Pathogenic members of the enterobacteriaceae usually will ___________?

Pathogenic members of the Enterobacteriaceae family are known for certain metabolic characteristics that help in their identification and classification. While it is true that many enteric bacteria are capable of fermenting glucose, the significant aspect regarding pathogenic members is their inability to ferment lactose.

This characteristic is particularly useful in laboratory diagnostics, especially with media like MacConkey agar, where lactose fermenters will produce pink colonies due to acid production from lactose fermentation, while non-fermenters will remain colorless. Pathogenic Enterobacteriaceae, such as Salmonella and Shigella, typically do not ferment lactose, differentiating them from some non-pathogenic strains, like E. coli, which can do so.

Choices involving fermentation of sucrose or breaking down proteins are not definitive characteristics associated with the pathogenicity of Enterobacteriaceae. While some may have the ability to ferment sucrose and degrade proteins, these traits do not serve as reliable indicators of pathogenicity within this group. Thus, the inability to ferment lactose is a more critical marker for identifying potential pathogens among the Enterobacteriaceae. This is why the option indicating that pathogenic members usually do not ferment lactose is the correct one.