The effect of antimicrobial compounds on the gastrointestinal microflora of rainbow trout, Salmo gairdneri Richardson

Populations of aerobic and anaerobic heterotrophic bacteria occurring in the gastrointestinal tract of healthy rainbow trout were estimated using a dilution plate technique. Data revealed a progressive decline in numbers of aerobic bacteria along the digestive tract from oesophagus to lower intestine. However, the highest numbers were recovered from the intestinal contents and faeces. Anaerobes were generally restricted to the upper intestine and intestinal contents. The aerobic component of the bacterial microflora from the digestive tract was equated with Acinetobacter calcoaceticus, Aeromonas hydrophila, Bacillus circulans, Bac. megaterium , coryneforms, Grampositive irregularly shaped rods, Flavobacterium sp., Kurthia sp., Microhacterium sp., Providencia stuartii, Pseudomonas spp., Ps.fluorescens and Ps. pseudoalcaligenes . Evidence from scanning electron microscopy pointed to a general lack of colonization of the gut wall: instead, microorganisms were abundant in the intestinal contents. Antimicrobial compounds, i.e. oxolinic acid, oxytetracycline and sulphafurazole (which are commonly used to combat infections by Gramnegative bacterial fish pathogens), caused an increase in bacterial numbers throughout the digestive tract, with maximal numbers in the lower intestine. The bacteria, comprising an essentially different range of taxa, were generally resistant to the antibiotics in use. Conversely, erythromycin and penicillin G, which are used to treat some diseases caused by Gram-positive bacteria, caused a rapid reduction in bacterial numbers within the gastrointestinal tract.