FIG. 7.

Effect of nitrogen supplementation on the growth of E. coli O157:H7 strain H1827R on middle leaves sampled from harvested romaine lettuce heads grown in the field. The leaves were inoculated with a suspension of the pathogen in buffer only (white bars) or in buffer containing 6 mM NH₄NO₃ (black bars) or 0.1% glucose (cross-hatched bars). The results of two replicate experiments are shown. The data represent the means of the log-transformed values of the bacterial population sizes per gram tissue of 16 replicate leaves; error bars indicate ±1 standard error of the mean. Data labeled with the same letter are not significantly different by the Tukey-Kramer multiple comparison test (P < 0.05).