Quantitative analysis of bacterial medium-chain-length poly([R]-3-hydroxyalkanoates) by gas chromatography

Current trends in medium-chain-length polyhydroxyalkanoates: Microbial production, purification, and characterization

Polyhydroxyalkanoates (PHAs) have gained interest recently due to their biodegradability and versatility. In particular, the chemical compositions of medium-chain-length (mcl)-PHAs are highly diverse, comprising different monomers containing 6-14 carbon atoms. This review summarizes different feedstocks and fermentation strategies to enhance mcl-PHA production and briefly discusses the downstream processing. This review also provides comprehensive details on analytical tools for determining the composition and properties of mcl-PHA. Moreover, this study provides novel information by statistically analyzing the data collected from several reports on mcl-PHA to determine the optimal fermentation parameters (specific growth rate, PHA productivity, and PHA yield from various structurally related and unrelated substrates), mcl-PHA composition, molecular weight (MW), and thermal and mechanical properties, in addition to other relevant statistical values. The analysis revealed that the median PHA productivity observed in the fed-batch feeding strategy was 0.4 g L-1 h-1, which is eight times higher than that obtained from batch feeding (0.05 g L-1 h-1). Furthermore, 3-hydroxyoctanoate and -decanoate were the primary monomers incorporated into mcl-PHA. The investigation also determined the median glass transition temperature (-43°C) and melting temperature (47°C), which indicated that mcl-PHA is a flexible amorphous polymer at room temperature with a median MW of 104 kDa. However, information on the monomer composition or heterogeneity and the associated physical and mechanical data of mcl-PHAs is inadequate. Based on their mechanical values, the mcl-PHAs can be classified as semi-crystalline polymers (median crystallinity 23%) with rubber-like properties and a median elongation at break of 385%. However, due to the limited mechanical data available for mcl-PHAs with known monomer composition, identifying suitable processing tools and applications to develop mcl-PHAs further is challenging.

Strict anoxic conditions significantly impact the metabolism of particulate and colloidal organic matter and bio-P compared to aerobic conditions

Fully anoxic suspended growth treatment of domestic wastewater is rarely performed in practice at large scale. However, recent advances in membrane aerated biofilm reactor (MABR) technology can enable the "hybrid" concept that couples nitrification in the MABR with anoxic suspended growth for biological nitrogen removal. Small scale sequencing batch reactors were constructed to compare high-rate anoxic metabolization of influent carbon and biological phosphorus removal side-by-side with a conventional aerated system in a low-strength domestic wastewater (COD/TN ratio of approximately 6). Little differences existed in the oxidation of soluble readily biodegradable organic material between the two systems, but hydrolysis of particulate and colloidal organic matter in the anoxic reactor over a range of solid retention times was 60 % of the aerobic reactor. Reduced hydrolysis limited the amount of carbon available to ferment to volatile fatty acid (VFA), adversely impacting anoxic biological phosphorus removal (bio-P) process rates, and ortho-P removal performance was diminished by more than half at equivalent SRTs. At optimal growth conditions, i.e., an SRT of approximately 8 days and with supplementary VFA, ortho-P removal from the influent averaged roughly 75 %. Experimentation with supplemented acetic acid showed reduced anoxic metabolic efficiency, quantified via a P/O ratio of 0.90 versus 1.7 for the aerobic system, although overall anoxic bio-P removal demonstrably increased with external carbon.

Marine biodegradation of tailor-made polyhydroxyalkanoates (PHA) influenced by the chemical structure and associated bacterial communities

Over recent years, biodegradable polymers have been proposed to reduce environmental impacts of plastics for specific applications. The production of polyhydroxyalkanoates (PHA) by using diverse carbon sources provides further benefits for the sustainable development of biodegradable plastics. Here, we present the first study evaluating the impact of physical, chemical and biological factors driving the biodegradability of various tailor-made PHAs in the marine environment. Our multidisciplinary approach demonstrated that the chemical structure of the polymer (i.e. the side chain size for short- vs. medium-chain PHA) which was intrinsically correlated to the physico-chemical properties, together with the specificity of the biofilm growing on plastic films (i.e., the associated 'plastisphere') were the main drivers of the PHA biodegradation in the marine environment.

