Which of the following genetically engineered organisms was used by Eli Lilly to prepare human insulin?

The correct answer is: Bacterium.

Eli Lilly produced human insulin using recombinant DNA technology. The process involved the following steps:

1. Gene identification: The DNA sequence coding for human insulin was determined and isolated.

2. Cloning into a plasmid: The insulin gene was inserted into a circular DNA vector (plasmid) that can replicate inside a host cell.

3. Transformation of a bacterial host: The recombinant plasmid was introduced into Escherichia coli (a bacterium). This bacterium serves as a living factory that can express the insulin gene.

4. Protein expression: Inside the bacterial cells, the insulin gene is transcribed and translated, producing the insulin protein.

5. Harvesting and purification: Bacterial cultures are grown in large fermenters, and the insulin protein is extracted and purified to pharmaceutical grade.

Other organisms listed (yeast, virus, phage) were not used by Eli Lilly for this product. Yeast and viral systems are employed by other companies for different recombinant proteins, while phages are typically used as vectors in molecular biology but not for large‑scale protein production.

Therefore, Eli Lilly’s human insulin was prepared using a bacterium as the expression system.