Why does beta-2-Thienylalanine inhibit bacterial growth?
&beta;-2-thienylalanine is an analogue of phenylalanine. It is incorporated into proteins in place of phenylalanine, and the resulting proteins are inactive because of the substitution. Consequently growth medium supplemented with &beta;-2-thienylalanine will not support bacterial growth.

Addition of phenylalanine to a medium containing &beta;-2-thienylalanine will result in a competition between the two compounds - if there is an excess of phenylalanine then this will out-compete the analogue and incorporation of the analogue will be minimised, allowing growth. Blood from a patient with PKU contains phenylalanine, so the addition of this to the growth medium will relieve inhibition of growth.

Here is a link to a patent application in which a screen for E. coli strains overproducing phenylalanine was based upon the same principle.