Determination of optimal growth conditions for gram-positive bacterium Gulosibacter sp. BS38, destructor of toxic xenobiotic epsilon-caprolactam

Caprolactam is one of the widespread pollutants; its annual world production is in millions of tons. In the process of production and polymerization of CAP, wastes are generated that contain a certain amount of caprolactam and low molecular weight fractions of oligomers. Currently, industrial waste is incinerated or disposed of, which leads to pollution of soil and groundwater with toxic pollutants. A lack of systematic studies on microbial destructors of caprolactam is a limiting factor in the development of cost-effective technologies for biological treatment of industrial wastewaters. The aim of this research was to explore the growth pattern of the strain-destructor Gulosibacter sp. BS38 in a mineral medium containing caprolactam as a sole carbon and energy source at different substrate concentrations, pH and temperatures. In order to determine the optimum growth conditions, liquid mineral medium was used at temperature range, pH value or substrate concentration according to the objective of the experiment. When studying the effect of cultivation conditions on the strain growth, such parameters as optical density (OD), the length of the lag-phase and the maximum specific growth rate of the culture (μ_max) were assessed. Cells were cultured in 750 ml Erlenmeyer flasks, containing 100 ml of the medium, during 100120 hours in an incubator shaker at 180 rpm. As the inoculum, a culture grown in the same medium (рН 7.5, cultivation temperature 28°С, CAP concentration 1.0 g/l) was used up to an optical density equivalent to 0.5. The intensity of culture growth was estimated spectrophotometrically by the change in the optical density (wavelength = 590 nm); the curve of growth patterns was used to calculate the length of the lag-phase and the maximum specific growth rate. According to literature data, the rate and degree of caprolactam degradation in wastewater, first, depends on the amount of this compound, hence the tolerance of the microorganisms-destructors to high concentrations of caprolactam can be critical in practical use. The strain BS38 was able to grow on this xenobiotic compound, the concentration of which was widely ranging from 0.5 to 12.0 g/l. In the presence of 0.5 g/l, the length of the growth lag-phase was the shortest (5 hours) compared to those observed at higher concentrations (See Fig. 1A), but the optical density was not high (0.55). The maximum specific growth rate was 0.057 h^-1. When substrate concentration was 1.0 and 2.0 g/l, the optical density was maximum, 0.71 and 0.73; it was the same for the specific growth rate (0.094 and 0.086 h^-1, respectively). A further increase in caprolactam level leads proportionally to extended lag time, slow specific growth rate and decreased optical density of the culture. The highest level of the caprolactam in the medium at which bacterial growth continued was 12.0 g/l. The study of the effect of pH and temperature on growth patterns of the strain BS38 showed that the optimum growth of the bacterium occurred at 28°С and pH 7.5 (See Fig. 1, B, C). It was found that the temperature mostly affected the specific growth rate of the culture (0.094, 0.052, and 0.038 h^-1 at a temperature of 28°C, 37°C, and 15°C, respectively) while the change in pH also caused an extension in lag time and a decrease in optical density (Fig. 1, B). The paper contains 1 Figure and 16 References.

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