Antibacterial spectrum of synthetic herbal-based polyphenols against Vibrio parahaemolyticus isolated from diseased Pacific whiteleg shrimp (Penaeus vannamei) in Thailand

Acute hepatopancreatic necrosis disease (AHPND) was first reported as a new shrimp disease in 2009, which
caused serious disease in shrimp culture with global economic losses and high rates of mortality. Vibrio parahaemolyticus (VP) was recently identified as the leading causative agent of AHPND that contains Pir toxin bearing
plasmid coding toxins (PirA and PirB). Classic antibiotic treatments have been used for the control of infections,
however, the frequent use of antibiotics has led to the emergence of antibiotic-resistant bacterial strains of public health concern. The present study aimed to investigate the potential use of synthetic herbal-based polyphenols compounds as natural, eco-friendly antimicrobial agents for disease control. Polyphenols and their active ingredients were investigated against 96 VP isolates retrieved from Pacific whiteleg shrimp (Penaeus vannamei) in
Thailand and compared with various commercial antibiotics using a broth microdilution method, time-kill assay,
and scanning electron microscope (SEM). All VP isolates were identified using conventional bacteriology and
molecular-based tools. Bacteriological examination revealed that all isolates harbored toxR gene with specific
amplicons of 368 bp. Only 56 isolates were positive to AP3, a toxin-encoding gene of AHPND-VP with a fragment
size of 336 bp. In terms of susceptibility, all isolates were entirely resistant to ampicillin and amoxicillin, whereas they showed different susceptibility patterns to the other tested antibiotics. Among the polyphenols, there was no apparent activity of syringic acid and rutin, while vanillic acid showed higher bactericidal activity against VP
isolates at MICs and MBCs of 1024–2048 μg mL− 1
. Pyrogallol exhibited the strongest antibacterial activity even at very low concentrations with a MIC and MBC of 32–256 μg mL− 1 , and the majority of cells showed disruption
and an indistinguishable cell wall structure by SEM analysis. The present study offers pyrogallol as a potential
synthetic herbal-based antimicrobial agent for the adoption of inventory management practices and control of
V. parahemolyticus causing AHPND in shrimp producing sectors.