Which Statement Shows an Example of a Commensal Species in One

Which statement shows an example of a commensal species in one host becoming a pathogenic species in another host?

A) C. difficile flourishes in the large intestine after antibiotic therapy.
B) New infectious diseases are due to increases in host tropism.
C) Shigella flexneri preferably infects the tissues of humans and other primates.
D) E. coli lives harmlessly in the appendix but will be attacked by the immune system if it invades the abdominal cavity.
E) Up to 30% of pregnant women harbor Group B streptococci in the vagina to no ill effect, but when transmitted to their newborns, a proportion of babies develop serious infection.