Poly(4-hydroxybutyrate) (P4HB) production in recombinant Escherichia coli: P4HB synthesis is uncoupled with cell growth

BACKGROUND: Poly(4-hydroxybutyrate) (P4HB), belonging to the family of bacterial polyhydroxyalkanoates (PHAs), is a strong, flexible and absorbable material which has a large variety of medical applications like tissue engineering and drug delivery. For efficient production of P4HB recombinant Escherichia coli has been employed. It was previously found that the P4HB synthesis is co-related with the cell growth. In this study, we aimed to investigate the physiology of P4HB synthesis, and to reduce the total production cost by using cheap and widely available xylose as the growth substrate and sodium 4-hydroxybutyrate (Na-4HB) as the precursor for P4HB synthesis. RESULTS: Six different E. coli strains which are able to utilize xylose as carbon source were compared for their ability to accumulate P4HB. E. coli JM109 was found to be the best strain regarding the specific growth rate and the P4HB content. The effect of growth conditions such as temperature and physiological stage of Na-4HB addition on P4HB synthesis was also studied in E. coli JM109 recombinant in batch culture. Under the tested conditions, a cellular P4HB content in the range of 58 to 70% (w w(-1)) and P4HB concentrations in the range of 2.76 to 4.33 g L(-1) were obtained with a conversion yield (Y(P4HB/Na-4HB)) of 92% w w(-1) in single stage batch cultures. Interestingly, three phases were identified during P4HB production: the “growth phase”, in which the cells grew exponentially, the “accumulation phase”, in which the exponential cell growth stopped while P4HB was accumulated exponentially, and the “stagnation phase”, in which the P4HB accumulation stopped and the total biomass remained constant. CONCLUSIONS: P4HB synthesis was found to be separated from the cell growth, i.e. P4HB synthesis mainly took place after the end of the exponential cell growth. High conversion rate and P4HB contents from xylose and precursor were achieved here by simple batch culture, which was only possible previously through fed-batch high cell density cultures with glucose.