Bioconversion of glycerol into polyhydroxyalkanoates through an atypical metabolism shift using Priestia megaterium during fermentation processes: A statistical analysis of carbon and nitrogen source concentrations

Polyhydroxyalkanoates (PHA) are bioplastics which are well known as intracellular energy storage compounds and are produced in a large number of prokaryotic species. These bio-based inclusions are biodegradable, biocompatible and environmental friendly. Industrial production of, short chain and medium chain length PHA, involves the use of microorganisms and their enzymes. Priestia megaterium previously known as Bacillus megaterium is a well-recognized bacterium for producing short chain length PHA. This study focuses to characterize this bacterium for the production of medium chain length PHA, and a novel blend of both types of monomers having enhanced properties and versatile applications. Statistical analyses and simulations were used to demonstrate that cell dry weight can be derived as a function of OD600 and PHA content. Optimization of growth conditions resulted in the maximum PHA production as: 0. 05 g. g-x. H-1, where the rate of PHA production was 0.28 g L-1. H-1 and PHA concentration was 4.94 g. L-1. This study also demonstrated FTIR to be a semi quantitative tool for PHA production. Moreover, conversion of scl-PHA to mcl-PHA with reference to time intermissions using GC-FID are shown.

Constant fed-batch cultivation with glucose and propionate as co-substrate: A strategy to fine-tune polyhydroxyalkanoates monomeric composition in Pseudomonas spp

Pseudomonas sp. LFM693 is a 2-methylisocitrate lyase (prpB) disrupted mutant. This enzyme catalyzes a step in the 2-methylcitrate cycle, the only known and described pathway for propionate oxidation in this organism. The affected mutants can efficiently produce PHA containing even and odd-chain length hydroxyalkanoates (HAeven/odd) in the presence of propionate and glucose. In this study, a constant fed-batch configuration was utilized to control the composition of PHA and decrease the toxicity of propionate. The incorporation of HAodd into the copolymer was linear, ranging from 7 to approximately 30 %, and correlated directly with the propionate/glucose molar ratio in the feeding solution. This allowed for the molecular composition of the mclPHA to be fine-tuned with minimum process monitoring and control. The average PHA content was 52 % cell dry weight with a molar composition that favored 3-hydroxyalkanoates containing C8, C9, and C10. The conversion factor of propionate to HAodd varied between 0.36 and 0.53 mol·mol-1 (YHAodd/prop.), which are significantly lower than the theoretical maximum efficiency (1.0 mol·mol-1). These results along with the lack of 2-methylisocitrate as a byproduct provides further support for the evidence that the mutant prpB- is still capable of oxidizing propionate.

The earth-star basidiomycetous mushroom Astraeus odoratus produces polyhydroxyalkanoates during cultivation on malt extract

Polyhydroxyalkanoates (PHAs) including poly-3-hydroxybutyrate (P3HB) as secondary metabolisms were in vitro produced by the edible basidiomycetous mushroom Astraeus odoratus during its growth on malt agar extract. Various carbon and nitrogen sources containing cellulose, glucose, glycerol, rice straw powder, soybean meal and peptone were investigated for the growth of basidiomycetous mushrooms. During cultivation, the A. odoratus culture exudated the considerably extracellular fluid up to approx. 2.3 ml on 2% malt extract agar plate within 7 days. The chemical compounds of the exudated fluid were further investigated by Fourier-transform infrared spectroscopy (FTIR) and gas chromatography/mass spectrometry (GC/MS); and its morphology of the lyophilized sample was observed by scanning electron microscope (SEM). FTIR results showed the characteristic bands of OH at 3445 cm^-1, CH/CH₂/symmetric CH₃ (stretch) at 2923 and 2852 cm^-1, C=O at 1730 cm^-1, asymmetric CH₃ (bend) at 1454 and 1414 cm^-1, C-O of COO- at 1396 cm^-1 and C-O-C at 1223, 1160, 1116, 1058 and 1019 cm^-1 which were similar to the absorptive characteristics of P3HB. Methyl ester derivatives of GC/MS results identified 7 compounds including: 3-hydroxybutanoic (monomer of PHB), aminobenzoic, salicylic, hexadecenoic, octadecadienoic, octadecenoic and octadecanoic acids. SEM images revealed a fibriform and porous materials. Hence, the occurrence of PHAs was first described in a basidiomycetous mushroom A. odoratus. Thus, PHAs could be found not only in bacteria and but also in basidiomycetous mushroom, which can be promising target for bioplastics and green environmental studies.

