Microbial resolution of baclofen by a new isolate of Streptomyces halstedii

We attempted to isolate microorganisms able to metabolize (S)-baclofen while showing little or no activity with (R)-baclofen. A culture collection of 218 microbial isolates was established from several soil samples using various approaches. Subsequent chiral HPLC analysis revealed that only two cultures showed promise. Initial shake flask experiments demonstrated a correlation between culture growth and (S)-baclofen metabolism. (S)-baclofen was metabolized much more rapidly than (R)-baclofen. The best culture, identified as Streptomyces halstedii, was used to scale up the process in small-scale fed-batch fermentations using pulse addition of racemic baclofen to yield extracellular fractions showing enantiomeric excess (e.e.) values of greater than 90% in favour of the (R)-isomer. Resting cells also were able to carry out the wanted reaction but initial attempts using cell free extracts were negative. It is speculated that the initial attack on (S)-baclofen may be catalyzed by a mono- or dioxygenase.