Technical Abstract: We evaluated several plant essential oils (cinnamon bark, cinnamon Cassia, cinnamon leaf, clove bud, lemon, lemon grass, orange bitter, orange Mandarin, oregano Spanish, palmarosa, rose Geranium, and thyme) and their major components (carvacrol, cinnamaldehyde, citral, eugenol, geraniol, limonene, and perillaldehyde) for anti-microbial activity in a pH 7 PBS buffer and in apple juice (pH 3.7) against Escherichia coli O157:H7 (E. coli) and Salmonella enterica (S. enterica) at ~5 min and 21 oC and after incubation for 60 min at 37 oC. Dose-response plots were used to determine the % of the sample that resulted in a 50% decrease in the colony forming units as compared to a control sample (BA50). The test compounds were more active in apple juice than in the buffer. In apple juice at 60 min, BA50 values for E. coli ranged from 0.023 to 0.14 and for S. enterica from 0.0053 to 0.15. The enhanced activity in apple juice compared to the pH 7 buffer was greater for S. enterica than for E. coli. Lemon oil induced the highest enhancement, 6.9-fold for E. coli and 90.6-fold for S. enterica. Studies with citrate and maleate buffers of the same pH as apple juice suggest that the acidity of the juice contributes to the bactericidal activity. Analysis of the oils by HPLC showed that the bactericidal results are related to the composition of the oils. These studies provide information about possible new ways to protect apple juice and possibly other foods against infection by human pathogens.