Identification of novel sulfur-containing bacterial polyesters: biosynthesis of poly(3-hydroxy-S-propyl-omega-thioalkanoates) containing thioether linkages in the side chains

From Residues to Added-Value Bacterial Biopolymers as Nanomaterials for Biomedical Applications

Bacterial biopolymers are naturally occurring materials comprising a wide range of molecules with diverse chemical structures that can be produced from renewable sources following the principles of the circular economy. Over the last decades, they have gained substantial interest in the biomedical field as drug nanocarriers, implantable material coatings, and tissue-regeneration scaffolds or membranes due to their inherent biocompatibility, biodegradability into nonhazardous disintegration products, and their mechanical properties, which are similar to those of human tissues. The present review focuses upon three technologically advanced bacterial biopolymers, namely, bacterial cellulose (BC), polyhydroxyalkanoates (PHA), and γ-polyglutamic acid (PGA), as models of different carbon-backbone structures (polysaccharides, polyesters, and polyamides) produced by bacteria that are suitable for biomedical applications in nanoscale systems. This selection models evidence of the wide versatility of microorganisms to generate biopolymers by diverse metabolic strategies. We highlight the suitability for applied sustainable bioprocesses for the production of BC, PHA, and PGA based on renewable carbon sources and the singularity of each process driven by bacterial machinery. The inherent properties of each polymer can be fine-tuned by means of chemical and biotechnological approaches, such as metabolic engineering and peptide functionalization, to further expand their structural diversity and their applicability as nanomaterials in biomedicine.

Engineering Native and Synthetic Pathways in Pseudomonas putida for the Production of Tailored Polyhydroxyalkanoates

Growing environmental concern sparks renewed interest in the sustainable production of (bio)materials that can replace oil-derived goods. Polyhydroxyalkanoates (PHAs) are isotactic polymers that play a critical role in the central metabolism of producer bacteria, as they act as dynamic reservoirs of carbon and reducing equivalents. PHAs continue to attract industrial attention as a starting point toward renewable, biodegradable, biocompatible, and versatile thermoplastic and elastomeric materials. Pseudomonas species have been known for long as efficient biopolymer producers, especially for medium-chain-length PHAs. The surge of synthetic biology and metabolic engineering approaches in recent years offers the possibility of exploiting the untapped potential of Pseudomonas cell factories for the production of tailored PHAs. In this article, an overview of the metabolic and regulatory circuits that rule PHA accumulation in Pseudomonas putida is provided, and approaches leading to the biosynthesis of novel polymers (e.g., PHAs including nonbiological chemical elements in their structures) are discussed. The potential of novel PHAs to disrupt existing and future market segments is closer to realization than ever before. The review is concluded by pinpointing challenges that currently hinder the wide adoption of bio-based PHAs, and strategies toward programmable polymer biosynthesis from alternative substrates in engineered P. putida strains are proposed.

Biodegradable Polymeric Micro/Nano-Structures with Intrinsic Antifouling/Antimicrobial Properties: Relevance in Damaged Skin and Other Biomedical Applications

Bacterial colonization of implanted biomedical devices is the main cause of healthcare-associated infections, estimated to be 8.8 million per year in Europe. Many infections originate from damaged skin, which lets microorganisms exploit injuries and surgical accesses as passageways to reach the implant site and inner organs. Therefore, an effective treatment of skin damage is highly desirable for the success of many biomaterial-related surgical procedures. Due to gained resistance to antibiotics, new antibacterial treatments are becoming vital to control nosocomial infections arising as surgical and post-surgical complications. Surface coatings can avoid biofouling and bacterial colonization thanks to biomaterial inherent properties (e.g., super hydrophobicity), specifically without using drugs, which may cause bacterial resistance. The focus of this review is to highlight the emerging role of degradable polymeric micro- and nano-structures that show intrinsic antifouling and antimicrobial properties, with a special outlook towards biomedical applications dealing with skin and skin damage. The intrinsic properties owned by the biomaterials encompass three main categories: (1) physical–mechanical, (2) chemical, and (3) electrostatic. Clinical relevance in ear prostheses and breast implants is reported. Collecting and discussing the updated outcomes in this field would help the development of better performing biomaterial-based antimicrobial strategies, which are useful to prevent infections.

Synthetic metabolism: engineering biology at the protein and pathway scales

Biocatalysis has become a powerful tool for the synthesis of high-value compounds, particularly so in the case of highly functionalized and/or stereoactive products. Nature has supplied thousands of enzymes and assembled them into numerous metabolic pathways. Although these native pathways can be use to produce natural bioproducts, there are many valuable and useful compounds that have no known natural biochemical route. Consequently, there is a need for both unnatural metabolic pathways and novel enzymatic activities upon which these pathways can be built. Here, we review the theoretical and experimental strategies for engineering synthetic metabolic pathways at the protein and pathway scales, and highlight the challenges that this subfield of synthetic biology currently faces.

