In positive selection, only those mutant cells that can survive in a medium containing a selective agent such as penicillin can survive.
 
 
 
 
 
 
 


In negative selection,  nutritional mutants called auxotrophs cannot grow on a minimal medium lacking a key nutrient.  The mutants are identified by the replicate organism direct agar contact (RODAC) technique.
 


The Ames Test utilizes a histidine auxotroph of Salmonella to determine if a chemical agent is a mutagen.  Though some spontaneous back mutations (a reversion back to the strain of Salmonella that can synthesize histadine) can occur, if many colonies of the microbe appear on the minimal plate after the addition of the test chemical, this is an indication the the chemical is a mutagen.