Extracellular particles of polymeric material formed in n-hexadecane fermentation by Pseudomonas aeruginosa

Two-step conversion of polyethylene into recombinant proteins using a microbial platform

BACKGROUND

The increasing prevalence of plastic waste combined with the inefficiencies of mechanical recycling has inspired interest in processes that can convert these waste streams into value-added biomaterials. To date, the microbial conversion of plastic substrates into biomaterials has been predominantly limited to polyhydroxyalkanoates production. Expanding the capabilities of these microbial conversion platforms to include a greater diversity of products generated from plastic waste streams can serve to promote the adoption of these technologies at a larger scale and encourage a more sustainable materials economy.

RESULTS

Herein, we report the development of a new strain of Pseudomonas bacteria capable of converting depolymerized polyethylene into high value bespoke recombinant protein products. Using hexadecane, a proxy for depolymerized polyethylene, as a sole carbon nutrient source, we optimized media compositions that facilitate robust biomass growth above 1 × 109 cfu/ml, with results suggesting the benefits of lower hydrocarbon concentrations and the use of NH4Cl as a nitrogen source. We genomically integrated recombinant genes for green fluorescent protein and spider dragline-inspired silk protein, and we showed their expression in Pseudomonas aeruginosa, reaching titers of approximately 10 mg/L when hexadecane was used as the sole carbon source. Lastly, we demonstrated that chemically depolymerized polyethylene, comprised of a mixture of branched and unbranched alkanes, could be converted into silk protein by Pseudomonas aeruginosa at titers of 11.3 ± 1.1 mg/L.

CONCLUSION

This work demonstrates a microbial platform for the conversion of a both alkanes and plastic-derived substrates to recombinant, protein-based materials. The findings in this work can serve as a basis for future endeavors seeking to upcycle recalcitrant plastic wastes into value-added recombinant proteins.

Achieving “Non-Foaming” Rhamnolipid Production and Productivity Rebounds of Pseudomonas aeruginosa under Weakly Acidic Fermentation

The rhamnolipid production of Pseudomonas aeruginosa has been impeded by its severe foaming; overcoming the bottleneck of foaming has become the most urgent requirement for rhamnolipid production in recent decades. In this study, we performed rhamnolipid fermentation under weakly acidic conditions to address this bottleneck. The results showed that the foaming behavior of rhamnolipid fermentation broths was pH-dependent with the foaming ability decreasing from 162.8% to 28.6% from pH 8 to 4. The “non-foaming” rhamnolipid fermentation can be realized at pH 5.5, but the biosynthesis of rhamnolipids was significantly inhibited. Further, rhamnolipid yield rebounded from 8.1 g/L to 15.4 g/L after ultraviolet and ethyl methanesulfonate compound mutagenesis. The mechanism study showed that the species changes of rhamnolipid homologs did not affect the foaming behavior of the fermentation but had a slight effect on the bioactivity of rhamnolipids. At pH 8.0 to 5.0, increased surface tension, decreased viscosity and zeta potential, and aggregation of rhamnolipid molecules contributed to the “non-foaming” rhamnolipid fermentation. This study provides a promising avenue for the “non-foaming” rhamnolipid fermentation and elucidates the mechanisms involved, facilitating the understanding of pH-associated foaming behavior and developing a more efficient strategy for achieving rhamnolipid production.

Foaming of rhamnolipids fermentation: impact factors and fermentation strategies

Rhamnolipids have recently attracted considerable attentions because of their excellent biosurfactant performance and potential applications in agriculture, environment, biomedicine, etc., but severe foaming causes the high cost of production, restraining their commercial production and applications. To reduce or eliminate the foaming, numerous explorations have been focused on foaming factors and fermentation strategies, but a systematic summary and discussion are still lacking. Additionally, although these studies have not broken through the bottleneck of foaming, they are conducive to understanding the foaming mechanism and developing more effective rhamnolipids production strategies. Therefore, this review focuses on the effects of fermentation components and control conditions on foaming behavior and fermentation strategies responded to the severe foaming in rhamnolipids fermentation and systematically summarizes 6 impact factors and 9 fermentation strategies. Furthermore, the potentialities of 9 fermentation strategies for large-scale production are discussed and some further strategies are suggested. We hope this review can further facilitate the understanding of foaming factors and fermentation strategies as well as conducive to developing the more effective large-scale production strategies to accelerate the commercial production process of rhamnolipids.

