Final answer:
The most difficult coliforms to differentiate from Salmonella and Shigella are non-lactose fermenters, and lactose fermentation on MacConkey agar is the primary characteristic used for differentiation.
Step-by-step explanation:
The coliform bacteria that are the most difficult to distinguish from the Salmonella-Shigella pathogens tend to be the non-lactose fermenters. The primary characteristic used to differentiate these bacteria is their ability to ferment lactose.
This is seen on MacConkey agar, where lactose-fermenting bacteria such as Escherichia coli produce pink colonies due to lactose fermentation, whereas Salmonella and Shigella spp. which do not ferment lactose, produce colorless colonies. This differential media is a key tool in distinguishing between the potentially pathogenic and non-pathogenic bacteria or between different types of pathogenic bacteria.