HIGH CELL-DENSITY PRODUCTION OF POLY-BETA-HYDROXYBUTYRATE (PHB) FROM METHANOL BY METHYLOBACTERIUM-EXTORQUENS - PRODUCTION OF HIGH-MOLECULAR-MASS PHB

Methylobacterium extorquens ATCC 55366 was successfully cultivated at very high cell densities in a fed-batch fermentation system using meth anol as a sole carbon and energy source and a completely minimal cultu re medium for the production of poly-beta-hydroxybutyrate (PHB). Cell biomass levels were between 100 g/l and 115 g/l (dry weight) and cells contained between 40% and 46% PHB on a dry-weight basis. PHB with hig her molecular mass values than previously reported for methylotrophic bacteria was obtained under certain conditions. Shake-flask and fermen ter experiments showed the importance of adjusting the mineral composi tion of the medium for improved biomass production and higher growth r ates. High-cell-density cultures were obtained without the need for ox ygen-enriched air; once the oxygen transfer capacity of the fermenter was reached, methanol was thereafter added in proportion to the amount of available dissolved oxygen, thus preventing oxygen limitation. Con trolling the methanol concentration at a very low level (less than 0.0 1 g/l), during the PHB production phase, led not only to prevention of oxygen limitation but also to the production of very high-molecular-m ass PHB, in the 900-1800 kDa range. Biomass yields relative to the tot al methanol consumed were in the range 0.29-0.33 g/g, whereas PHB yiel ds were in the range 0.09-0.12 g/g. During the active period of PHB sy nthesis, PHB yields relative to the total methanol consumed were betwe en 0.2 g/g and 0.22 g/g. M. extorquens ATCC 55366 appears to be a prom ising organism for industrial PHB production.