Continuous Fermentation of a Prodigiosin-Producing Serratia marcescens Strain Isolated from Soil

A red-pigmented bacterial strain NS-17 was isolated from soil, and physiological and biochemical characteristics and 16S rDNA analysis established this organism as a strain of Serratia marcescens. The red pigment isolated from cells of NS-17 was identified as prodigiosin. By analyzing factors affecting the production of prodigiosin, modified medium and culture conditions were set up, and a continuous fermentation method was carried out to take advantage of the mass production of foam during fermentation. Continuous fermentation was achieved in the following conditions: 32 ̊C, 1:1 ventilation ratio, and medium with a nutrient concentration 5-fold higher than that of the fermentation medium (with half the inorganic salt concentration) supplied as a feed medium at a flow rate of 8 mL/min. For the first time of continuous fermentation of Serratia marcescens coupling with foam flotation, a high yield of prodigiosin was obtained. After 56 h of culturing, the total harvest of cells was enhanced 2.33-fold compared to that of batch fermentation, the total amount of prodigiosin was enhanced 2.70-fold compared to that of batch fermentation, and cells and prodigiosin were concentrated in the efflux broth automatically.

Serratia marcescens is the most widely used PG-producing bacterium due to its high production of both biomass and prodigiosin. The traditional production of PG was by batch fermentation, but the yield was not sufficiently high, and because the fermentation process produces considerable foam, operators must add a large amount of defoamer, which is not conducive to downstream extraction and purification. In this study, a red-pigmented bacterium strain NS-17 isolated from soil was identified as S. marcescens according to its physiological and biochemical characteristics and 16S rDNA sequence, and the red pigment, PG, was characterized by LC/MS, FT-IR and NMR. Willenbacher [13] and Cui [14] got high yield of product by coupling the fermentation process and foam flotation.
To improve the yield of PG and to avoid requiring defoamer, the S. marcescens cells were continuously separated from the broth by the foam produced during fermentation, and the efflux broth was collected.

Bacterial Strain and Media
A red-pigmented bacterium NS-17 was isolated from the soil of the Nanchang area with a nutrient agar plate. NS-17 was incubated in liquid medium containing 4 g of glycerol, 16 g of peptone, 4 g of NaCl and 7 g of KCl at 28˚C and shaken at 160 rpm for 24 h in a shaking incubator.

Identification of NS-17
Morphological and biochemical tests were done using standard procedures [15].
Ultrastructural studies were performed by scanning electron microscopy.
The 16S rDNA of NS-17 was isolated and sequenced. The sequence was compared with available 16S sequences in GenBank at the National Center for Biotechnology Information (NCBI). A phylogenetic tree was constructed with the Mega program, and 1000 bootstrap resampling was applied to evaluate the robustness of the phylogenetic tree.

Extraction and Isolation of Pigment
After fermentation, NS-17 cells were collected by centrifugation and washed twice with distilled water. Acidified ethanol (pH 3.0) was added to the cell mass to extract pigment, and the mixture was centrifuged. The pigment in ethanol was isolated using phase separation with water and chloroform. The chloroform

Characterization of Red Pigments
The purified pigment was dissolved in ethanol, with pH = 5.0 or 9.0 (adjusted by

Optimization of Flask Culture Conditions
The purified prodigiosin was used to establish a formula between OD 535 and mass concentration.
The culture broth was mixed with acidic methanol (pH 3.0). The volume of methanol was adjusted according to the OD 535 of the mixture to ensure the optical density was in the range of 0. 2-1.0.
Because NS-17 produced minimal pigment in LB broth (the most widely used medium for production of prodigiosin), we tested several media reported for fermentation of prodigiosin [1] [9] [16] [17] [18] and chose the initial medium as follows: glycerol 4 g/L, peptone 16 g/L, NaCl 3 g/L, and KCl 4 g/L; pH 7.0. The initial culture conditions were as follows: inoculum size: 1.0%, 100 mL of medium in flask (250 mL), culture temperature 28˚C, and rotation speed 160 rpm.
The production of prodigiosin was measured after 24 h culture.   All the results showed that NS-17 was a member of S. marcescens, and the strain was therefore named S. marcescens NS-17.

Characterization of the Pigment
Purified Serratia marcescens strain NS-17 was fermented in a shaken flask to produce pigments. After separation with column chromatography and purification by TLC, the red pigment was obtained.
The UV-Vis spectrum of the pigment (Figure 3) was similar to the typical spectrum of prodigiosin [2]. The TOFMS analysis result is shown in Figure 4.    UV-Vis, TOFMS, FT-IR and NMR analyses suggested that the red pigment isolated from NS-17 was PG. The pigment structure is shown in Figure 5.

Flask Culture
The strain NS-17 produced minimal red pigment in LB broth, weakly responded to supplementation with Tween-80 and oil and showed a high production of prodigiosin under the temperature of 30˚C -32˚C, different from most reported S. marcescens strains [2] [3] [19]. The results of single-factor experiments are shown in Table 1, with the maximum values presented in bold. Experiments were carried out according to the order in Table 1 with the factors optimized in the previous steps.
In the investigated factors and concentrations, the optimal conditions for NS-17 to produce prodigiosin were as follows: maltose 6 g/L; peptone 12 g/L; NaCl 1 g/L; KCl 2 g/L; Tween-80 1% (w/v); soybean oil 4% (w/v); pH of medium 8; inoculum size ≥ 1.5%; volume of medium in a 250 mL flask ≤ 50 mL; rotational rate ≥ 190 rpm; and cultivation temperature 32˚C. Figure 6 shows the yield of prodigiosin according to time for an inoculum size of 1.5%, volume of medium of 50 mL and rotational rate of 190 rpm. The concentration of prodigiosin reached a relatively high value (60.51 mg/L) within 32 h of incubation, and the pH of the culture broth was stable at approximately 8.0, which is supposed to be good for the production of prodigiosin [19] [20] [21].
Oxygen supply significantly affected the concentration of prodigiosin in investigations of the volume of medium and rotational rate. Sufficient oxygen supplementation was the essential condition for prodigiosin fermentation.

Continuous Fermentation
With a 250 rpm rotor speed and 1:1 ventilation ratio in a bioreactor (GUJS-50L, EASTBIO Ltd., China), a control growth culture was repeated three times under the optimized conditions of the flask culture with 5 g/L of propoxylated glycerin as a defoamer. Group 1 was carried out three times under the same conditions as those of the control group but with no defoamer, and Group 2 was carried out  and the weight of the efflux broth is shown in Figure 6. The PG concentrations in the bioreactor and efflux broth are shown in Figure 7 and Figure 8.   Because the process of foam collection was subject to contamination, the continuous fermentation stage of S. marcescens was restricted to less than 72 h, and no contamination occurred in 56 h.

Conclusion
S. marcescens NS-17 isolated from soil produced a considerable amount of PG in a flask culture under the following conditions: maltose 6 g/L; peptone 12 g/L; NaCl 1 g/L; KCl 2 g/L; Tween-80 1% (w/v); pH 8; inoculum size ≥ 1.5%; and cultivation temperature 32˚C. Based on the foam flotation method, continuous fermentation was performed by removing the foam as soon as it reached the top of the bioreactor; the cells and PG were automatically concentrated in the foam, making the extraction easier and achieving a high production of PG. After 56 h of culture, the total cell harvest was enhanced 2.33-fold compared to that of batch fermentation, and the total amount of prodigiosin was enhanced 2.70-fold compared to that of batch fermentation. However, due to the risk of contamination, long-term culture remains a challenge.