Structural and Physicochemical Characterization of Rhamnolipids produced by Pseudomonas aeruginosa P6

Rhamnolipids are important biosurfactants for application in bioremediation, enhanced oil recovery, pharmaceutical, and detergent industry. In this study, rhamnolipids extracted from P. aeruginosa P6 were characterized to determine their potential fields of application. Thin-layer chromatographic analysis of the produced rhamnolipids indicated the production of two homologues: mono- and di-rhamnolipids, whose structures were verified by 1H and 13C nuclear magnetic resonance spectroscopy. Additionally, high performance liquid chromatography-mass spectrometry identified seven different rhamnolipid congeners, of which a significantly high proportion was di-rhamnolipids reaching 80.16%. Rha-Rha-C10-C10 was confirmed as the principal compound of the rhamnolipid mixture (24.30%). The rhamnolipids were capable of lowering surface tension of water to 36 mN/m at a critical micelle concentration of 0.2 g/L, and exhibited a great emulsifying activity (E24 = 63%). In addition, they showed excellent stability at pH ranges 4-8, NaCl concentrations up to 9% (w/v) and temperatures ranging from 20 to 100 °C and even after autoclaving. These results suggest that rhamnolipids, produced by P. aeruginosa P6 using the cheap substrate glycerol, are propitious for biotechnology use in extreme and complex environments, like oil reservoirs and hydrocarbon contaminated soil. Moreover, P. aeruginosa P6 may be considered, in its wild type form, as a promising industrial producer of di-RLs, which have superior characteristics for potential applications and offer outstanding commercial benefits.

Recent progress towards industrial rhamnolipids fermentation: Process optimization and foam control

The global market for rhamnolipids production holds great promise, and is in need of an economically viable mass-production scheme. Accordingly, several strategies have been employed to improve the efficiency of rhamnolipid production in the past few decades. Currently, rhamnolipids can be produced by Pseudomonas aeruginosa at a high yield (over 70 g/L) when vegetable oil is used as the carbon source under optimized fed-batch cultivations. However, severe foaming during rhamnolipid fermentation inhibits scaling-up and production efficiency. Stop valve was found to effective break the extremely stable rhamnolipids foams during fermentation, and production efficiency of rhamnolipids was highly improved, while its scale-up mechanism needs further study. In addition, the combination of both chemical and mechanical approaches is likely to be more efficiently resolving the foam problem existed in rhamnolipids fermentation than either chemical or mechanical methods alone.

Rhamnolipid production by a gamma ray-induced Pseudomonas aeruginosa mutant under solid state fermentation

Solid-state fermentation has a special advantage of preventing the foaming problem that obstructs submerged fermentation processes for rhamnolipid production. In the present work, a 50:50 mixture of sugarcane bagasse and sunflower seed meal was selected as the optimum substrate for rhamnolipid production using a Pseudomonas aeruginosa mutant 15GR and an impregnating solution including 5% v/v glycerol. Using Box-Behnken design, the optimum fermentation conditions were found to be an inoculum size 1% v/v, temperature 30 °C and unlike other studies, pH 8. These optimized conditions yielded a 67% enhancement of rhamnolipid levels reaching 46.85 g rhamnolipids per liter of impregnating solution, after 10 days, which was about 5.5 folds higher than that obtained by submerged liquid fermentation. Although maximum rhamnolipids concentration was obtained after 10 days of incubation, rhamnolipids concentration already reached high levels (41.87 g/l) after only 6 days. This rhamnolipid level was obtained in a shorter time and using lower carbon source concentrations than most studies reported so far. The findings obtained indicate an enormous potential for employing solid-state fermentation for rhamnolipid production by the studied isolate.

