Tender morsels for microbes

Direct observation of polyhydroxyalkanoate chains by atomic force microscopy

Atomic force microscopy in the tapping mode was used to investigate aqueous acetone-treated polyhydroxyalkanoate (PHA) inclusions freshly isolated from a recombinant bacterium. The PHA is a copolymer containing about 95 mol% 3-hydroxybutyrate units while the rests are units of 3-hydroxyhexanoate, 3-hydroxyoctanoate, 3-hydroxydecanoate, and 3-hydroxydodecanoate. Polymer chains extending to several micrometers in length were observed on glass cover slips upon the evaporation of the aqueous acetone. The polymer chains seem to exist in the form of fibrillar aggregates. The height of the microfibrils was about 1 nm. Upon prolonged standing at ambient conditions, the microfibrils dissociated into finer strands of about 0.5 nm in height. The results suggest that biosynthesized PHA are stored in the inclusions in an amorphous state but with minimal chain entanglement. This is possible because the PHA chains exist in the form of fibrillar aggregates that may be the product of a special biosynthesis mechanism.

Properties, modifications and applications of biopolyesters

Poly(hydroxyalkanoates) (PHAs), of which poly(hydroxybutyrate) (PHB) is the most common, can be accumulated by a large number of bacteria as energy and carbon reserve. Due to their biodegradability and biocompatibility these optically active biopolyesters may find industrial applications. A general overview of the physical and material properties of PHAs, alongside with accomplished applications and new developments in this field is presented in this chapter. The properties of PHAs are dependent on their monomer composition and therefore it is of great interest that recent research has revealed that, in addition to PHB, a large variety of PHAs can be synthesized microbially. The monomer composition of PHAs depends on the nature of the carbon source and microorganism used. PHB is a typical highly crystalline thermoplastic whereas medium chain length PHAs are elastomers with low melting points and a relatively lower degree of crystallinity. By (chemical) modification of the PHAs, the ultimate properties of the materials can be adjusted even further, when necessary. Applications that have been developed from PHB and related materials (e.g. Biopol) can be found in very different application areas and cover packaging, hygienic, agricultural and biomedical products. Recent application developments based on medium chain length PHAs range from high solid alkyd-like paints to pressure sensitive adhesives, biodegradable cheese coatings and biodegradable rubbers. Technically, the prospects for PHAs are very promising. When the price of these materials can be further reduced, application of biopolyesters will also become economically very attractive.

Differential scanning calorimetric study of poly(3-hydroxyoctanoate) inclusions in bacterial cells

Medium chain length polyhydroxyalkanoates, MCL-PHAs, produced by bacteria as inclusion bodies or granules were analyzed in situ by differential scanning calorimetry (DSC) without isolation from the cells. The kinetic DSC study of PHA granules, which contained mostly 3-hydroxyoctanoate units (PHO), in Pseudomonas putida BM01 cells showed that the polymer within the granules existed in an amorphous state, but it crystallized after dehydration of the cells under freeze-drying condition (below -50 degrees C) followed by annealing at ambient temperature. In this manner, PHO within the cells readily crystallized to the maximum degree of crystallinity within 24 h at room temperature, which was much faster than for the same polymer isolated by solvent extraction. This observation suggests that the polymer within the cellular granules may be well organized. The DSC endothermic melting peak areas for the room-temperature annealed polymers within the cells were directly proportional to the amount of polymer in the cell, and the results from this type of quantitative analysis were essentially identical to those obtained by gas chromatographic and gravimetric analysis of the polymers. X-Ray diffraction analysis of the polymer in the freeze-dried, whole cells and of the isolated, fully crystallized polymer showed that the two types of PHO samples had similar crystal structures, but the polymer in the granules exhibited better side-chain packing and higher crystallinity.

Biomimetic, amorphous granules of polyhydroxyalkanoates: composition, mobility, and stabilization in vitro by proteins

It is a remarkable feature of poly(3-hydroxybutyrate) (PHB) that although the isolated polymer is highly crystalline, native PHB storage granules in cells are only found in an amorphous, mobile state. It has recently been proposed that the failure of PHB granules to crystallize is simply the result of slow nucleation kinetics that are operative for small, isolated particles. In support of this new model, we present here a straightforward procedure by which pure crystalline PHB can be reconstituted into submicron-size, detergent-coated artificial granules. The artificial granules are amorphous and stable in suspension, and they are essentially indistinguishable from their native counterparts in terms of size, morphology, molecular mobility, and density. Furthermore, when the surfactant coating is removed from the artificial granules by dialysis, the granules crystallize, verifying the nucleation hypothesis. In vivo, the PHB granule surface is likely to consist of both polypeptide and lipid; in vitro it is possible to prepare amorphous PHB granules that are stabilized solely by phospholipids or by any of several common proteins (serum albumin, casein, or ovalbumin). Artificial amorphous granules may be prepared from a variety of different bacterial PHAs and from blends of incompatible polyesters.Key words: polyhydroxyalkanoate, poly(3-hydroxybutyrate), granules, nucleation, latex.