Why use IPTG for lac operon in laboratory?

Question

Although lactose (and thus allolactose) is the natural inducer for lac operon, however, in the laboratory, an artificial inducer IPTG is always being used. Why?

Answer 1

Mavi is right.  Bacteria can use lactose as a carbon source so as they "eat" the lactose less and less of it would be around which would slowly shut off gene expression.  IPTG is a chemical not metabolized by the bugs so once you add it at a certain concentration....that concentration stays the same and gene expression stays on.

Answer 2

Isopropylthioglucose (IPTG) is not metabolized in the same way that lactose, which it mimics, would be metabolized.  It thus maintains de-repression of the lac operon.  In contrast, use of lactose would even cause repression to occur quickly.

For more info, look here:

http://en.wikipedia.org/wiki/IPTG