Mechanical Properties of Comonomer-Compositionally Fractionated Poly[(3-hydroxybutyrate)-co-(3-mercaptopropionate)] with Low 3-Mercaptopropionate Unit Content

The mechanical properties, such as Young's modulus, yield strength, and the elongation at breakage, were investigated for several sulfur-containing biopolymers P(3HB-co-3MP). A series of P(3HB-co-3MP) samples with 3MP unit content ranging from 6.6 to 39.1 mol-% was biosynthesized by fermentation using the PHA-synthesizing bacteria Cupriavidus necator. For comparison, the bacterially synthesized P(3HB) and P(3HB-co-3HP) with the 3HP unit content ranging from 13.1 to 21.1 mol-% were also investigated. It was found that the sulfur-containing P(3HB-co-3MP) is much more durable to stretching. Notably, P(3HB-co-3MP) with the 3MP unit content of only 6.6 mol-% was found to show excellent mechanical properties.

Metabolic Engineering for Microbial Production and Applications of Copolyesters Consisting of 3-Hydroxybutyrate and Medium-Chain-Length 3-Hydroxyalkanoates

Poly(hydroxyalkanoate)s (PHAs) are a class of microbially synthesized polyesters that combine biological properties, such as biocompatibility and biodegradability, and non-bioproperties such as thermoprocessability, piezoelectricity, and nonlinear optical activity. PHA monomer structures and their contents strongly affect the PHA properties. Using metabolic engineering approaches, PHA structures and contents can be manipulated to achieve controllable monomer and PHA cellular contents. This paper focuses on metabolic engineering methods to produce PHA consisting of 3-hydroxybutyrate (3HB) and medium-chain-length 3-hydroxyalkanoates (3HA) in recombinant microbial systems. This type of copolyester has mechanical and thermal properties similar to conventional plastics such as poly(propylene) and poly(ethylene terephthalate) (PET). In addition, pathways containing engineered PHA synthases have proven to be useful for enhanced PHA production with adjustable PHA monomers and contents. The applications of PHA as implant biomaterials are briefly discussed here. In the very near term, metabolic engineering will help solve many problems in promoting PHA as a new type of plastic material for many applications.

Chemo-enzymatic preparation of copolymeric polythioesters containing branched-chain thioether groups

Branched-chain copolymeric polythioesters (PTE) were formed in good yield (approximately 87%) by chemoenzymatic reactions including thiyl radical-induced addition of 1,6-hexanedithiol to the >C=C< double bond of dimethyl 1,18-octadec-9-enedioate and transthioesterification of polyfunctional dimethyl 1,18-octadec-9-enedioate with bifunctional 1,6-hexanedithiol catalyzed by immobilized lipase from Rhizomucor miehei. The reactions were performed in vacuo at 80 degrees C without a solvent. PTE was extracted from the reaction mixture using methyl-t-butylether and precipitated from i-hexane. The polymer structure of the i-hexane-insoluble PTE precipitate was elucidated by GPC/SEC showing an average molecular mass (Mw) of 1,857 Da corresponding to a molecular weight range of up to 24,000 Da and a maximum degree of polymerization of up to 50 monomer units. Chemical derivatization with TMSH demonstrated the formation of up to approximately 58 mol% of a branched-chain thio(S)ether, i.e., dimethyl S-9-(6-mercaptohexylthio)-1,18-octadecanedioate, and small proportions (approximately 8 mol%) of a dimeric disulfide formed therefrom. The chemical structures of various low-molecular weight (<900 Da) reaction products formed by transthioesterification, addition reaction or disulfide formation of the reactants or reaction intermediates, e.g., 1,18-octadec-9-enedioic acid methyl(O)ester 6'-S-mercaptohexyl thio(S)ester, dimethyl S-9-(6-mercaptohexylthio)-1,18-octadecanedioate, were elucidated by GC-MS. Similarly, dimethyl S-9-(6-S-methylthiohexylthio)-1,18-octadecanedioate and dimethyl 11,18,19,26-tetrathia-10,27-di-(7-carboxymethyl-heptyl)hexatriacontane-1,36-dioate were detected in the reaction mixtures after derivatization with trimethylsulfonium hydroxide.

Poly(3-mercaptopropionate): a nonbiodegradable biopolymer?

Polythioesters (PTEs) represent a novel class of biopolymers, which basically can be synthesized with polyhydroxyalkanoate (PHA) biosynthesis systems. Albeit technical applications of PTEs have not been elucidated yet, biodegradability might be an important property of this new thermoplastic material. In this study, extensive approaches were employed to isolate microorganisms capable of degrading poly(3-mercaptopropionate), poly(3MP), as a model compound of PTEs. Screening of 74 different environmental samples using various enrichment techniques were applied, but neither bacteria nor fungi could be isolated hydrolyzing poly(3MP). Furthermore, microcosms such as soil, compost, or activated sludge were applied to search for poly(3MP) degrading microorganisms, considering microbial communities and/or nonculturable bacteria, and the poly(3MP) material was exposed for more than half a year. However, no poly(3MP) degrading organisms were found, indicating an unexpected persistence of this biologically produced polymer.

