Abstract

Pongsak Rattanachaikunsopon* and Parichat Phumkhachorn

Carvacrol, a major compound naturally present in oregano and thyme and its precursor, cymene, were tested in vitro for their antimicrobial activity against Edwardsiella tarda, a fish pathogenic bacteria causing edwardsiellosis. When used alone, carvacrol, but not cymene, inhibited the bacterium with the minimal inhibitory concentration (MIC) of 20 ppm. However, cymene was shown to be able to enhance the inhibitory ability of carvacol indicated by the reduction of MIC to 5 ppm when used with 2.5 ppm of cymene. Based on mortalities in 2 weeks after intraperitoneal E. tarda injection, the median lethal dose (LD50) of E. tarda for tilapia (Oreochromis niloticus) was 5.0 × 10 2 CFU/g of fish. Fish diets supplemented with carvacrol and cymene were also tested for their protective effect against E. tarda infection in tilapia. The results showed that carvacrol (200 ppm) but not cymene (200 ppm) when fed prophylactically decreased the mortality in E. tarda-challenged tilapia. However, carvacrol at the same concentration could cause no mortality of E. tarda infected fish when used in conjunction with 200 ppm of cymene.