FACTORS AFFECTING THE GERMINATION OF SPORES OF CLOSTRIDIUM BIFERMENTANS

Conservation of the "Outside-in" Germination Pathway in Paraclostridium bifermentans

Clostridium difficile spore germination is initiated in response to certain bile acids and amino acids (e.g., glycine). Though the amino acid-recognizing germinant receptor is unknown, the bile acid germinant receptor is the germination-specific, subtilisin-like pseudoprotease, CspC. In C. difficile the CspB, CspA, and CspC proteins are involved in spore germination. Of these, only CspB is predicted to have catalytic activity because the residues important for catalysis are mutated in the cspA and cspC sequence. The CspB, CspA, and CspC proteins are likely localized to the outer layers of the spore (e.g., the cortex or the coat layers) and not the inner membrane where the Ger-type germinant receptors are located. In C. difficile, germination proceeds in an “outside-in” direction, instead of the “‘inside-out” direction observed during the germination of Bacillus subtilis spores. During C. difficile spore germination, cortex fragments are released prior to the release of 2,4-dipicolinic acid (DPA) from the spore core. This is opposite to what occurs during B. subtilis spore germination. To understand if the mechanism C. difficile spore germination is unique or if spores from other organisms germinate in a similar fashion, we analyzed the germination of Paraclostridium bifermentans spores. We find that P. bifermentans spores release cortex fragments prior to DPA during germination and the DPA release from the P. bifermentans spore core can be blocked by high concentrations of osmolytes. Moreover, we find that P. bifermentans spores do not respond to steroid-like compounds (unlike the related C. difficile and P. sordellii organisms), indicating that the mere presence of the Csp proteins does permit germination in response to steroid compounds. Our findings indicate that the “outside in” mechanism of spore germination observed in C. difficile can be found in other bacteria suggesting that this mechanism is a novel pathway for endospore germination.

Germinants and Their Receptors in Clostridia

Many anaerobic spore-forming clostridial species are pathogenic, and some are industrially useful. Although many are strict anaerobes, the bacteria persist under aerobic and growth-limiting conditions as multilayered metabolically dormant spores. For many pathogens, the spore form is what most commonly transmits the organism between hosts. After the spores are introduced into the host, certain proteins (germinant receptors) recognize specific signals (germinants), inducing spores to germinate and subsequently grow into metabolically active cells. Upon germination of the spore into the metabolically active vegetative form, the resulting bacteria can colonize the host and cause disease due to the secretion of toxins from the cell. Spores are resistant to many environmental stressors, which make them challenging to remove from clinical environments. Identifying the conditions and the mechanisms of germination in toxin-producing species could help develop affordable remedies for some infections by inhibiting germination of the spore form. Unrelated to infectious disease, spore formation in species used in the industrial production of chemicals hinders the optimum production of the chemicals due to the depletion of the vegetative cells from the population. Understanding spore germination in acetone-butanol-ethanol-producing species can help boost the production of chemicals, leading to cheaper ethanol-based fuels. Until recently, clostridial spore germination is assumed to be similar to that of Bacillus subtilis However, recent studies in Clostridium difficile shed light on a mechanism of spore germination that has not been observed in any endospore-forming organisms to date. In this review, we focus on the germinants and the receptors recognizing these germinants in various clostridial species.

The ability of Clostridium bifermentans strains to lactic acid biosynthesis in various environmental conditions

Clostridium bifermentans strains, isolated from a manure, were examinated for their ability to produce lactic acid from PY medium with glycerol under different pH conditions and when PY medium was supplemented with saccharides such as fructose, sorbitol, glucose, mannose, mannitol, maltose, xylose, raffinose, and arabinose. In the last test performed, the ability of investigated strains to produce lactic acid from mixed carbon source (glycerol plus saccharide) was checked. The strains of Cl. bifermentans, designated as CB 371, CB 374, and CB 376 grew and produced lactic acid on PY medium irrespective of pH and the carbon source used. The optimal lactic acid production on PY medium with glycerol was obtained at pH of 7.0 in case of CB 371 and 376 (19.63 g/L and 16.65 g/L, accordingly) and at pH 8.0 in case of CB 374 (13.88 g/L). The best productivity of lactic acid on PY media by CB 371, CB 374, and CB 376 (above 30 g/L) was observed when mannitol was used as a carbon source. The mixed carbon source did not increase productivity of lactic acid by Cl. bifermentans. The yield of lactic acid was approximately equal to the yield of lactic acid obtained on the medium with only glycerol and lower than in medium with only mannitol. Thus, from the environmental point of view it is more beneficial to use the medium with waste-type material only, such as glycerol.

