Features of the Synthesis of Extracellular Cytotoxic Lectin Bacillus subtilis IMV B-7724, Depending on the Cultivation Conditions in the Laboratory Fermenter

Abstract

The level of oxygen mass transfer (KV) is an important parameter influencing the growth rate of aerobic microorganisms and the synthesis of metabolites. It is mainly determined by the agitation and the aeration rates in the fermenter. Aim. To study changes in pH, optical density (OD), and hemagglutinating (lectin) activity (HAA) of culture fluid (CF) of Bacillus subtilis strain IMV B-7724, a producer of extracellular cytotoxic lectin (ECL), during its cultivation in a laboratory fermenter at different agitation and aeration rates as well as to determine and compare the HAA, carbohydrate specifi city, and cytotoxic properties of the corresponding samples of the preparation isolated from CF. Methods. Batch antifoam-free fermentations were performed by culturing the strain in the modified Gause medium with galactose in two identical lab-scale fermenters with a working volume of 2.5 L at 37ºC for 48—72 h according to three fermentation variants. Variant 1: n — 400 rpm for the whole cultivation, the air supply to the CF — through a sparger at 0.1 vvm until the 39th h with further gradual decrease, KV — 4.2±0.3 g O2·L−1·h−1. Variant 2: n — 400 rpm for the first 24 h, then a gradual decrease to 200 rpm, air supply — through a sparger at 0.1 rpm for the first 12 h, followed by its switching into the fermenter free space, corresponding KV — from 4.2±0.3 to 0.3±0.1 g O2·L−1·h−1. Variant 3: n — 400 rpm and air supply to the fermenter free space during the whole cultivation, KV — 4.0±0.3 g O2·L−1·h−1. A number of biological properties of strain CF and isolated lectin samples were evaluated by biochemical, spectrophotometric, immunological, and culture methods. Statistical analysis was performed using Student’s t-test. Results. The maximum increase in the OD of CF relative to the initial values (28 and 21-fold) at the end of the period of the rapid growth of the strain (at 9th h), the μmax values of 0.33 and 0.41 h−1, and pH not lower than 6.7 and 6.3 units were observed for fermentation variants 1 and 2, respectively. In the case of variant 2, the HAA of CF reached 32 hemagglutinating units (HAU), and the samples isolated from it had a lectin activity of 512±64 HAU, whereas for variant 1 such values were lower:16 and 32±8 HAA, respectively; carbohydrate specificity of preparations to bovine submandibular gland mucin was the same, i.e. 0.2±0.1 mg/mL. In contrast to the above, a slower increase in the OD of the CF, a decrease in μmax, and significant acid formation (15-fold at the 9th h, 0.25 h−1, and pH decrease to 5.8 units, respectively) were observed for variant 3; in this case, the level of HAA of CF was minimal (2—4 HAU) and was absent in the corresponding isolated samples. The probable reason for such differences was the limited mass transfer in the CF due to the isolating effect of the foam layer on its surface formed as a result of intensive agitation. Conclusions. The rapid growth of the strain and an increase in the HAA of CF were observed during cultivation in a lab-scale fermenter by maintaining the maximum level of oxygen mass transfer with air supply into the CF through a sparger until the maximum OD was reached and the subsequent gradual decrease in the specifi ed level during further cultivation started.