The optimal combination of Nile red identification, colony polymerase chain reaction, and gas chromatography detection methods in screening for polyhydroxyalkanoicate-producing bacteria

The study devised a detection process combining Nile red-containing media, polymerase chain reaction (PCR), and gas chromatography (GC) to evaluate the possibility of microbes becoming polyhydroxyalkanoate (PHA) producers. The Nile red and PCR detection steps of designating PHA producers had true positive rates of 39.4% and 40%, respectively, and the use of GC analysis as the final step yielded accurate results for the production types and yields of PHAs. When the number of screening samples was up to 10², connecting all three inspection methods in tandem generated economic benefits. When up to 10⁵ samples were screened, the use of all three detection methods reduced the cost to 3% of the cost and the time consumed 6% of using just Nile red plus GC or PCR plus GC. However, when the sum of samples exceeded 10⁸, the cost of combining the three methods exceeds 1 million US dollars and was excessive; here, the combination of Nile red plus PCR could be considered, even though the true positive rate was only 30.7%.

Polyhydroxyalkanoates (PHAs) as Biomaterials in Tissue Engineering: Production, Isolation, Characterization

Polyhydroxyalkanoates (PHAs) are biodegradable and biocompatible biopolymers. These biomaterials have grown in importance in the fields of tissue engineering and tissue reconstruction for structural applications where tissue morphology is critical, such as bone, cartilage, blood vessels, and skin, among others. Furthermore, they can be used to accelerate the regeneration in combination with drugs, as drug delivery systems, thus reducing microbial infections. When cells are cultured under stress conditions, a wide variety of microorganisms produce them as a store of intracellular energy in the form of homo- and copolymers of [R]—hydroxyalkanoic acids, depending on the carbon source used for microorganism growth. This paper gives an overview of PHAs, their biosynthetic pathways, producing microorganisms, cultivation bioprocess, isolation, purification and characterization to obtain biomaterials with medical applications such as tissue engineering.

New model development for qualitative and quantitative analysis of microbial polyhydroxyalkanoates: A comparison of Fourier Transform Infrared Spectroscopy with Gas Chromatography

FT-IR spectroscopy is a non-destructive testing technique that requires minimal sample preparation time and allows the rapid characterization of structural features of complex, polymeric material. This technique has been frequently used in the past for the detection of PHA (Polyhydroxyalkanoates) but rarely for their quantification. In this work, by using cluster and discriminant statistical analysis of FT-IR data, different models are proposed for rapid identification of PHA monomers produced under different growth conditions by bacterial strains, and for their semi quantification. The results on the ability to produce large amounts of PHA (of 21 strains) in different environmental conditions of medium, substrates and deficiency of nutrients are presented. The spectral data analysis gives qualitative and semi quantitative information about the PHA produced in the samples. Models are proposed to test a large number of cultural conditions of strains and substrates in the field of screening and for identifying best conditions of PHA production in lab scale bioreactor and on industrial scale.

Polyhydroxyalkanoates: Next generation natural biomolecules and a solution for the world's future economy

Petrochemical plastics have become a cause of pollution for decades and finding alternative plastics that are environmental friendly. Polyhydroxyalkanoate (PHA), a biopolyester produced by microbial cells, has characteristics (biocompatible, biodegradable, non-toxic) that make it appropriate as a biodegradable plastic substance. The different forms of PHA make it suitable to a wide choice of products, from packaging materials to biomedical applications. The major challenge in commercialization of PHA is the cost of manufacturing. There are a lot of factors that could affect the efficiency of a development method. The development of new strategic parameters for better synthesis, including consumption of low cost carbon substrates, genetic modification of PHA-producing strains, and fermentational strategies are discussed. Recently, many efforts have been made to develop a method for the cost-effective production of PHAs. The isolation, analysis as well as characterization of PHAs are significant factors for any developmental process. Due to the biodegradable and biocompatible properties of PHAs, they are majorly used in biomedical applications such as vascular grafting, heart tissue engineering, skin tissue repairing, liver tissue engineering, nerve tissue engineering, bone tissue engineering, cartilage tissue engineering and therapeutic carrier. The emerging and interesting area of research is the development of self-healing biopolymer that could significantly broaden the operational life and protection of the polymeric materials for a broad range of uses. Biodegradable and biocompatible polymers are considered as the green materials in place of petroleum-based plastics in the future.

