Mixed Cultures

Mixed cultures are used extensively in food fermentations and in biodegradation of complex substances. Mutualism, commensalism, amensalism, competition, and prey-predator relationships are the main interactions observed in mixed cultures (16). Mutualism occurs when both species benefit from the interaction, whereas commensalism describes processes where only one species benefits and the other is not affected. Amensalism occurs when one species restricts another, whereas competition affects both species because of nutrient limitations. The predator consumes the prey in prey-predator interactions. In actual situations, more than two species are often present, and several types of interaction may be taking place.

In the recent book on mixed cultures by Zeikus and Johnson (20), most of the contributions are related to either food fermentations or environmental microbiology.

There are food fermentation chapters on bread, milk, oriental foods, wine, and vegetables and environmental microbiology chapters related to degradation of polysaccha-rides, methanogenesis, detoxification of hazardous waste, corrosion, and leaching processes in mineral biotechnology.

In San Francisco sourdough bread, Lactobacillus san-franciscoferments only the maltose, whereas Candidamil-leri uses all the other free sugars but not maltose. Lactic acid, acetic acid, and carbon dioxide are the main products of the fermentation; however, small amounts of propionic, isobutyric, butyric, a-methyl-n-butyric, isovaleric, andval-eric acids have been found in commercial San Francisco sourdough breads (21).

Anaerobic cellulose degradation is accomplished by a mixed culture of Clostridium cellulolytic bacteria that produce cellobiose, glucose, and cellodextrins; fermentative bacteria that produce propionate, butyrate and other fermentation products from glucose and other intermediates; syntrophic acetogenic bacteria that convert the fermentation products to acetate, hydrogen, and carbon dioxide; homoacetogens that convert hydrogen and carbon dioxide to acetate; and methanogens that produce methane from acetate, formate, and hydrogen (22). Leschine (22) reports that Clostridium papyrosolvens grow mutualistically in co-culture with a noncellulytic Klebsiella; C. papyrosolvens hydrolyzes cellulose for the Klebsiella, whereas Klebsiella excretes vitamins required by the C. papyrosolvens. The soluble sugar products of cellulose hydrolysis provide substrates for many noncelluloytic commensal organisms that depend on the cellulolytic bacteria, but do not provide known compounds in return.

In the natural environment, the human environment, and even in the industrial environment, mixed cultures of bacteria, yeast, mold, viruses, protozoa, nematodes, and higher organisms live in antagonistic, cooperative, or inert coexistence. Pure culture systems are the result of understanding of the fundamental role of microbes in a particular process and the isolation and cultivation of such cultures for specific reactions. This section deals with single-culture processes mixed pure-culture processes, and mixed natural-culture processes, using food systems as examples. This information certainly can be applied to other industrial and fermentation processes.

Other publications deal adequately with the subject of single-cell process. The key success of single-culture process is to provide the culture with a sterile substrate and environment with no contamination during the process. Single-cell process is a manmade situation classified as a controlled process because the substrate is prepared and processed in such a way as to minimize contamination. Examples of this type of process are wine making, beer making, bread making, single-culture dairy product fermentation, and vinegar production. The kinetics of growth and product formation are easier to control and monitor.

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