Submitted to: Biomacromolecules
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/31/2005
Publication Date: 6/1/2005
Citation: Ashby, R.D., Solaiman, D., Foglia, T.A. 2005. Synthesis of short-/medium-chain-length poly(hydroxyalkanoate) blends by mixed culture fermentation of glycerol. Biomacromolecules. 6:2106-2112. Interpretive Summary: Poly(hydroxyalkanoates) (PHA) are a class of environmentally benign bacterial polymers that can be classified as either rigid or elastic based on their origin and chemical composition. This property variation allows these polymers to be earmarked for many different applications. However, the high costs associated with bacterial PHA production have impeded industrial application of these materials. Glycerol is a relatively inexpensive, large volume chemical that is produced from many different industrial processes (particularly those that involve animal fats and/or vegetable oils) and is commonly used in oral-care products, tobacco, cosmetics, food and beverages. Despite its widespread use, there is a growing need to find additional non-food outlets for its use because of the increased production anticipated from new sources. In this study we took advantage of the different genetic systems of two bacterial strains that grew and produced PHA from glycerol. This led to the production of two different types of PHA polymer (one rigid and the other elastic) under identical growth conditions. By using these two bacterial strains in a single culture it was possible to produce blends or mixtures of rigid and elastomeric PHA polymers and ultimately control the ratio of one polymer type to the other by varying the time of inoculation of one bacterial strain to the other, increasing the concentration of glycerol in the growth media and controlling the duration of incubation of the culture. The use of an inexpensive substrate (i.e., glycerol) may go a long way to decrease the production costs of PHA polymers thus expanding the market potential of these polymers.
Technical Abstract: Glycerol was used as a substrate in the bacterial production of poly(hydroxyalkanoates) (PHAs) in an effort to establish an alternative outlet for glycerol and at the same time produce a value-added product. Both Pseudomonas oleovorans NRRL B-14682 and Pseudomonas corrugata 388 grew and synthesized PHA from glycerol at concentrations up to 5% (v/v). Interestingly, under identical growth conditions, P. oleovorans synthesized poly(3-hydroxybutyrate) (PHB) whereas P. corrugata synthesized a medium-chain-length PHA (mcl-PHA) consisting primarily of 3-hydroxydecanoic acid (C10:0; 44+/-2 mol%) and 3-hydroxydodecenoic acid (C12:1; 31+/-2 mol%). Increased glycerol concentrations had little effect on the cell growth and PHA yields from P. oleovorans, but growth and PHA production in P. corrugata were adversely affected at glycerol concentrations above 2%. Results showed that cellular productivity maximized at 40% for P. oleovorans with 5% glycerol and 20% for P. corrugata in 2% glycerol after 72 hours. Interestingly, in both cases as the glycerol concentration increased the molar masses (Mn) of the resulting PHB and mcl-PHA polymers decreased by 61% and 72%, respectively, after 72 hours' incubation time. In addition to molar mass control, the growth patterns of P. oleovorans and P. corrugata as a function of glycerol concentration allowed their use as mixed cultures to control the blend ratios of the natural PHB/mcl-PHA polymers. By incorporating a staggered inoculation pattern, PHB/mcl-PHA blend ratios were achieved that varied from 34:66 to 96:4, depending on the duration of cell growth.