Conditions Affecting Germination of Clostridium botulinum 62A Spores in a Chemically Defined Medium

Spores of Clostridium botulinum type 62A were germinated in a chemically defined medium (8 mm l-cysteine, 11.9 mm sodium bicarbonate, 4.4 mm sodium thioglycolate; buffered with 100 mm TES, pH 7.0). The rate and extent of germination were increased when an aqueous spore suspension was heated sublethally (80 C, 60 min) before addition to the germination medium. Neither sublethal nor lethal doses of gamma radiation had any marked effect on subsequent germination. Maximum germination (>90% in 2 hr) in the defined medium occurred in the pH range of 6.5 to 7.5, at 30 to 37 C, with an l-cysteine level of 8 mm. Increasing l-cysteine to 32 mm increased the rate (over that with 8 mm l-cysteine) but not the extent of germination. The rate and extent of germination increased with NaHCO(3) addition to 8.3 mm, but increasing levels to 11.9 mm had no further effect. For maximum germination, 2.2 mm sodium thioglycolate was required and higher levels (to 8.8 mm) had no further enhancing or inhibitory effect. Under optimal conditions for germination, 97% of the spores had become heat sensitive; 98% had become sensitive to radiation; 88 and 91% had become phase dark and stainable, respectively, and the spore suspension had lost 46% of its initial optical density by 2 hr. Loss of heat resistance preceded loss of radiation resistance, acquisition of stainability, and phase darkening by about 12 min.

Enrichment, isolation, and cultural characteristics of marine strains of Clostridium botulinum type C

Terrestrial strains of Clostridium botulinum type C, designated 468 and 571, were used to screen various media for growth and sporulation at 30 C. Of the various formulations tested, only egg meat medium fortified with 1% additions of yeast extract, ammonium sulfate, and glucose (FEM medium) gave good growth and satisfactory sporulation. FEM medium was used to recover four marine type C isolates from inshore sediments collected along the Atlantic, the Gulf of Mexico, and the Pacific coasts of the United States. The isolation techniques involved repeated transfer of cultures showing type C toxin in FEM medium and purification by a deep tube method. The medium used for purification was beef infusion-agar supplemented with 0.14% sodium bicarbonate and 0.1% l-cysteine hydrochloride. l-Cysteine was adopted in preference to sodium thioglycolate, because some lots of the latter were definitely inhibitory for growth. The addition of bicarbonate markedly increased viable spore counts of both the marine and terrestrial strains. Various cultural and biochemical characteristics of the marine and the terrestrial strains were compared. With the exception of some variations in their fermentation patterns, both groups showed similar characteristics. Of 23 fermentable compounds tested, the terrestrial strains attacked only glucose and mannose. The marine strains fermented glucose, mannose, galactose, and ribose actively; dextrin, inositol, maltose, and melibiose were weakly fermented.

The effect of temperature and of cetrimide on the rate of loss of refractility of spores of Bacillus megaterium

The rate of germination of spores of Bacillus megaterium at 30° is not significantly different from the rate at 37° but the onset of germination is delayed; outgrowth is normal. At 45° germination of some spores occurs but the rate is much slower than at 37°, and there is no lag; emergence occurs from only a proportion of the germinated spores and after 3 or 4 vegetative cells have been produced, replication ceases. A single regression equation can represent the germination rate of the spores at 37° in the presence of from 0.0005 to 0.02% w/v of cetrimide and in its absence. In 0.0005% w/v of cetrimide, germ cells emerge from some of the germinated spores but many of them become swollen and disintegrate. Concentrations of 0.00125% w/v or more progressively inhibit swelling and completely inhibit emergence.