Variances in cellular sedimentation behavior as an effective enrichment method of hydrocarbon-overproducing Micrococcus luteus strains

BACKGROUND

Aliphatic hydrocarbons of microbial origin are highly interesting candidate biofuels because these molecules are identical or very similar to the main components of petroleum-based gasoline and diesel fuels. The high-GC Gram-positive bacterium Micrococcus luteus is capable of naturally synthesizing long-chain, iso- and anteiso-branched alkenes which are formed via the head-to-head condensation of fatty acid thioesters by a dedicated enzyme system. The present study describes the relation we observed between olefin production and cell buoyancy in Micrococcus luteus and the use of this phenotype to simply and efficiently separate cells from a mixture based on their hydrocarbon content.

METHODS

We generated M. luteus mutants producing different amounts of olefins and used them in mixing and sedimentation experiments, olefin content analysis by GC-MS and in equilibrium centrifugation in Percoll gradients.

RESULTS

We found well-detectable differences in the buoyant densities of the examined strains, which correlated with the amounts of hydrocarbons produced by the cells. We also demonstrate how our observations can be used to simply and efficiently fractionate cells based on their hydrocarbon content.

CONCLUSIONS

In summary, we show that cultures of M. luteus cells sediment at distinct rates depending on the amounts of alkenes produced. Our results indicate that buoyant cell density is the primary cause for the observed differences in sedimentation behaviour. The simple separation strategy described here can be a valuable tool in various mutagenesis and enrichment protocols, aimed at generating and isolating strains with increased olefin productivity.

Mammalian cell retention devices for stirred perfusion bioreactors

Within the spectrum of current applications for cell culture technologies, efficient large-scale mammalian cell production processes are typically carried out in stirred fed-batch or perfusion bioreactors. The specific aspects of each individual process that can be considered when determining the method of choice are presented. A major challenge for perfusion reactor design and operation is the reliability of the cell retention device. Current retention systems include cross-flow membrane filters, spin-filters, inclined settlers, continuous centrifuges and ultrasonic separators. The relative merits and limitations of these technologies for cell retention and their suitability for large-scale perfusion are discussed.

Contamination of a high-cell-density continuous bioreactor

Continuous fermentations were carried out with a recombinant flocculent Saccharomyces cerevisiae strain in an airlift bioreactor. Once operating under steady state at a dilution rate of 0.45 h(-1), the bioreactor was contaminated with Escherichia coli cells. The faster growing E. coli strain was washed out of the bioreactor and the recombinant, slower growing flocculating S. cerevisiae strain remained as the only species detected in the bioreactor. Flocculation, besides allowing for the realization of high-cell-density systems with corresponding unusual high productivity, may be used as a selective property for controlling some contamination problems associated with prolonged continuous operation.