The main purpose of aseptic technique is to prevent the sterile medium, petri dishes and other laboratory utensils used for cultivation from being exposed to miscellaneous bacteria.

To accomplish this, the following must be noted:

1.The working environment must be cleaned with disinfectants to reduce possible miscellaneous bacteria;

2.Sterilization packs should be kept dry to avoid capillary phenomenon, and pay attention to the expiration date.

3. Non-sterile substances should be kept more than 2 inches away from the sterile area. Do not talk, cough or sneeze into the sterile area.

4. Unsterilized items or hands should not cross the sterile surface.

5.The transplanting tools must be sterile (for example, the inoculation loop should be sterilized by burning it with an alcohol lamp before and after transplanting);

6.Actions are performed quickly and efficiently to reduce exposure time during which the medium or researcher can be contaminated.

7.Sterile and non-sterile items should not be in contact. When sterile items are suspected to be contaminated, they are deemed non-sterile and should be replaced.

What sort of contamination are you seeing? Is it fungal or bacterial?

I would suggest getting all fresh medium (liquid culture, plates) and seeing what happens if you do a mock transformation (to see if your cells were contaminated to start with).

Do you use a frozen stock of competent cells or make them fresh? One time I had to re-make competent E. coli stocks because ours were contaminated with what looked like Staphlococcus (the person who made the cell stocks had a really bad cold)