This approach is useful as a teaching aid and for quality control when initial conjecture is compared with final identification and antimicrobial susceptibility testing. Bile salts selectively inhibit Gram-positive organisms. When Gram-negative bacilli ferment lactose, mixed acid byproducts are formed. These acids cause a localized decrease in pH, shown by Neutral Red, producing pink-red colonies. NFL produce no color.
MacConkey agar is a commonly used primary plating medium in many clinical microbiology laboratories.
Since this medium is so common, and because it can provide timely clues as to the identification of some Gram-negative bacilli, it behooves microbiologists to be efficient in interpreting colonial growth. Initial conjecture as to the possible identification of an organism can then be compared with the final identification and antimicrobial susceptibility results as part of a quality control process. Our main goal was to develop a means for facilitating accurate preliminary grouping of some common lactose-fermenting Enterobacteriaceae, primarily for use by inexperienced laboratory technicians. Final pH of uninoculated medium was 7.1. Quality control of the medium with known strains of Escherichia coli and Staphylococcus aureus showed that it worked properly throughout the study.