The recent discovery of naturally occurring plasmids that confer a Cit+ phenotype on E. coli strains (14, 15, 35) has provided an explanation for the periodic isolation of bacteria that would have been called E. coli except for their ability to utilize citrate.So the only reason that these other bacteria are NOT called "E. coli" is due to their ability to utilize citrate.
Plasmids are often swapped between bacteria, and an e. coli may well come across a plasmid that has the enzymes to digest citrate (yet without the ability to absorb citrate it probably looses it just as fast); this is hardly the same as developing the ability on its own WITHOUT being given a plasmid. Do you understand the difference?