A review on natural surfactants

Natural surfactants or biosurfactants are amphiphilic biological compounds, usually extracellular, produced by a variety of microorganisms from various substances including waste materials.

Biosurfactant-mediated biodegradation of straight and methyl-branched alkanes by Pseudomonas aeruginosa ATCC 55925

Accidental oil spills and waste disposal are important sources for environmental pollution. We investigated the biodegradation of alkanes by Pseudomonas aeruginosa ATCC 55925 in relation to a rhamnolipid surfactant produced by the same bacterial strain. Results showed that the linear C11-C21 compounds in a heating oil sample degraded from 6% to 100%, whereas the iso-alkanes tended to be recalcitrant unless they were exposed to the biosurfactant; under such condition total biodegradation was achieved. Only the biodegradation of the commercial C12-C19 alkanes could be demonstrated, ranging from 23% to 100%, depending on the experimental conditions. Pristane (a C19 branched alkane) only biodegraded when present alone with the biosurfactant and when included in an artificial mixture even without the biosurfactant. In all cases the biosurfactant significantly enhanced biodegradation. The electron scanning microscopy showed that cells depicted several adaptations to growth on hydrocarbons, such as biopolymeric spheres with embedded cells distributed over different layers on the spherical surfaces and cells linked to each other by extracellular appendages. Electron transmission microscopy revealed transparent inclusions, which were associated with hydrocarbon based-culture cells. These patterns of hydrocarbon biodegradation and cell adaptations depended on the substrate bioavailability, type and length of hydrocarbon.

Rhamnolipids as affinity foaming agent for selective collection of β-glucosidase from cellulase enzyme mixture

Selective and effective separation can potentially be achieved with affinity foam fractionation using simple foaming setup and operation. In this study the use of affinity foam fractionation for selective collection and enrichment of β-glucosidase from a cellulase enzyme mixture was evaluated. Rhamnolipids, a group of glycolipids produced most commonly by Pseudomonas aeruginosa, were used as the affinity foaming agent, because of their foaming property and the presence of dirhamnose moiety (a potential substrate analog for β-glucosidase) in some rhamnolipids. The effects of aeration rate, medium pH, cellulase concentration and rhamnolipid concentration on the foam fractionation performance were examined. Among the pH studied (3.1, 5.0, 7.0 and 9.0), pH 5 was clearly the optimal for selective enrichment of β-glucosidase, presumably corresponding to the high binding affinity between the enzyme and the substrate analog (dirhamnose). With adequate rhamnolipid concentrations (≥0.1 g/L), the aeration rate of 0.1L/min (i.e., 2VVM) for 50 ml test samples was found to give the highest enrichment; higher aeration rates produced wetter foam and, thus, lower (diluted) enzyme activity in the foamate. The enrichment increased with the increasing rhamnolipids-to-cellulase ratio, in the range of 0-2 (w/w) investigated in this study. The finding indicated that rhamnolipids were the limiting compounds in these systems so that the amount of surfactant-enzyme complexes formed and removed into the foam phase would increase when more rhamnolipids were added. At the rhamnolipids-to-cellulase ratio of 2, the β-glucosidase activity in the foamate was about 9 times as high as the activity in the original sample of cellulase mixture and about 17 times the activity in the remaining solution (after foaming). The overall FPU (filter paper unit, a measurement of total cellulase activity) and the activities of endo- and exo-glucanases were only enriched 70-150%. The feasibility of affinity foam fractionation was demonstrated.

