Cellulolytic bacteria

The degradation of cellulose by bacterial systems occurs both aerobically and anaerobically. The aerobic bacteria include Acidothennus cellulofyticus, CeUulomonas sp.. Bacillus sp.. Pseudomonas sp., CelMbrio sp., Cytophaga sp., Microbispora sp., and Thermomonospora sp. The anaerobic bacteria arc Acetivibrio cellulofyticus, Bacteroides sp., Clostridium sp., Ruminococcus sp., and Micromonospora sp. This review will cover some mesophilic and thermophilic cellulolytic bacteria, some of which have been reviewed by Ljungdahl and Eriksson (i), Coughlan and Ljungdahl (2), and Robson and Chambliss (5). 

Cellulolytic bacteria can be found which produce only cell-bound cellulase such as Cytophaga (12), only cell-free cellulase, such as CelMbrio vulgaris (21), Bacillus sp. (22), Clostridium sp. (23), Acetivibrio cellulofyticus (24), and Thermoactinomyces (25,26), and both cell-bound and cell-free cellulase such as Pseudomonas (27), Bacteroides succinogenes (28), and CelMbrio futvus (29). However, the location of cellulase in bacteria is also dependent upon the environments in which the bacteria are grown and the age of the culture (29,27). 

A summary of the key information has been compiled on the anaerobic and aerobic bacteria discussed above. Comparison of the substrates for growth of these organisms (Table I) show that all utilize cellobiose and various forms of cellulose. The two species belonging to Bacteroides have different specificity for substrates, while those for Ruminococcus, Cellulomonas and Thermomonospora were the same. Table I also allows comparison of the behavior of the 13 species of cellulolytic bacteria toward cellobiose. More variability is noted in this regard and no correlation between induction/repression can be made with the mechanism of cellobiose degradation. 

Table I. Substrates for Growth of Cellulose Bacteria (a) and Means by Which Cellobiose is Utilized by Cellulolytic Bacteria (b) (a)...

Shi, Y., and P. J. Weimer. 1996. Utilization of individual cellodextrins by three predominant ruminal cellulolytic bacteria. Applied Environmental Microbiology 62 1084-1088. 

Weimer, P.J. and Odt, C.L., Cellulose degradation by ruminal microbes physiological and hydrolytic diversity among ruminal cellulolytic bacteria, in Enzymatic Degradation of Insoluble Carbohydrates, Saddler, J.N. and Penner, M.H., Eds., ACS Symposium Series, No. 618, American Chemical Society, Washington, D.C., 1995, chap. 18. 

Oince cellulose is insoluble high polymers under physiological condi- tions, cellulase which is destined to attack it has been expected to be an extracellular enzyme. In fact, most of cellulolytic microorganisms secrete some cellulase components into the culture medium, and almost all work on the cellulase have been performed using these extracellular components. In the cultures of cellulolytic bacteria, cellulases are not only found in their culture filtrates, but also are generally obtainable from the cells by treatment with autolytic agents—e.g., toluene (9, 19, 30). These facts indicate that at least certain components are existent within the bacterial cells and that their physiological function may be distinct from that of the extracellular components. 

P. J. Van Soest I don t know. But we do know that in the rumen most of the cellulolytic bacteria are actually attached to the fibers. ... 

Although mammals don t have enzymes to digest cellulose, certain bacteria do have these enzymes. In fact, these cellulolytic bacteria thrive by using cellulose as their primary food source. Capitalizing on this feature, evolution has come to the rescue. 

Over millions of years, some animals evolved a specialized pouch in their digestive tracts to house these bacteria. This pouch—a fermentation sac—can either be located at the front end of the digestive tract, where it is called the rumen, or at the far end of the digestive tract, where it is called the large intestine or cecum. In these fermentation sacs, the cellulolytic bacteria happily do their thing and in turn, produce nutritional products that can be absorbed and used by the host animal for its own metabolism. Thus the ruminant or horse or alpaca or any animal housing these bacteria (including, to a small extent, humans) can obtain nutritional value from cellulose. 

Actually, mammals aren t the only animals who evolved fermentation sacs for cellulolytic bacteria. Termites have done it too, for exactly the same reason. 

The direct fermentation method is based on the direct fermentation of cellulose by cellulolytic bacteria, without acidic or alkali pretreatment. It has the features of simple equipment and low cost, but with low ethanol yield and production of organic acid and other by-products. The problems can be solved partly by mixed strain fermentation. 

Products that are used to modify rumen fermentation include buffers, which regulate rumen pH and favour the activity of cellulolytic bacteria, compounds to suppress methane production, and bloat-preventing compounds, which prevent the build-up of gas trapped in foam in the rumen fluid. 

Celluloses.—Papers presented at a symposium on the thermal uses and properties of polysaccharides and lignins have discussed the pyrolysis of cellulosic materials. Phenols and carboxylic acids present in condensates of the smoke of cigarettes containing a cellulosic tobacco substitute have been identified and measured. The solubility of holocelluloses in DMSO—paraformaldehyde has provided the basis for a rapid laboratory method for predicting the dry-matter digestibility of cellulosic materials. The amount of dry matter solubilized by this chemical system correlated with that digested by rumen fluid in vitro or by cellulolytic bacteria. 

R. M. Theater, and P. J. Wood, Use of Congo Red-Polysaccharide Interactions in Enumeration and Characterization of Cellulolytic Bacteria from the Bovine Rumen, App. Environ. Microbiol., 43 777 (1982). 

So far no significant evidence of cellulose digestion by bacteria has been found in termites which contain protozoans, and Baldacci could not demonstrate cellulolytic bacteria in defaunated termites. 

---