Antimicrobial properties of medicinal herbal extracts against pathogenic bacteria isolated from the infected grouper Epinephelus tauvina ARUNACHALAM PALAVESAM, LEELA SHEEJA and GRASIAN IMMANUEL* Marine Biotechnology Laboratory, Centre for Marine Science and Technology, M.S. University, Rajakkamangalam 629 502, Kanyakumari District, Tamilnadu, India Abstract A study was undertaken to investigate the antimicrobial properties of herbal extracts against bacterial pathogens isolated from infected grouper Epinephelus tauvina. The tissue and blood samples were aseptically collected from the infected grouper and the bacterial diversity was screened and identified up to species level. From the identified pathogens, bacterial species such as Streptococcus pyogenes, Staphylococcus aureus, Serratia salinaria, Alcaligen faecalis and Vibrio parahaemolyticus were selected for sensitivity study on the herbal compounds extracted from Withania somnifera, Tinospora cordifolia, Solanum xanthocarpum, Daemia extensa and Andrographis paniculata. The bacterial growth inhibitory effect was found to be dependant on the solvent used for extraction as well as specific to herbal extracts. The minimum inhibitory concentrations of selected herbal extracts were 50 ppm for W. somnifera in benzene on A. faecalis, 12.5 ppm for T. cordifolia in propanol on V. parahaemolyticus and 100 ppm for S. xanthocarpum in petroleum ether on S. salinaria. The study was also extended to find out the inhibitory effect of phenolic compounds of herbal extracts on bacterial pathogens and only W. somnifera showed a positive effect. The thin layer chromatography results showed that alkaloids such as piperine, concessine, codeine and morphine-like compounds may be responsible for the inhibitory effect of herbal products from T. cordifolia, S. xanthocarpum and A. paniculata.   < back .