The E. coli might be out competing the Salmonella, in which case you'll see one and not the other.  However, it is possible that the E. coli is producing an inhibitor that is negatively affecting the Salmonella.  Try streaking them side by side on a TSA plate, even cross streaking, and look for zones of inhibition.

Dear Wendell

Mixed cultures are much more difficult than they seem. We also had difficulty.  I suggest changing the ratio of the starter mix, using an excess of Salmonella.

Hi Liam, 

I changed the ratio even to the extreme of adding 1000ul of my cultured Salmonella into my E.coli mix which started out as 2:1 ratio ( 2ul Salmonella and 1ul E.coli).  Surprisingly I was able to detect both on multiplex qPCR same day but the next day E.Coli was only detectable. 

Thank you guys for your suggestions. 

I think it is depending on the medium. If you use non-selective medium, both organisms can grow in/on media. Many Salmonella selective media could inhibit E. coli growth.

Hi ,

E. coli grows more rapidly on glucose than Salmonella. I think there is a competition on nutriments. You can use selective media (XLD agar for Salmonella) .

there is also a quorum sensing that was well demonstrated between E coli and Salmonella. So your media, inoculum and time of harvesting (overnight, 8H or 24 H) can influence the groeth of each organism.

Your PCR conditions and primers specificity are questionnable too.

Both microbes could grow on non-selective media. You can differentiate them by colony characteristics. On the other hand, you can also use McConkey agar for differentiating them. As salmonella is non-lactose fermenter they will produce creamlike colony rather than pink in the media.

Hi, Kindly use selective media you will be home and dry.

Thank you all for your suggestions!

Hi, you can try to play with differents ratios, increase and dicrease E. coli or Samonella sample, or try with another medium, which gives enough nutrients for both bacteries.