Carbon content reduction in a model reluctant clayey soil: slurry phase n-hexadecane bioremediation

Clayey soils contaminated with organic pollutants are nowadays one of the important environmental issues as they are highly reluctant to conventional bioremediation techniques. In this study, biodegradability of n-hexadecane as a model contaminant in oil polluted clayey soil by an indigenous bacterium was investigated. Maximal bacterial growth was achieved at 8% (v/v) n-hexadecane as sole carbon and energy sources in aqueous phase. The predominant n-hexadecane uptake mechanism was identified to be biosurfactant-mediated using bacterial adhesion to hydrocarbon (BATH) test and surface tension measurements. The effect of n-hexadecane concentration, soil to water ratio, inoculum concentration and pH on total organic carbon (TOC) reduction from kaolin soil in slurry phase was investigated at two levels in shake flasks using full factorial experimental design method where 10,000 (mg n-hexadecane)(kg soil)(-1), soil-water ratio of 1:3, 10% (v/w) inoculum and pH of 7 resulted in the highest TOC reduction of 70% within 20 days. Additionally, slurry bioreactor experiments were performed to study the effect of various aeration rates on n-hexadecane biodegradation during 9 days where 2.5 vvm was found as an appropriate aeration rate leading to 54% TOC reduction. Slurry phase bioremediation is shown to be a successful method for remediation of clayey reluctant soils.

Methods for investigating biosurfactants and bioemulsifiers: a review

Microorganisms produce biosurfactant (BS)/bioemulsifier (BE) with wide structural and functional diversity which consequently results in the adoption of different techniques to investigate these diverse amphiphilic molecules. This review aims to compile information on different microbial screening methods, surface active products extraction procedures, and analytical terminologies used in this field. Different methods for screening microbial culture broth or cell biomass for surface active compounds production are also presented and their possible advantages and disadvantages highlighted. In addition, the most common methods for purification, detection, and structure determination for a wide range of BS and BE are introduced. Simple techniques such as precipitation using acetone, ammonium sulphate, solvent extraction, ultrafiltration, ion exchange, dialysis, ultrafiltration, lyophilization, isoelectric focusing (IEF), and thin layer chromatography (TLC) are described. Other more elaborate techniques including high pressure liquid chromatography (HPLC), infra red (IR), gas chromatography-mass spectroscopy (GC-MS), nuclear magnetic resonance (NMR), and fast atom bombardment mass spectroscopy (FAB-MS), protein digestion and amino acid sequencing are also elucidated. Various experimental strategies including static light scattering and hydrodynamic characterization for micelles have been discussed. A combination of various analytical methods are often essential in this area of research and a numbers of trials and errors to isolate, purify and characterize various surface active agents are required. This review introduces the various methodologies that are indispensable for studying biosurfactants and bioemulsifiers.

The production of rhamnolipid by a Pseudomonas aeruginosa strain isolated from a southern coastal zone in Brazil

The production and properties of a rhamnolipid-type biosurfactant, synthesized by the Pseudomonas aeruginosa LBM10 strain, isolated from a southern coastal zone in Brazil, were investigated. The assays were conducted in a rotary shaker at 30 degrees C and 180 rpm for a period of 96 h. Soybean oil and sodium nitrate were the best sources of carbon and nitrogen, respectively. A nitrogen-limiting condition (C/N ratio of 100) was favorable to biosurfactant production. The formation of stable emulsions was better in saline concentrations below 0.5%, pH values in the range from 6 to 9 and temperatures in the range from 35 to 40 degrees C, maintaining about 80% of its original activity for salinity up to 3% and 120 min of exposure at 100 degrees C. The biosurfactant may be produced with this microorganism using renewable substrates that are readily available, reaching values of 1.42 g l(-1) measured as rhamnose. This biosurfactant has interesting and useful properties for many industrial applications.

