Polyhydroxybutyrate (PHB) biodegradation using bacterial strains with demonstrated and predicted PHB depolymerase activity

The biodegradation of polyhydroxybutyrate (PHB) has been broadly investigated, but studies typically focus on a single strain or enzyme and little attention has been paid to comparing the interaction of different PHB depolymerase (PhaZ)-producing strains with this biopolymer. In this work, we selected nine bacterial strains-five with demonstrated and four with predicted PhaZ activity-to compare their effectiveness at degrading PHB film provided as sole carbon source. Each of the strains with demonstrated activity were able to use the PHB film (maximum mass losses ranging from 12% after 2 days for Paucimonas lemoignei to 90% after 4 days for Cupriavidus sp.), and to a lower extent Marinobacter algicola DG893 (with a predicted PhaZ) achieved PHB film mass loss of 11% after 2 weeks of exposure. Among the strains with proven PhaZ activity, Ralstonia sp. showed the highest specific activity since less biomass was required to degrade the polymer in comparison to the other strains. In the case of Ralstonia sp., PHB continued to be degraded at pH values as low as pH 3.3-3.7. In addition, analysis of the extracellular fractions of the strains with demonstrated activity showed that Comamonas testosteroni, Cupriavidus sp., and Ralstonia sp. readily degraded both PHB film and PHB particles in agar suspensions. This study highlights that whole cell cultures and enzymatic (extracellular) fractions display different levels of activity, an important factor in the development of PHB-based applications and in understanding the fate of PHB and other PHAs released in the environment. Furthermore, predictions of PhaZ functionality from genome sequencing analyses remain to be validated by experimental results; PHB-degrading ability could not be proven for three of four investigated species predicted by the polyhydroxyalkanoates (PHA) depolymerase engineering database.