Thesis (M.Sc.)--Chulalongkorn University, 2014

Vibrio parahaemolyticus (VP) is an emerging pathogen causing vast economic losses in shrimp production. Using antibiotics to control disease may have resulted antibiotics resistance. This study aimed to investigate susceptibility of VP to 8 antibiotics, and 4 polyphenols, potential alternative against bacterial species. VP were isolated from Pacific white shrimp (Litopenaeus vannamei) in central and southern parts of Thailand, and identified by phenotypic-based and molecular-based methods. Pathogenic isolates (Acute Hepatopancreatic Necrosis Disease, AHPND-VP) were confirmed by PCR targeting toxin gene. Susceptibility to antibiotics and polyphenols was determined by broth microdilution method. Effects of polyphenols on VP was further evaluated by time-kill curve. The results showed that all VP isolates were resistant to ampicillin, and amoxicillin at high concentrations, but susceptible to 6 other antibiotics. Polyphenols demonstrated antimicrobial effects on VP isolates. However, pyrogallol exhibited outstanding activity compared to others. Further investigation proved that pyrogallol possessed time and dose dependent bactericidal activity on VP isolates. In conclusion, all tested antibiotics except ampicillin and amoxicillin have high potential against VP isolates. Additionally, pyrogallol showed highest efficacy against VP isolates among polyphenols.