Influence of Unusual Co-substrates on the Biosynthesis of Medium-Chain-Length Polyhydroxyalkanoates Produced in Multistage Chemostat

A two-stage chemostat cultivation was used to investigate the biosynthesis of functionalized medium-chain-length polyhydroxyalkanoate (mcl-PHA) in the β-oxidation weakened strain of Pseudomonas putida KTQQ20. Chemostats were linked in sequence and allowed separation of biomass production in the first stage from the PHA synthesis in the second stage. Four parallel reactors in the second stage provided identical growth conditions and ensured that the only variable was the ratio of decanoic acid (C10) to an unusual PHA monomer precursor, such as 10-undecenoic acid (C11:1) or phenylvaleric acid (PhVA). Obtained PHA content was in the range of 10 to 25 wt%. When different ratios of C10 and C11:1 were fed to P. putida, the produced PHA had a slightly higher molar ratio in favor of C11:1-based 3-hydroxy-10-undecenoate. However, in case of PhVA a significantly lower incorporation of 3-hydroxy-5-phenylvalerate over 3-hydroxydecanoate took place when compared to the ratio of their precursors in the feed medium. A result that is explained by a less efficient uptake of PhVA compared to C10 and a 24% lower yield of polymer from the aromatic fatty acid ( y P H A - M P h V A = 0.25). In addition, PHA isolated from cultivations with PhVA resulted in the number average molecular weight M n ¯ two times lower than the PHA produced from C10 alone. Detection of products from PhVA metabolism in the culture supernatant showed that uptaken PhVA was not entirely converted into PHA, thus explaining the difference in the yield polymer from substrate. It was concluded that PhVA or its related metabolites increased the chain transfer rate during PHA biosynthesis in P. putida KTQQ20, resulting in a reduction of the polymer molecular weight.

Exploring nutrient limitation for polyhydroxyalkanoates synthesis by newly isolated strains of Aeromonas sp. using biodiesel-derived glycerol as a substrate

Aeromonas spp. strains isolated from activated sludge in a municipal wastewater treatment plant were found to be able to synthesize polyhydroxyalkanoates (PHA) utilizing pure and crude glycerol. The 16S rRNA gene sequence of the isolates exhibited similarity to Aeromonas hydrophila, A. aquatica, and A. salmonicida. Our results confirmed that the adequate supply of nitrogen and phosphorus during culture in 250-ml shake flasks did not stimulate the synthesis of PHAs. The results indicate that the PHA content of cells was higher under a phosphorus-limiting environment compared to nitrogen starvation. In the two-stage cultivation using glucose (in the first step) and crude glycerol from biodiesel industry (in the second step) as a component of the growth medium, the analyzed strains grew to 3.06 g/l of cell dry weight containing up to 22% of PHAs. Furthermore, during the same culture strategy up to 42% of PHAs were extracted, when in the second step of the process, Aeromonas sp. AC_03 was grown on pure glycerol under phosphorus limitation. The purified biopolymer was confirmed to be polyhydroxybutyrate. Aeromonas sp. AC_02 was also capable to accumulate the poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer when pure glycerol was added as a substrate under nitrogen-deficiency one-step bioprocess. Our results confirm that due to the biopolymer productivity, newly isolated strains could be exploited for obtaining valuable biopolymers using wastes generated from biodiesel industry.

Improved intracellular PHA determinations with novel spectrophotometric quantification methodologies based on Sudan black dye

The presence of intracellular polyhydroxyalkanoates (PHAs) is usually studied using Sudan black dye solution (SB). In a previous work it was shown that the PHA could be directly quantified using the absorbance of SB fixed by PHA granules in wet cell samples. In the present paper, the optimum SB amount and the optimum conditions to be used for SB assays were determined following an experimental design by hybrid response surface methodology and desirability-function. In addition, a new methodology was developed in which it is shown that the amount of SB fixed by PHA granules can also be determined indirectly through the absorbance of the supernatant obtained from the stained cell samples. This alternative methodology allows a faster determination of the PHA content (involving 23 and 42 min for indirect and direct determinations, respectively), and can be undertaken by means of basic laboratory equipment and reagents. The correlation between PHA content in wet cell samples and the spectra of the SB stained supernatant was determined by means of multivariate and linear regression analysis. The best calibration adjustment (R² = 0.91, RSE: 1.56%), and the good PHA prediction obtained (RSE = 1.81%), shows that the proposed methodology constitutes a reasonably precise way for PHA content determination. Thus, this methodology could anticipate the probable results of the above mentioned direct PHA determination. Compared with the most used techniques described in the scientific literature, the combined implementation of these two methodologies seems to be one of the most economical and environmentally friendly, suitable for rapid monitoring of the intracellular PHA content.

