An engineered strain of Stenotrophomonas maltophilia (SMCD1) is capable of the direct extracellular biomineralization of CdS quantum dot nanocrystals from buffered aqueous solution of cadmium acetate and L-cysteine without the addition of a chemically reactive precursor. Nanocrystal synthesis is strongly influenced by both the L-cysteine/cadmium acetate ratio and pH of the solution. The observed trends are consistent with L-cysteine acting as both a sulfur source and nanocrystal capping agent. Enzymatic turnover of L-cysteine by a putative cystathionine gamma-lyase forms reactive sulfur in solution, removing the requirement for addition of reactive sodium sulfide typical of most other biomineralization approaches. The utility of the biomineralized, quantum dots is demonstrated by phase transfer from the aqueous to the organic phase and subsequent incorporation into a quantum dot sensitized solar cell and chemical growth of a ZnS shell onto the biomineralized CdS core.