Polyhydroxybutyrate accumulation and cadmium adsorption in Cupriavidus taiwanensis EJ02 and its cadmium-sensitive mutants

Polyhydroxyalkanoates (PHAs) are biopolymers accumulated by various bacteria under unbalanced growth conditions when the carbon substrate is in excess of other nutrients such as nitrogen or phosphorus. Bacteria belonging to the genus Cupriavidus including C. necator and C. taiwanensis are well known polyhydroxybutyrate (PHB) producers.   A cadmium resistant C. taiwanensis (strain EJ02) was isolated from sediments of a heavily polluted river in southern Taiwan and used to study its ability for PHB biosynthesis and heavy metal resistance.    Our previous report had suggested that the resistance of C. taiwanensis EJ02 to cadmium stress might stem from its ability to synthesize PHB.  In the present study, we generated cadmium-sensitive mutants of C. taiwanensis EJ02 by transposon mutagenesis.  Two cadmium-sensitive mutant strains (strains K33 and K93) were selected for comparison against the wildtype strain EJ02. Both mutant strains were still able to synthesize significant amount of PHB. The content of PHB accumulation in wildtype (strain EJ02) and the cadmium-sensitive mutants (strains K33 and K93) were comparable in preliminary examination under the same culture conditions.  However, biofilm formation was less apparent in these two mutant strains compared to the wildtype strain EJ02 and the mutants were less efficient in its ability to remove cadmium ions from the culture medium (presumably by adsorption) compared to C. taiwanensis EJ02.  Genes disrupted in strains K33 and K93 are currently under investigated.  Additionally, bioaccumulation of PHB by these strains will be further evaluated and compared to each other under different growth conditions.