Wisconsin State Health Authorities Alerted the CDC About an E

Multiple Choice

Wisconsin state health authorities alerted the CDC about an E. coli O157:H7 outbreak. Soon thereafter, officials from two other states reported that they also had cases of illness associated with this pathogen. Within days, the CDC determined that the E. coli strains from all three states were the same, and the likely source of the outbreak was a particular brand of pre-packaged fresh spinach. The CDC then issued a press release advising people not to eat bagged fresh spinach.
-E. coli is a Gram-negative bacillus that is found as intestinal normal microbiota. Which of the following would not be useful for an initial step in identifying E. coli O157:H7 strains in a stool sample?

A) Gram staining, because E. coli strains will appear identical using this approach.
B) Serology, because E. coli strains all express the same antigens.
C) Growth on selective media, because all strains of E. coli will have the same metabolic capabilities.
D) RFLPs, because E. coli strains will be identical at the genome level. 
E) These methods can all be used to differentiate between E. coli strains.