Production of a novel medium chain length poly(3-hydroxyalkanoate) using unprocessed biodiesel waste and its evaluation as a tissue engineering scaffold

This study demonstrated the utilization of unprocessed biodiesel waste as a carbon feedstock for Pseudomonas mendocinaCH50, for the production of PHAs. A PHA yield of 39.5% CDM was obtained using 5% (v/v) biodiesel waste substrate. Chemical analysis confirmed that the polymer produced was poly(3‐hydroxyhexanoate‐co‐3‐hydroxyoctanoate‐co‐3‐hydroxydecanoate‐co‐3‐hydroxydodecanoate) or P(3HHx‐3HO‐3HD‐3HDD). P(3HHx‐3HO‐3HD‐3HDD) was further characterized and evaluated for its use as a tissue engineering scaffold (TES). This study demonstrated that P(3HHx‐3HO‐3HD‐3HDD) was biocompatible with the C2C12 (myoblast) cell line. In fact, the % cell proliferation of C2C12 on the P(3HHx‐3HO‐3HD‐3HDD) scaffold was 72% higher than the standard tissue culture plastic confirming that this novel PHA was indeed a promising new material for soft tissue engineering.

The Bistable Behaviour of Pseudomonas putida KT2440 during PHA Depolymerization under Carbon Limitation

Poly(hydroxyalkanoates) (PHAs) are bacterial polyesters offering a biodegradable alternative to petrochemical plastics. The intracellular formation and degradation of PHAs is a dynamic process that strongly depends on the availability of carbon and other nutrients. Carbon excess and nitrogen limitation are considered to favor PHA accumulation, whereas carbon limitation triggers PHA depolymerization when all other essential nutrients are present in excess. We studied the population dynamics of Pseudomonas putida KT2440 at the single cell level during different physiological conditions, favoring first PHA polymerization during growth on octanoic acid, and then PHA depolymerization during carbon limitation. PHAs accumulate intracellularly in granules, and were proposed to separate preferentially together with nucleic acids, leading to two daughter cells containing approximately equal amounts of PHA. However, we could show that such P. putida KT2440 cells show bistable behavior when exposed to carbon limitation, and separate into two subpopulations: one with high and one with low PHA. This suggests an asymmetric PHA distribution during cell division under carbon limitation, which has a significant influence on our understanding of PHA mobilization.

Tailor-made PAT platform for safe syngas fermentations in batch, fed-batch and chemostat mode with Rhodospirillum rubrum

Recently, syngas has gained significant interest as renewable and sustainable feedstock, in particular for the biotechnological production of poly([R]‐3‐hydroxybutyrate) (PHB). PHB is a biodegradable, biocompatible polyester produced by some bacteria growing on the principal component of syngas, CO. However, working with syngas is challenging because of the CO toxicity and the explosion danger of H₂, another main component of syngas. In addition, the bioprocess control needs specific monitoring tools and analytical methods that differ from standard fermentations. Here, we present a syngas fermentation platform with a focus on safety installations and process analytical technology (PAT) that serves as a basis to assess the physiology of the PHB‐producing bacterium Rhodospirillum rubrum. The platform includes (i) off‐gas analysis with an online quadrupole mass spectrometer to measure CO consumption and production rates of H₂ and CO₂, (ii) an at‐line flow cytometer to determine the total cell count and the intracellular PHB content and (iii) different online sensors, notably a redox sensor that is important to confirm that the culture conditions are suitable for the CO metabolization of R. rubrum. Furthermore, we present as first applications of the platform a fed‐batch and a chemostat process with R. rubrum for PHB production from syngas.

Photoautotrophic production of polyhydroxyalkanoates in a synthetic mixed culture of Synechococcus elongatus cscB and Pseudomonas putida cscAB

BACKGROUND

One of the major challenges for the present and future generations is to find suitable substitutes for the fossil resources we rely on today. Cyanobacterial carbohydrates have been discussed as an emerging renewable feedstock in industrial biotechnology for the production of fuels and chemicals, showing promising production rates when compared to crop-based feedstock. However, intrinsic capacities of cyanobacteria to produce biotechnological compounds are limited and yields are low.

