Final answer:
The coliform bacteria most difficult to distinguish from Salmonella-Shigella pathogens are certain strains of E. coli, especially those producing Shiga toxin.
Step-by-step explanation:
The coliform bacteria that are the most difficult to distinguish from the Salmonella-Shigella pathogens are Escherichia coli. E. coli is a member of the Enterobacteriaceae family, which includes both coliforms and noncoliforms.
Coliforms, such as E. coli, are characterized by their ability to completely ferment lactose, producing both gas and acid as byproducts. This fermentation results in pink colonies when grown on media like MacConkey agar. On the other hand, noncoliforms, including pathogens like Salmonella spp. and Shigella spp., either cannot ferment lactose or do so incompletely, leading to colorless colonies on the same medium.
Moreover, certain strains of E. coli produce a Shiga toxin, which is similar to the toxin produced by Shigella dysenteriae. This resemblance can complicate differentiation between the two bacteria. These E. coli strains, known as enterohemorrhagic E. coli (EHEC), possess virulence factors that were acquired through horizontal gene transfer, such as the Shiga toxin genes transduced from bacteriophages that originally infected Shigella.
Despite their mutualistic relationships with humans under normal circumstances, these pathogenic E. coli can cause severe symptoms similar to those caused by Shigella, including bloody diarrhea and hemolytic uremic syndrome. Due to the similarity in disease presentation and challenges in identification, specifically when they do not demonstrate typical lactose fermentation characteristics, these EHEC strains are particularly difficult to differentiate from Salmonella and Shigella pathogens in a clinical setting.