Aims: The objective of this study was to investigate the effect of growing conditions on the glutamate-, arginine- and lysine-dependent acid resistance (AR) systems of Escherichia coli O157:H7. Methods and Results: Seven E. coli O157:H7 strains were grown in five different media at neutral or acidic pH under aerobic or anaerobic conditions, and the survival rate after acid shocks (pH 2.0, 1 h, 37degreesC) in the presence of glutamate, arginine and lysine was determined. Six strains induced the glutamate-dependent AR at stationary phase, and maximal survival were observed (greater than or equal to10%) when grown in pH 5- Luria-Bertani media with glucose (LBG) and in pH 4.5-anaerobic media. The arginine- and lysine-dependent systems were also present, but were only induced if cells had grown in LBG. For strain ATCC 43895, the minimum glutamate concentration that resulted in at least 10% survival rate was 10 mu mol l(-1), but it required at least 10-fold more arginine and lysine. Conclusions: The lysine-dependent AR system could be as important as the arginine-mediated one, but the contribution of both systems to E. coli O157:H7 overall AR response might be minor compared with the glutamate-dependent system. Significance and Impact of the Study: Under typical environmental conditions, the glutamate-dependent AR system might be solely responsible for protecting cells against acidic pH.