Comparison of two commercial formulations of the MacConkey agar test for mycobacteria.

Recent evaluations of the MacConkey agar test for differentiation of rapidly growing mycobacteria have revealed that certain strains of Mycobacterium fortuitum and M. chelonei that were expected to grow on MacConkey agar failed to do so. Investigation of two formulations of MacConkey agar showed that these two species grew better on the medium when the crystal violet dye was omitted. Several possible reasons for this difficulty are discussed. It is recommended that clinical laboratories engaged in differential identification of mycobacteria utilize commercial MacConkey agar without crystal violet when testing rapidly growing species of this genus.