A culture of E. coli is grown on minimal medium plates supplemented with either tryptophan, histidine, or lysine. The resulting colonies are replica plated to three minimal medium plates containing no supplements. The next day, colonies grow on the minimal medium replica plated from the histidine and lysine plates, but colonies do not grow on the tryptophan replica plate. This indicates that the E. coli strain is a.

tryptophan auxotroph

Explanation:

Tryptophan is known to be a crystaline essential amino acid that is widely distributed or mostly found in proteins.

An auxotroph is an organism that is not able to synthesize a compound needed  for its growth.

In this case, E. coli is described as a tryptophan auxotroph because it was unable to synthesize or make use of the tryptophan vy growing in the plate containing it.

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