RESULTS

Here, we present an approach to circumvent these problems by employing a synthetic bacterial co-culture for the carbon-neutral production of polyhydroxyalkanoates (PHAs) from CO₂. The co-culture consists of two bio-modules: Bio-module I, in which the cyanobacterial strain Synechococcus elongatus cscB fixes CO₂, converts it to sucrose, and exports it into the culture supernatant; and bio-module II, where this sugar serves as C-source for Pseudomonas putida cscAB and is converted to PHAs that are accumulated in the cytoplasm. By applying a nitrogen-limited process, we achieved a maximal PHA production rate of 23.8 mg/(L day) and a maximal titer of 156 mg/L. We will discuss the present shortcomings of the process and show the potential for future improvement.

CONCLUSIONS

These results demonstrate the feasibility of mixed cultures of S. elongatus cscB and P. putida cscAB for PHA production, making room for the cornucopia of possible products that are described for P. putida. The construction of more efficient sucrose-utilizing P. putida phenotypes and the optimization of process conditions will increase yields and productivities and eventually close the gap in the contemporary process. In the long term, the co-culture may serve as a platform process, in which P. putida is used as a chassis for the implementation of synthetic metabolic pathways for biotechnological production of value-added products.

Robust at-line quantification of poly(3-hydroxyalkanoate) biosynthesis by flow cytometry using a BODIPY 493/503-SYTO 62 double-staining

Poly(3-hydroxyalkanoates) (PHAs) are bio-based and biodegradable polyesters which have been considered as a promising alternative to petrol-based plastics. Their bacterial production is a dynamic process in which intracellular polymerization and depolymerization are closely linked and depend on the availability of carbon substrates and other nutrients. These dynamics require a fast and quantitative method to determine the optimal harvest-time of PHA containing cells or to adjust carbon supply. In principle, flow cytometry (FCM) is an ideal tool that suits these requirements and, in addition, provides data on the PHA content of different cell populations. However, FCM-based PHA quantification methods have often relied on laborious sample preparation including washing steps and long incubation times. Here, we introduce a fast method based on double-staining using BODIPY 493/503 for PHA staining and SYTO 62 for DNA that allows acquiring reliable fluorescence and cell count data in <10min. Finally, fed-batch experiments with Pseudomonas putida KT2440 and Rhodospirillum rubrum S1 revealed that the method was robust and independent of the strain and type of PHA (medium-chain-length [mcl-] and short-chain-length [scl-] PHA, respectively). Interestingly, the specific PHA fluorescence was in case of mcl-PHA larger than for scl-PHA, probably reflecting the different material properties (e.g., specific density, hydrophilicity and crystallinity).

Interactions of biodegradable poly([R]-3-hydroxy-10-undecenoate) with 1,2-dioleoyl-sn-glycero-3-phosphocholine lipid: a monolayer study

Polyhydroxyalkanoates (PHAs) are biodegradable, biocompatible polyesters and very attractive candidates for biomedical applications as materials for tissue engineering. They have a hydrophobic character, but some are able to spread at the air-water interface to form monomolecularly thin films (Langmuir monolayers). This is a very convenient model to analyze PHA self-assembly in two dimensions and to study their molecular interactions with other amphiphilic compounds, which is very important considering compatibility between biomaterials and cell membranes. We used the Langmuir monolayer technique and Brewster angle microscopy to study the properties of poly([R]-3-hydroxy-10-undecenoate) (PHUE) films on the free water surface in various experimental conditions. Moreover, we investigated the interactions between the polymer and one of the main biomembrane components, 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). The addition of lipid to a polymer film does not change the monolayer phase behavior; however, the interactions between these two materials are repulsive and fall in two composition-dependent regimes. In summary, this is the first systematic study of the monolayer behavior of PHUE, thus forming a solid basis for a thorough understanding of material interactions, in particular in the context of biomaterials and implants.

A reduction in growth rate of Pseudomonas putida KT2442 counteracts productivity advances in medium-chain-length polyhydroxyalkanoate production from gluconate

Background

The substitution of plastics based on fossil raw material by biodegradable plastics produced from renewable resources is of crucial importance in a context of oil scarcity and overflowing plastic landfills. One of the most promising organisms for the manufacturing of medium-chain-length polyhydroxyalkanoates (mcl-PHA) is Pseudomonas putida KT2440 which can accumulate large amounts of polymer from cheap substrates such as glucose. Current research focuses on enhancing the strain production capacity and synthesizing polymers with novel material properties. Many of the corresponding protocols for strain engineering rely on the rifampicin-resistant variant, P. putida KT2442. However, it remains unclear whether these two strains can be treated as equivalent in terms of mcl-PHA production, as the underlying antibiotic resistance mechanism involves a modification in the RNA polymerase and thus has ample potential for interfering with global transcription.