Directed evolution of copy number of a broad host range plasmid for metabolic engineering

Random mutagenesis and directed evolution has been successfully used to improve desired properties of enzymes for biocatalysis and metabolic engineering. Here we employ the method to increase copy number of a pBBR-based broad host range plasmid, which can be used to express desired enzymes in a variety of microbial hosts. Localized random mutagenesis was performed in the replication control region of a pBBR-derived plasmid containing a beta-carotene reporter. Mutant plasmids were isolated that showed increased beta-carotene production. Real-time PCR analysis confirmed that the copy number of the mutant plasmids increased 3-7 fold. Sequence of the 10 mutant plasmids indicated that each plasmid contained single or multiple mutations in the rep gene or the flanking regions. Single amino acid change of serine to leucine at codon 100 of the replication protein and single nucleotide change of C to T at 46 bp upstream of the rep gene caused the increase of plasmid copy number. The utility of the mutant plasmids for metabolic engineering were further demonstrated by increased beta-carotene production, when an isoprenoid pathway gene (dxs) was co-expressed on a compatible plasmid. The mutant plasmids were tested in Agrobacterium tumefaciens. Increase of plasmid copy number and beta-carotene production was also observed in the non-Escherichia coli host.

Process design for microbial plastic factories: metabolic engineering of polyhydroxyalkanoates

Implementing several metabolic engineering strategies, either individually or in combination, it is possible to construct microbial plastic factories to produce a variety of polyhydroxyalkanoate (PHA) biopolymers with desirable structures and material properties. Approaches include external substrate manipulation, inhibitor addition, recombinant gene expression, host cell genome manipulation and, most recently, protein engineering of PHA biosynthetic enzymes. In addition, mathematical models and molecular methods can be used to elucidate metabolically engineered systems and to identify targets for performance improvement.

Novel precursor substrates for polythioesters (PTE) and limits of PTE biosynthesis in Ralstonia eutropha

A novel class of biopolymers referred to as polythioesters (PTE) was recently detected when the polyhydroxyalkanoate (PHA) accumulating bacterium Ralstonia eutropha was cultivated in the presence of 3-mercaptopropionic acid (3MP) or 3,3'-thiodipropionic acid (TDP). In this study, 3,3'-dithiodipropionic acid (DTDP) and 3-mercaptovaleric acid (3MV) were identified as two additional precursor carbon sources for in vivo biosynthesis of PTE in R. eutropha. Biosynthesis of copolymers of 3-hydroxybutyrate (3HB) and 3MP, which contributed 19-25% of cell dry matter, was compared referring to the different precursor substrates. Using DTDP as carbon source, which is probably cleaved into two molecules 3MP, yielded an about 2.3-fold higher molar 3MP content of the copolyester than TDP, which is probably cleaved into only one molecule 3MP. Furthermore, cultivation of R. eutropha in the presence of 3MV resulted in biosynthesis of copolymers consisting predominantly of 3HB with low amounts of 3MV and 3-hydroxyvalerate, each contributing less than 5 mol% of the constituents. In contrast, 4-mercaptobutyric acid could be not incorporated into PHAs, although - as documented in this study - five different strategies, various precursor substrates, R. eutropha and also a recombinant strain of Escherichia coli were employed. Therefore, this study not only extended the range of substrates suitable for PTE biosynthesis and also the range of PTE constituents in R. eutropha, it also demonstrates limits for PTE biosynthesis in this bacterium.

Biosynthesis of novel thermoplastic polythioesters by engineered Escherichia coli

The development of non-petrochemical sources for the plastics industry continues to progress as large multinationals focus on renewable resources to replace fossil carbon. Many bacteria are known to accumulate polyoxoesters as water-insoluble granules in the cytoplasm. The thermoplastic and/or elastomeric behaviour of these biodegradable polymers holds promise for the development of various technological applications. Here, we report the synthesis and characterization of microbial polythioesters (PTEs), a novel class of biopolymers of general technological relevance. Biosynthesis of PTE homopolymers was achieved using a recombinant strain of Escherichia coli that expressed a non-natural pathway consisting of a butyrate kinase, a phosphotransbutyrylase, and a PHA synthase. Different homopolymers were produced, consisting of either 3-mercaptopropionate, 3-mercaptobutyrate, or 3-mercaptovalerate repeating units, if the respective mercaptoalkanoic acids were provided as precursor substrates to the fermentative process. The PTEs contributed up to 30% (w/w) of the cellular dry weight and were identified as hydrophobic inclusions in the cytoplasm. The chemical and stereochemical homogeneity of the purified PTEs were identified by different methods, and the estimated physical properties were compared to the oxypolyester equivalents, revealing low crystalline order and, for the poly(3-mercaptopropionate) improved thermal stability. The ability to produce PTEs through a biosynthetic route opens up new avenues in the field of biomaterials.