Increased and accelerated global economic activities over the past century have led to interlinked problems that require urgent attention. The current patterns of production and consumption have raised serious concerns. In this context, greater emphasis has been put on the concept of sustainable economic systems that rely on technologies based on and supporting renewable sources of energy and materials. Average UK households produce around 3.2 million tonnes of packaging waste annually whereas 150 million tonnes of packaging waste is generated annually by industries in the UK. Hence, the development of biologically derived biodegradable polymers is one important element of the new economic development. Key among the biodegradable biopolymers is a class known as polyhydroxyalkanoates. Polyhydroxyalkanoates (PHAs) are a family of polyhydroxyesters of 3-, 4-, 5-and 6-hydroxyalkanoic acids, produced by a variety of bacterial species under nutrient-limiting conditions with excess carbon. These water-insoluble storage polymers are biodegradable, exhibit thermoplastic properties and can be produced from renewable carbon sources. Thus, there has been considerable interest in the commercial exploitation of these biodegradable polyesters. In this review various applications of polyhydroxyalkanoates are discussed, covering areas such as medicine, agriculture, tissue engineering, nanocomposites, polymer blends and chiral synthesis. Overall this review shows that polyhydroxyalkanoates are a promising class of new emerging biopolymers.

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Polyhydroxyalkanoates (PHAs) have recently been the focus of attention as a biodegradable and biocompatible substitute for conventional non degradable plastics. The cost of large-scale production of these polymers has inhibited its widespread use. Thus, economical, large-scale production of PHAs is currently being studied intensively. Various bacterial strains, either wild-type or recombinant have been utilized with a wide spectrum of utilizable carbon sources. New fermentation strategies have been developed for the efficient production of PHAs at high concentration and productivity. With the current advances, PHAs can now be produced to a concentration of 80 g L −1 with productivities greater than 4 g PHA L −1 h −1 . These advances will further lower the production cost of PHAs and allow this family of polymers to become a leading biodegradable polymer in the near future. This review describes the properties of PHAs, their uses, the various attempts towards the production of PHAs, focusing on the utilization of cheap substrates and the development of different fermentation strategies for the production of these polymers, an essential step forward towards their widespread use.

Plants show physiological and morphological responses to a range of physical and chemical factors known as 'elicitors'. These responses have been considered as defence reactions 'elicited' by the plants' biochemical factory to ensure their survival, persistence and competitiveness. Recently examples have been cited of elicitation in some fungal and bacterial cultures. Through a chronological survey, this Review considers examples of elicitors and elicitation and describes suggested mechanisms of elicitation in plants and microbial cell cultures. The majority of research in this field has been carried out on the plant systems using complex (undefined) biotic elicitors. Carbohydrates are the main class of compounds used as defined elicitors. This Review focuses on carbohydrates as compounds initiating a defence response in cell cultures. Physiological changes brought about on the plant and microbial cultures include expression of novel metabolites and overproduction of already known products. Recent reports confirming elicitation in microbial cultures are of potential importance, as the relative ease of fermentation and scale-up could open an opportunity for the introduction of useful novel metabolites as well as enhancement of commercially useful bioproducts. In this context, a sound knowledge of the elicitor molecules' structure-function relationships and mechanisms of elicitation is essential.