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3
Microbial Synthesis of Biodegradable Polyesters: Processes,
Products, Applications
Bernd H.A. Rehm

3.1
Introduction

Bacterial polyhydroxyalkanoate (PHA) granules are naturally found spherical
inclusions, which are considered as source for renewable and biodegradable
plastics; they are also increasingly recognized as possible functionalized beads for
use in biotechnological and biomedical applications. PHAs are polyesters which
serve as carbon and energy storage for bacteria, becoming deposited as insoluble
spherical inclusions in the cytoplasm. Many bacterial genera and members of
the family of Halobacteriaceae belonging to Archaea are known to synthesize
PHA [1–4], which is produced in conditions of nutrient limitation but where
carbon is available in excess [5]. Bacteria have the capacity to produce about 80%
of their dry weight in PHA [6] with reversal of the PHA polymerization process
in conditions of carbon starvation [7, 8]. Poly(3-hydroxybutyrate) (PHB), one
of the most common PHAs, is synthesized from 3-hydroxybutyrate (3HB) but
different bacteria use hydroxy fatty acids of varying chain length, generating a
range of PHAs.
Because of traits such as biocompatibility, biodegradability, and production
from renewable resources, the interest in the possible application of PHAs is sig-
nificant. Using chemical modification or the creation of copolymers, a variety of
material properties can be attained. For example, PHAs as plastics are less brittle
and more flexible while retaining tensile strength. Developed for use in industrial
or medical applications, these polymers have been revealed to be well tolerated
by mammalian systems. Because of the comparatively high production costs,
PHAs are presently mainly appealing for use in the medical field, for example, for
sutures or implants such as heart valves, stents, and bone scaffolding.
The keyenzymefor PHAbiosynthesisisthe PHA synthase. This enzyme
polymerizes (R)-3-hydroxyacyl-CoA thioester monomers into polyester with the
release of Co A.
Depending on the organism, there are different classes of PHA synthases using
different (R)-3-hydroxyacyl-CoA precursors that can be provided by different
avenues [9]. In Ralstonia eutropha, the most scrutinized PHB producer [10],

Biodegradable Polyesters, First Edition. Edited by Stoyko Fakirov.
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2015 by Wiley-VCH Verlag GmbH & Co. KGaA.