Drug resistance of bacteria isolated from the intestine of yellowtail cultured with feeds containing various drugs

In order to ascertain whether the use of drugs as feed additive for fish ma y cause drug resistance of bacteria in the intestinal tract, viable counts and bacterial flora in the intestine of yellowtail cultivated with the feed s in the presence or absence of various drugs, drug resistance of the flora were determined. Viable counts in the intestine were lowered remarkably by the administration of each drug, suggesting that bacterial growth in the i ntestinal tract was inhibited by various drugs. Bacterial flora in the inte stine consisted mainly of coryneform bacteria, Staphylococcus and Bacillus regardless of whether drugs were administered or not. In these floras, sign ificant differences could not be observed for their resistance to respectiv e drugs among the isolates from fish cultivated with feeds containing or no t containing each drug. In addition, these floras mainly showed resistance to various combinations comprising from one to seven drugs; moreover, the m ajority of resistant strains were multidrug-resistant.