4. Microorganisms and fermentation strategies
To effectively produce PHAs with microorganisms, several factors
must be considered. Although these include the ratio of carbon
source to nitrogen source, the presence of macro- and microelements,
physical parameters including the period of cultivation, the
temperature of cultivation, and the optimum pH for cultivation (Lee
et al., 2004), the most important factor is the PHA producer, which
is usually a species of bacteria. At this time, at least a few hundred
bacterial species have been studied that possess the ability to
accumulate PHAs. Most of them, however, can not be regarded as
promising biopolymer producers, because their ability to synthesize
and accumulate PHAs is insufficient.
When considering microorganisms for the industrial production
of PHAs, several factors need to be considered, such as the ability of
the cell to utilize an inexpensive carbon source and to achieve good
rates of growth and polymer synthesis. Only some of the bacterial
species that have been studied can accumulate PHAs in satisfactory
amounts and at a low cost. These include Alcaligenes latus, Azotobacter
vinelandii, B. megaterium, Paracoccus denitrificans, Protomonas
extorquens, Cupriavidus necator and Pseudomonas oleovorans
(Chanprateep, 2010). The general scheme presenting PHAs production process is
shown on Fig. 2. Several PHA production strategies have been described, including batch, fed-batch and continuous processes.
Each of them can be conducted under a variety of conditions. Batch and fed-batch fermentations are widely used in industrial fermentation processes. For batch cultivation, two approaches have been developed: one-stage and two-stage cultivation. In one stage cultivation, microorganisms are used that can simultaneously grow and accumulate PHAs. Two-stage cultivation is performed when
microorganisms need nutrient limitations. The first stage consists
of a cell growth phase which is carried out in a separate nutrientenriched
medium to ensure sufficient biomass. The cells are then
transferred to a nutrient limited medium for the PHA accumulation
phase. During this nutrient limitation stage, the cells are unable to
multiply and their number remains almost constant. However, the
cells begin to increase in size and weight due to the intracellular
accumulation of PHAs as a storage product (Lee, 1996).