Why might it be necessary to subculture both pink and colorless colonies from an initial stool culture on MacConkey agar?

Why might it be necessary to subculture both pink and colorless colonies from an initial stool culture on MacConkey agar?

• A. Most Shigella strains are lactose positive
• B. Most Salmonella strains are maltose negative
• C. Most Proteus spp. are lactose negative
• D. Pathogenic E. coli can be lactose positive or lactose negative

Answer: (D)

Subculturing both pink and colorless colonies from an initial stool culture on MacConkey agar is essential because pathogenic E. coli can be either lactose positive or lactose negative. MacConkey agar is designed to differentiate lactose fermenters, which produce acid and turn the medium pink, from non-lactose fermenters, which do not produce acid and remain colorless. The presence of pink colonies indicates lactose fermentation, which can include strains of E. coli that are either non-pathogenic or pathogenic. Colorless colonies indicate non-lactose fermenters, which could also include pathogenic organisms like certain strains of E. coli or other Enterobacteriaceae. By subculturing both types of colonies, further tests can be performed to identify the specific organism and assess its pathogenic potential accurately. This dual approach is crucial in identifying outbreaks of diseases caused by Enterobacteriaceae, as different strains have varying implications for treatment and public health.

Subculturing both pink and colorless colonies from an initial stool culture on MacConkey agar is essential because pathogenic E. coli can be either lactose positive or lactose negative. MacConkey agar is designed to differentiate lactose fermenters, which produce acid and turn the medium pink, from non-lactose fermenters, which do not produce acid and remain colorless.

The presence of pink colonies indicates lactose fermentation, which can include strains of E. coli that are either non-pathogenic or pathogenic. Colorless colonies indicate non-lactose fermenters, which could also include pathogenic organisms like certain strains of E. coli or other Enterobacteriaceae. By subculturing both types of colonies, further tests can be performed to identify the specific organism and assess its pathogenic potential accurately.

This dual approach is crucial in identifying outbreaks of diseases caused by Enterobacteriaceae, as different strains have varying implications for treatment and public health.