology - Schlegel, H. G. 1987

Prokaryotes: A Systematic Review
Gliding Bacteria (Group 2)

Only a few Bacteria possess The ability to move by gliding. They are grouped together as "gliding bacteria" and divided into several subgroups:

I. Sulfur-accumulating bacteria, including trichome-forming genera (Beggiatoa, Thiothrix) and unicellular forms (Achromatium).

II. Non-sulfur-accumulating trichome-forming bacteria, such as Vitreoscilla, Leucothrix, and Saprospira, as well as the "oral oscillatorians" Simonsiella and Allysiella (see Fig. 3.16).

III. Unicellular rod-shaped bacteria, which include myxobacteria, the Cytophaga group, and the Flexibacter group.

IV. The gliding filamentous bacterium Chloroflexus; it will be discussed among the anoxygenic phototrophic bacteria (Section 12.1).

V. Cyanobacteria, when motile at all, also move by gliding. They are described in Section 3.21.

Subgroup I. Beggiatoa is a colorless, filamentous sulfur bacterium. It forms trichomes consisting of Cells of uniform width and closely resembles the cyanobacterium Oscillatoria in Structure. Several species are distinguished based on filament diameter (ranging from 1.5 to 35 µm). The cells are typically packed with sulfur droplets, giving the colorless filaments a white appearance. The filaments move by gliding. Beggiatoa is an aerobic Organism that forms web-like mats on the black mud of slow-flowing waters or in marine environments where H₂S-containing Water comes into contact with air. The sulfide is oxidized to sulfate, with elemental sulfur occasionally accumulating temporarily as an intermediate product inside the cells. Through his research on Beggiatoa, Winogradsky (1888) established METABOLISM/2.html">THE CONCEPT OF chemolithoautotrophic metabolism, demonstrating that a Cell can derive energy by oxidizing Reduced Inorganic Compounds while synthesizing organic matter from CO₂. However, some natural strains of Beggiatoa require organic substrates.

Thiothrix cells are incapable of free movement. Their trichomes are attached in bundles by their bases to a substrate, with each trichome tapering gradually from 5 µm at the base to 2 µm at the apex. Reproduction occurs via gonidia, which are formed by the rounding off of apical cells; these gonidia exhibit the ability to glide over solid surfaces. Typically, several gonidia settle simultaneously, subsequently developing into multicellular filaments. Bacteria of the genus Thiothrix are significantly more widespread than Beggiatoa, thriving in all aquatic environments where the decomposition of organic matter generates hydrogen sulfide.

Subgroup II. Vitreoscilla is a colorless, aerobic bacterium that forms multicellular filaments, moves by gliding, and reproduces by filament fragmentation (Fig. 3.11). It can be isolated from cow dung. Leucothrix grows as an epiphyte on marine Algae and, in its growth habit, can be regarded as an organotrophic counterpart to Thiothrix. The filaments commonly grow in tufts attached by their bases to solid surfaces.

The helically coiled cyanobacterium Spirulina can be compared with the morphologically similar sulfur bacterium Thiospirillopsis floridana and the organotrophic bacterium Saprospira grandis. Figure 3.12 illustrates morphologically similar representatives of three metabolic types (phototrophic cyanobacteria, colorless hydrogen sulfide-oxidizing bacteria, and organic-oxidizing bacteria). They are arranged in the figure so that outwardly similar forms appear side by side.

Subgroup III. Myxobacteria are strictly aerobic chemoheterotrophs that move by gliding. They are typical soil bacteria. In nature, they are notable for forming fruiting bodies, which are nevertheless very small (less than 1 mm). These fruiting bodies are found on decomposing plant material, rotting wood, tree bark, and herbivore dung pellets. The bacteria can be isolated from animal droppings (such as mice or hares) by keeping the pellets on moist soil for several weeks.

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Fig. 3.11. Filament-forming bacteria. A. Colony of Vitreoscilla sp. B. Alysiella filiformis. C. Colony of Simonsiella crassa prior to "creeping." D. Filaments of S. crassa lying flat and on edge, some of which are "creeping." (Bright-field photomicrographs by V. V. D. Skerman.)

On solid media, myxobacteria form flat colonies that spread outward. As soon as vegetative cells aggregate at multiple points within the colony, The formation of fruiting bodies begins; these structures vary in shape, size, and pigmentation. These characteristics are used to identify different genera and species of myxobacteria (Figs. 3.13 and 3.14). As they mature, the cells within the fruiting bodies transform into resting cells known as myxospores. Myxospores may be spherical (in Myxococcus) or rod-shaped (Fig. 3.13) and can be enclosed within cysts.

Fig. 3.12. Comparison of several cyanobacteria and morphologically similar non-phototrophic bacteria.

Depending on their nutrient sources, bacteriolytic and cellulolytic species are distinguished. Most myxobacteria are capable of lysing other bacteria using exoenzymes. Cellulolytic species are found exclusively within the genus Polyangium.

The Cytophaga group comprises the genera Cytophaga and Sporocytophaga. They are well known for their ability to decompose Cellulose under aerobic conditions in the soil. Much like myxobacteria, they move by gliding but do not form fruiting bodies. Some species are facultative anaerobes and ferment glucose with the Production of organic acids.

The spindle-shaped vegetative cells of Sporocytophaga can transform into long-lasting, capsule-enclosed round cells. These resemble the myxospores of Myxococcus (Fig. 3.15) but are referred to as microcysts. Enrichment cultures of Cytophaga and Sporocytophaga are easily obtained using cellulose (filter paper) as a substrate (Section 14.1).

Flexibacter species are aquatic bacteria. They consist of long, unsegmented, highly flexible cells (Fig. 3.15). During cultivation, they form progressively shorter filaments that subsequently break down into coccoid forms. Many forms contain carotenoids and exhibit yellow, pink, or orange pigmentation. Recently, the fish pathogen F. columnaris (Chondrococcus columnaris) has also been assigned to the genus Flexibacter.

Fig. 3.13. Fruiting bodies, microspores, and vegetative cell shapes of selected myxobacteria. The fruiting bodies are shown schematically and at various scales. In the genera represented in the top two rows, vegetative cells are spindle-shaped (rods with pointed ends), whereas in Chondromyces and Polyangium they are rod-shaped with blunt ends. Mcn — myxospores; ВК — vegetative cells. (Reichenbach H., Biologie in unserer Zeit, 4 [1974], 33.)

Fig. 3.14. Developmental cycle involving fruiting body formation in Chondromyces apiculatus. Following cell aggregation, The Development of a fruiting body begins, consisting of a mucous stalk and cysts. The cysts function as dissemination Organs and, upon maturation, release myxospores, which give rise to new vegetative cells.

Fig. 3.15. Cell shapes in Representatives of the genera Sporocytophaga, Cytophaga, and Flexibacter. In young cultures of Flexibacter, filaments up to 100 μm long are formed consisting of several cells with almost no separating cross-walls; these filaments exhibit active movement. Upon prolonged incubation, they break down into shorter filaments and subsequently into coccoid cells. (Reichenbach H. Biologie in unserer Zeit, 4 [1974], 33).