The neuroendocrine hormone noradrenaline has previously been shown to increase the growth of Gram-negative bacteria. To determine whether these results were due to noradrenaline-induced genetic rearrangements, arbitrarily-primed PCR utilizing a large number of primers of varying G + C content was performed on Escherichia coli O157:H7 grown in serum-based media at low initial inocula in order to approximate in vivo conditions. In addition to increased growth, bacteria grown in noradrenaline supplemented media displayed a different molecular fingerprint compared with growth in non-noradrenaline supplemented media with a minority of the primers tested. Differences in fingerprint patterns between noradrenaline supplemented and non-supplemented growth bacteria were greatest in the late logarithmic to early stationary phase of growth compared with the late stationary phase. These results suggest that the neuroendocrine hormonal environment of the host may provide environmental signals which lead to genetic rearrangements in the infectious agent and possibly alterations in subsequent pathogenicity.