I would just use each strain alone as your control since you are primarily trying to determine if there is no background colonies from the donor or recipient strain. If all works correctly your control plates should be clean and you should have colonies on your conjugation plate. 

Ok,

thanks. I don't have a protocol for this kind of experiment. I read that I have to prpepare serial dilutions of the mating mixture. What diluition is better to prepare? 1:10?I didn't understand how I treat the control strains (in terms of volumes to be used, diluitions etc..). if I understood correctly I take the broth cultures that I used to prepare the mating mix..is it correct? Do I have to diluite them too before plating them? 

Thanks

I would plate your controls undiluted, and 1:10 and 1:100. I would plate your experimental mating at the same dilutions and perhaps also 1:1000. It really depends upon the donor and recipient strains, i.e. are they closely related or not, and whether the plasmid is highly conjugal or not. If the strains are very similar and the plasmids highly conjugal, then you might even need lower dilutions to get single colonies. But most of the time those dilutions would likely cover the range you need. 

Ok, thank you very much