Assay using marked microbial host cell strains

A process for enumerating individual colonies in a mixture of cultured microbial host cell strains contained in a single culturing vessel which strains utilize more than one carbohydrate source as a substrate and have been genetically engineered by altering the strains' genotypes so that they do not utilize a set of n- 1 carbohydrate sources as substrates, where n is the number of said microbial host cell strains, which set of carbohydrate sources is different for each genetic marking employed, which process comprises:

• (a) culturing in the vessel the host cell strains;
• (b) diluting an isolated sample of culture until a countable number of viable cells will be present when the sample is plated;
• (c) plating the diluted sample of culture onto culture media supplemented with a carbohydrate source only utilized by one of the host cell strains as a substrate;
• (d) incubating the plated cells at a temperature and for a time sufficient for any positive colony to grow to a detectable level;
• and
• (e) counting the number of colonies growing on the supplemented culture media;
• wherein steps (c) through (e) are carried out for each colony of strain to be enumerated, and wherein the carbohydrate source supplementing the media is different for each strain.