Results

To assess PHA production in P. putida KT2440 and KT2442, we characterized the growth and PHA accumulation on three categories of substrate: PHA-related (octanoate), PHA-unrelated (gluconate) and poor PHA substrate (citrate). The strains showed clear differences of growth rate on gluconate and citrate (reduction for KT2442 > 3-fold and > 1.5-fold, respectively) but not on octanoate. In addition, P. putida KT2442 PHA-free biomass significantly decreased after nitrogen depletion on gluconate. In an attempt to narrow down the range of possible reasons for this different behavior, the uptake of gluconate and extracellular release of the oxidized product 2-ketogluconate were measured. The results suggested that the reason has to be an inefficient transport or metabolization of 2-ketogluconate while an alteration of gluconate uptake and conversion to 2-ketogluconate could be excluded.

Conclusions

The study illustrates that the recruitment of a pleiotropic mutation, whose effects might reach deep into physiological regulation, effectively makes P. putida KT2440 and KT2442 two different strains in terms of mcl-PHA production. The differences include the onset of mcl-PHA production (nitrogen limitation) and the resulting strain performance (growth rate). It remains difficult to predict a prioriwhere such major changes might occur, as illustrated by the comparable behavior on octanoate. Consequently, experimental data on mcl-PHA production acquired for P. putida KT2442 cannot always be extrapolated to KT2440 and vice versa, which potentially reduces the body of available knowledge for each of these two model strains for mcl-PHA production substantially.

Simultaneous accumulation and degradation of polyhydroxyalkanoates: futile cycle or clever regulation?

The regulation of medium-chain-length polyhydroxyalkanoates (mcl-PHA) metabolism in Pseudomonas putida GPo1 was studied by analysis of enzymes bound to PHA granules and enzymes involved in fatty acid oxidation. N-terminal sequencing of granule-bound enzymes revealed the presence of PHA polymerase (PhaC) and PHA depolymerase (PhaZ) and an acyl-CoA synthetase (ACS1), which recently was found to be associated with PHA granules by in vivo study. The acs1 knockout mutant accumulated 30-50% less PHA than its parental strain, confirming the involvement of ACS1 in PHA metabolism. Isolated PHA granules showed both PhaC and PhaZ activities. PhaC activity was found to be sensitive to the ratio of [R-3-hydroxyacyl-CoA]/[CoA] in which free CoA was a mild competitive inhibitor. Fatty acid oxidation was regulated by the [acetyl-CoA]/[CoA] and [NADH]/[NAD] ratios, with high ratios resulting in accumulation and low ratios leading to rapid oxidation of 3-hydroxyacyl-CoA. These results suggest that PHA metabolism is likely to be controlled by the [acetyl-CoA]/[CoA] and [NADH]/[NAD] ratios. The physiological roles of simultaneous PHA accumulation and degradation are also discussed.

Use of extracellular medium chain length polyhydroxyalkanoate depolymerase for targeted binding of proteins to artificial poly[(3-hydroxyoctanoate)-co-(3-hydroxyhexanoate)] granules

Polyhydroxyalkanoates (PHA), which are produced by many microorganisms, are promising polymers for biomedical applications due to their biodegradability and biocompatibility. In this study, we evaluated the suitability of medium chain length (mcl) PHA as surface materials for immobilizing proteins. Self-stabilized, artificial mcl-PHA beads with a size of 200-300 nm were fabricated. Five of six tested proteins adsorbed nonspecifically to mcl-PHA beads in amounts of 0.4-1.8 mg m(-2) bead surface area. The binding capacity was comparable to similar-sized polystyrene particles commonly used for antibody immobilization in clinical diagnostics. A targeted immobilization of fusion proteins was achieved by using inactive extracellular PHA depolymerase (ePHA(mcl)) from Pseudomonas fluorescens as the capture ligand. The N-terminal part of ePhaZ(MCL) preceding the catalytic domain was identified to comprise the substrate binding domain and was sufficient for mediating the binding of fusion proteins to mcl-PHA. We suggest mcl-PHA to be prime candidates for both nonspecific and targeted immobilization of proteins in applications such as drug delivery, protein microarrays, and protein purification.