GENERAL MICROBIOLOGY - T.P. Pyroh - 2004

5. STRUCTURE OF THE MICROBIAL CELL

5.4. ENDOSPORES AND OTHER RESTING STAGES IN BACTERIA

5.4.3. Other resting stages (cysts, exosomes, myxospores)

In addition to endospores, certain Bacteria form other resting stages, namely exospores and cysts. Exospores are produced by methane-oxidizing bacteria of the genus Methylosinus and the phototrophic purple bacterium Rhodomicrobium vannielii. Exospores are formed by budding of the mother Cell. While significantly less heat-resistant than endospores, exospores exhibit greater resistance to desiccation and ultraviolet irradiation than vegetative Cells. Exospores lack dipicolinic acid, are not resistant to Lysozyme, and stain poorly with various Dyes.

Some bacteria form spherical, thick-walled cells known as cysts. Cysts contain Cytoplasm with a nucleoid (and poly-$eta$-hydroxybutyric acid granules in Azotobacter vinelandii), surrounded by a cytoplasmic membrane and two layers of Cell wall. They arise in older cultures when nutrient resources are depleted, with the entire vegetative cell transforming into a cyst. Unlike vegetative cells, cysts contain high amounts of Lipids. Cysts are more resistant to desiccation, mechanical damage, and lysozyme than vegetative cells, yet they show low thermal resistance. Under favorable conditions (in the presence of carbon sources), cysts germinate. Cysts are formed by bacteria of the genera Azotobacter, Bdellovibrio, Methylococcus, as well as spirochetes. In methanotrophic bacteria of the genus Methylococcus, cysts develop under adverse conditions, such as oxygen deprivation. In many cases, cyst formation in these bacteria is accompanied by A change in cell pigmentation from yellow to dark brown. Cultures that do not form cysts remain cream-white. However, in Methylococcus thermophilus, cyst formation is not always accompanied by enhanced pigmentation.

In myxobacteria, The formation of cysts—referred to as myxospores—is a regular and natural stage of their life cycle. Following the active binary fission stage under conditions of nutrient scarcity, vegetative myxobacterial cells aggregate into a mass to form fruiting bodies of various shapes (Fig. 5.14) composed of mucus and myxospores. Only a fraction of the cells differentiate into myxospores, while the rest undergo cell death and lysis, serving as building material for The Development of the fruiting bodies.

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Fig. 5.14. Fruiting bodies of myxobacteria

Myxospores differ only slightly from vegetative cells, though some variants vary markedly in shape, resistance to desiccation, UV radiation, and Temperature, and possess additional cell wall layers. Such myxospores are termed microcysts.

In some prokaryotes, spores function simultaneously as resting structures and reproductive units. In actinomycetes of the genera Thermoactinomyces and Actinobifida, endogenous spore formation is observed, whereas in most actinomycete genera, spores form exogenously (arthrospores, conidiospores, etc.). Arthrospores are produced by the development of transverse septa within hyphae, dividing them into segments that form a chain of spores. Conidiospores develop On the surface of specialized hyphae known as conidiophores. Sporangiospores are spores formed within sporangia. Because sporangiospores develop internally within sporangia, they are considered endogenous. These sporangia are borne on specialized hyphae called sporangiophores.

Prokaryotic resting forms are characterized by a low level of metabolic activity, enhanced resistance to adverse environmental factors, and The ability to remain viable for prolonged periods of time.