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Mannheimia succiniciproducens LPK7 (knock-out: ldhA, pflB, ptaackA), a powerful succinate-producing strain was used in this study. Organic acids, succinic acid and pyruvic acid produced in fermentation have toxic effects on the cells that inhibit the cellular growth and metabolism. Therefore, experiments were designed and performed to determine harmful effects of the organic acids on fermentation processes. The resistance test of organic acids was composed of two steps; a viable cell count test, and a flask culture test of culturing cells at various organic acid concentrations. To figure out tolerance of the cells against organic acid, cellular growth was measured in several organic acid concentrations; 0%, 0.1%, 0.2%, 0.3%, 0.4%, and 0.5% for succinic acid and 0.25%, 0.75%, and 1.25% (w/v) for pyruvic acid. In the 0.1% succinic acid
concentration, cells were not affected significantly by the toxic effect of succinic acid. However, cell growth began to be inhibited when the succinic acid concentration reached 0.2% or higher levels. In addition, cells did not grow at all in the 0.5% medium. For the toxicity test of pyruvic acid, cell concentration was nearly one third in the 0.25% pyruvic acid medium compared to the control.