Prevention of Competitive Suppression in Microbial Plating Experiments

Abstract

AN assumption inherent in back-mutation experiments is that at the cell concentrations used ‘background’ prototrophs in a substantially auxotrophic population will be expressed quantitatively in the form of colonies or turbid culture when the mixed population is incubated on or in the basal medium. This is not always valid^1,2. Fewer prototrophs may appear than expected when high cell concentrations are used, because the growth of prototrophs is suppressed by the presence of sufficiently large numbers of auxotrophs. This is referred to as ‘competitive’ suppression³. On diluting a suspension of auxotrophic cells to different degrees in basal medium the frequency of back-mutant colonies which arises may increase as the dilution rate increases and the suppression decreases.