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Although aerobic, non-CO2 incubation is recommended for MacConkey agar, it has become common in many clinical microbiology laboratories to incubate all primary plates including MacCondey agar in a 5% CO2 incubator to keep all plates together. The problem with this is that some strains of Enterobacteriaceae may fail to grow or show partial inhibition when incuated in CO2 because of a decrease in pH of the medium. Becton-Dickinson has experiment with a new formulation called MacConkey III agar that was shown to provide superior recovery and colony size in both air and CO2 incubation (Kircher SM et al. CO2 incubation of MacConkey agar (MacConkey III). Abstr Ann Meet Am Soc Microbiol 2000; C274:194). Until this medium becomes commercially available you might want to consider incubating all MacConkey plates in a non-CO2 incubator.
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