Mixed Bacteria and Mold Interaction

Another example of mixed culture interaction is ensilage of crops. Dalamacio and Fung (35) and Dalmacio et al. (36) studied the influence of ammonia on bacteria and mold during ensilage of high-moisture corn. Bacteria were not affected by ammonia treatment as much as mold at the onset of the treatment. Bacterial populations remained detectable after treatment of 1.0, 1.5, and 2% ammonia. However, no mold was detected for corn treated with 1.0, 1.5, 2.0% ammonia. Corn treated with 0.5% ammonia had high bacterial and mold counts. After 2 months of storage, the

Figure 5. Interaction of mixed culture of S. cerevisiae and E. coli in terms of cell numbers monitored by direct count, electronic count, and viable cell count as well as product formation (alcohol and acid production). Source: Reprinted with permission from reference 39, page 528, by courtesy of Marcel Dekker, Inc.

Figure 5. Interaction of mixed culture of S. cerevisiae and E. coli in terms of cell numbers monitored by direct count, electronic count, and viable cell count as well as product formation (alcohol and acid production). Source: Reprinted with permission from reference 39, page 528, by courtesy of Marcel Dekker, Inc.

bacterial population increased to more than 107 g_1. Mold count increased to around 104 g "1 for the corn treated with 0.5% ammonia. After 4 months of storage, the bacterial count reached saturation level (higher than 1010 g_1) but the mold population remained at around 104 or 105 g_1. Not only did the bacteria and mold counts change in the fermentation, but the genera of bacteria and mold also changed concomitant to the fermentation stages. At the beginning, most of the bacteria isolated were Bacillus, but as the fermentation progressed most of the bacterial isolates belonged to Lactobacillus.

At the beginning of the storage of ammonia-treated corn (in the fall season), the predominant mold was Mucor, which is considered a field mold. As the storage continued into the cold winter, Penicillium isolates increased, and at the end of the storage in spring, Scopulariopsis predominated. These successions occurred for a variety of reasons. For example, the winter months favored the development of Penicillium, which is cold tolerant. As temperature increased in the spring, Penicillium population was overtaken by Scopulariopsis, which can use complex nitrogenous foods better than Penicillium.

The aforementioned examples of microbial interactions are just some of the typical interaction patterns in food and in nature. Countless other interactions can occur. The main point to emphasize is that mixed-culture interactions are very complex, but scientists can use this knowledge to optimize process control in obtaining desirable products in food and industrial microbiological processes.

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