Final answer:
Dark pink colonies on MacConkey agar indicate the presence of strong acid fermenters that are positive for lactose fermentation, such as E. coli. The color change is due to the pH indicator in the agar reacting to the acidic environment produced by the fermentation process. Selective and differential media like MacConkey and EMB agar are used to distinguish among bacteria based on their fermentation capabilities.
Step-by-step explanation:
When dark pink colonies and surrounding medium are present, this indicates the bacteria are both strong acid fermenters and are positive for lactose fermentation.
On MacConkey agar, lactose-fermenting bacteria such as E. coli produce acid as a result of lactose fermentation, which in turn causes the pH indicator in the medium to change color. Specifically, the pH indicator neutral red turns to hot pink at low pH levels, indicating the presence of strong acid fermenters. This differential media allows for the easy identification of bacteria based on their biochemical properties.
A similar diagnostic approach is used with EMB agar, which uses different dyes to distinguish between lactose fermenters and non-lactose fermenters. This is an example of selective and differential media, which is crucial in the isolation and identification of specific types of bacteria, especially in clinical settings.