Isolation and characterization of biosurfactant/bioemulsifier-producing bacteria from petroleum contaminated sites

Biosurfactant-producing bacteria were isolated from terrestrial and marine samples collected in areas contaminated with crude oil or its byproducts. Isolates were screened for biosurfactant/bioemulsifier production in different carbon sources (glucose, fructose, sucrose and kerosene) using the qualitative drop-collapse test. Glucose produced the highest number of positive results (17 of 185 isolates). All 17 isolates produced emulsions with kerosene and 12 exhibited high emulsion-stabilizing capacity, maintaining 50% of the original emulsion volume for 48 h. Eight of the 17 isolates reduced the growth medium surface tension below 40 mN m(-1) with 5 exhibiting this capacity in cell-free filtrates. Onset of biosurfactant production differed among the isolates, with some initiating synthesis during the exponential growth phase and others after the stationary phase was reached. Increasing temperature from 25 to 35 degrees C accelerated onset of biosurfactant production in only two isolates while pH (6.5-7.6) had no effect in any isolate tested. Isolation from petroleum contaminated sites using the screening protocol presented proved to be a rapid and effective manner to identify bacterial isolates with potential industrial applications.

Competition between oxygen and nitrate respirations in continuous culture ofPseudomonas aeruginosa performing aerobic denitrification

Continuous culture of P. aeruginosa was conducted with nitrate-containing media under the dilution rates (D) of 0.026, 0.06, and 0.13/h and the dissolved oxygen concentrations (DO) of 0-2.2 mg/L. The bacterium performed simultaneous O(2) and nitrate respiration in all of the systems studied. For each D, the (apparent) cell yield from glucose (Y(X/S)) was lower at zero DO, but did not change substantially with non-zero DO. In non-zero DO systems, Y(X/S) increased with increasing D, and when fit with a model considering cell death, gave the following parameters: maximum cell yield Y(X/S) (m) = 0.49, maintenance coefficient M(S) = 0.029 (/h), and cell decay constant k(d) = 0.014/h. The same model failed to describe the behaviors of zero-DO systems, where neither glucose nor nitrate was limiting and the limiting factor(s) remained unknown. The cell yield from accepted electron (Y(X/e)) was however relatively constant in all systems, and the energy yield per electron accepted via denitrification was estimated at approximately 69% of that via O(2) respiration. A closer examination revealed that increasing DO enhanced O(2) respiration only at extremely low DO ( <0.05 mg/L), beyond which the increasing DO only slightly increased its weak inhibition on denitrification. While O(2) was the preferred electron acceptor, the fraction of electrons accepted via denitrification increased with increasing D.

Stimulating in-soil rhamnolipid production in a bioslurry reactor by limiting nitrogen

A soil with aged contamination from lubricating oil (LO) and polychlorinated biphenyls (PCBs) was treated in a bioslurry reactor to investigate in-soil biosurfactant production by Pseudomonas aeruginosa, the most abundant indigenous, culturable, hydrocarbon-degrading microorganism. After 2 days of growth on LO, a depletion of nitrogen stimulated the production and accumulation of rhamnolipids to levels roughly 20 times the critical micelle concentration. Surface tensions and concentrations of monorhamnolipid and dirhamnolipid, PCBs, and total petroleum hydrocarbons (TPH) were measured in a slurry filtrate. Soil-bound PCBs and TPH were also quantified. Rhamnolipid production was observed within 1 to 2 days after nitrogen depletion in each of the 10 batches tested. By day 6, total rhamnolipid concentrations increased from below detection to average values over 1,000 mg/L, which caused over 98% of soil-bound PCBs and over 99% of TPH to be emulsified and recovered in the filtrate. After 70 days, rhamnolipid concentrations were only reduced by 15%, because of nitrogen-limited rates of rhamnolipid biodegradation. The results show that in-soil biosurfactant production can be stimulated in a controlled way with nutrient limitation and can be used to achieve soil washing.

Pyrene biodegradatin in aqueous solutions and soil slurries by Mycobacterium PYR-1 and enriched consortium

To better understand complex bioavailability issues, pyrene degradation was examined in aqueous and soil slurry solutions using pure Mycobacterium sp. PYR-1 and a microbial consortium. The intrinsic rates of the aqueous pyrene degradation were very similar, 1.3 x 10(-9) microg pyrene/CFU-h for Mycobacterium sp. PYR-1 and 1.1 x 10(-9) microg pyrene/CFU-h for the consortium. Rates were much lower with the soil-slurry experiments, ranging from 1.2 x 10(-12) to 7.8 x 10(-10) microg/CFU-h, depicting the strong negative effects of soils on bioavailability. Supernatants from the slurry experiments were found to increase the aqueous-phase pyrene solubility significantly. Pyrene solubility was increased from 120.5 to over 230 microg/l. However, the linear adsorption constants of pyrene on the soil were reduced.

