The antibacterial activity of ethanolic extracts of selected Thai medicinal plants (Rhodomyrtus tomentosa (Aiton) Hassk., Quercus infectoria G. Olivier, and Eleutherine americana Merr.) against Campylobacter spp. was investigated. Sixty-five Campylobacter, including 39 isolates from humans and 26 isolates from chicken samples, were tested. Reference Campylobacter spp. that are commonly encountered in gastroenteritis were included. The ethanolic extract of E. americana demonstrated good antibacterial activity against all the tested isolates. Inhibition zones ranged from 10 to 37 mm. Minimum inhibitory concentration (MIC) of the extract against Campylobacter isolates from humans and chicken samples ranged from 31.25 to 500 μg/mL and 62.50 to 1,000 μg/mL, respectively. The minimum bactericidal concentration ranged from 31.25 to 1,000 μg/mL for isolates from humans and 125 to 1,000 μg/mL from chicken isolates. The bactericidal activity of the ethanolic extracts of E. americana against important Campylobacter spp., including Campylobacter coli MUMT 18630, Campylobacter fetus ATCC 27374, Campylobacter jejuni ATCC 81176, Campylobacter lari ATCC 43675, and Campylobacter upsaliensis DMST 19055, were assessed at MIC, 2 MIC, and 4 MIC by counting viable cells after various time intervals. At 4 MIC, the level of the tested isolates decreased by 2 to 5 log-fold within 8 h. The ethanolic extract of E. americana demonstrated antibacterial activity against all Campylobacter spp. from both human and chicken isolates. Further investigation of this plant species may provide an alternative medicine for Campylobacter infection and an effective food additive to prevent the infection.

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Eleutherine americana, a well‐known Asian medicinal herb, has been claimed for many biological activities. In this study, potential of E. americana ethanolic extract against adhesion and invasion of Caco‐2 cells by commonly contaminated Campylobacter spp. was evaluated. MIC50 values of E. americana extract against 39 enteritis patient and 26 chicken isolates were 125 µg/ml. Two models were set up: Model I, Prevention: Preincubation of E. americana extract with campylobacters before adding to Caco‐2 cells. Adhesion to and invasion of Caco‐2 cells by campylobacters were completely inhibited within 12 hr. Model II, Treatment: Preincubation of the organisms with Caco‐2 cells before adding the extract. Adding the extract after the bacteria were mixed with Caco‐2 cells resulted in moderate effect on adhesion, but complete prevention of penetration of the organisms into Caco‐2 cells was observed. The extract at the highest concentration tested (250 μg/ml) demonstrated no cytotoxicity toward Caco‐2 cells. Practical applications Eleutherine americana bulb extract could be applied as an effective food preservative to control campylobacter contamination at initial stage in food chain. In addition, the extract provides a curative approach to campylobacter infections.