the positive control works, but the sample gives no product - (Jan/05/2012 )
I am trying to do PCR of DNA of a bacterial sample which was extracted in other lab and then sent to our lab. I performed it according to our PCR protocol which is always working very well. However, only my own positive control gives me a nice band. The received bacterial sample gives no band... The concentration of this received sample and my control in the PCR reaction was the same (measured by picogreen and also seen on the gel). The original DNA of the received sample seems to be OK when run on the agarose gel. However, I don't know which method was used for DNA extraction. Could be the extraction method the problem? I am having doubts whether it is really bacterial DNA... Do you have some similar experience with no PCR products in tested samples and good products in controls?
Maybe wrong DNA indeed. Maybe it contains inhibitors that block PCR. You can test the later if you mix sample and your positive control and see if it amplifies. Or you can try diluting the sample like 10x, 100x and 1000x.