Rhamnolipid production by Pseudomonas aeruginosa under denitrification: effects of limiting nutrients and carbon substrates

Being biosurfactants, rhamnolipids create severe foaming when produced in aerobic Pseudomonas aeruginosa fermentation. The necessary reduction of aeration causes oxygen limitation and restricts cell and product concentrations. In this study, we evaluate the new strategy of rhamnolipid production under denitrification conditions. Because hydrocarbons used in earlier aerobic fermentations were not metabolizable in the absence of oxygen, other potential C substrates were examined, including palmitic acid, stearic acid, oleic acid, linoleic acid, glycerol, vegetable oil, and glucose. All were found able to support cell growth under anaerobic denitrification. The growth on the two solid substrates (palmitic acid and stearic acid) was slower but could be enhanced substantially by initial addition of rhamnolipids (0.06 g/L). The effects of different limiting nutrients (N, P, S, Mg, Ca, and Fe) were also investigated. The commonly used N limitation could not be adopted in the denitrifying fermentation because the nitrate added for anaerobic respiration would also be assimilated for growth. P limitation was most effective, giving four- to fivefold higher specific productivity than the conventional N limitation. S limitation was comparable to N limitation; Mg limitation was much poorer. Ca and Fe were ineffective in limiting cell growth. The new strategy was further evaluated in a P-limited fermentation with palmitic acid as the substrate. The fermentation was first carried out under denitrification and later switched to aerobic condition. The specific productivity under denitrification was found to be about one-third that of the aerobic condition. The denitrification process was, however, free of foaming or respiratory limitation. Much higher cell concentrations may be employed to attain higher volumetric productivity and product concentrations, for more economical product recovery and/or purification.

Degradation of n-hexadecane and its metabolites by Pseudomonas aeruginosa under microaerobic and anaerobic denitrifying conditions

A strategy for sequential hydrocarbon bioremediation is proposed. The initial O(2)-requiring transformation is effected by aerobic resting cells, thus avoiding a high oxygen demand. The oxygenated metabolites can then be degraded even under anaerobic conditions when supplemented with a highly water-soluble alternative electron acceptor, such as nitrate. To develop the new strategy, some phenomena were studied by examining Pseudomonas aeruginosa fermentation. The effects of dissolved oxygen (DO) concentration on n-hexadecane biodegradation were investigated first. Under microaerobic conditions, the denitrification rate decreased as the DO concentration decreased, implying that the O(2)-requiring reactions were rate limiting. The effects of different nitrate and nitrite concentrations were examined next. When cultivated aerobically in tryptic soy broth supplemented with 0 to 0.35 g of NO(2)(-)-N per liter, cells grew in all systems, but the lag phase was longer in the presence of higher nitrite concentrations. However, under anaerobic denitrifying conditions, even 0.1 g of NO(2)(-)-N per liter totally inhibited cell growth. Growth was also inhibited by high nitrate concentrations (>1 g of NO(3)(-)-N per liter). Cells were found to be more sensitive to nitrate or nitrite inhibition under denitrifying conditions than under aerobic conditions. Sequential hexadecane biodegradation by P. aeruginosa was then investigated. The initial fermentation was aerobic for cell growth and hydrocarbon oxidation to oxygenated metabolites, as confirmed by increasing dissolved total organic carbon (TOC) concentrations. The culture was then supplemented with nitrate and purged with nitrogen (N(2)). Nitrate was consumed rapidly initially. The live cell concentration, however, also decreased. The aqueous-phase TOC level decreased by about 40% during the initial active period but remained high after this period. Additional experiments confirmed that only about one-half of the derived TOC was readily consumable under anaerobic denitrifying conditions.