Aims: To investigate the influence of the source of tryptone in the growth medium on the resistance of Escherichia coli to various types of oxidative stress. Methods and Results: Cultures of Escherichia coli MG1655 were grown in Luria-Bertani (LB) medium at 37degreesC to stationary phase, harvested, and subsequently subjected to various types of oxidative stress. A marked difference in oxidative stress sensitivity was observed depending on the origin of the tryptone in the LB medium used to grow the cultures. Cells harvested from LB containing tryptone from source x (LBx) were more sensitive to inactivation by the superoxide generating compound plumbagin and by t-butyl peroxide, and to growth inhibition by the lactoperoxidase enzyme system, than cells harvested from LB containing tryptone from source y (LBy). By monitoring expression of a panel of stress gene promotors linked to the gfp (green fluorescent protein) gene, and using Delta2-22 alkaline phosphatase as a probe for disulphide bridge formation from protein sulphydryl groups, it was demonstrated that a greater cytoplasmic oxidative stress existed in cells during growth in LBy than in LBx. Conclusions: Depending on the source of tryptone, bacteria may experience different levels of oxidative stress in tryptone-containing nonselective growth media. Although these levels of oxidative stress are subinhibitory, they may trigger a stress response that makes the bacteria more resistant to a subsequent exposure to a lethal or inhibitory level of oxidative stress. Significance and Impact of the Study: This work highlights the importance of controlling very subtle differences in composition of nonselective growth media in studies on bacterial physiology.