Influence of pH, microbial strain and sample type, on the microbiological detection of oxolinic acid and oxitetraciclyne in fish

Abstract

Simultaneously with the development of the salmoniculture, comes forth the need to use antimicrobial drugs to control fish disease thus, increasing the risk of residues in the edible product. 
Detection of residues of oxolinic acid and oxitetracycline in fish, carried out using 75 units of Salmo salar. 
With an average weight of 2.7 Kg. Fish were treated during 10 days with the dosis recommended by the manufacturer, for both antibiotics. 
Once the treatment was finished, blood serum samples and muscle samples after killing the fish, were taken on days 1 to 5, 10, 15, 20 and 30 of the posttreatment period. Samples were kept frozen and submitted later to bacterial inhibition method, using B. subtilis BGA and E. coli ATCC 11303 as sensitive strains included in a nutritive culture media at pH 6,0 and 8,0. 
Both samples, muscle and serum of fishes, were placed in duplicate on each Petri dish with the different substrates. Residues of antibiotic were detected after an incubation period of 24 h at 35 ºC when an inhibitory zone of the test microorganism appeared. The inhibition zone was measured in milimeters and recorded. 
Results show that the oxolinic acid and oxitetraciclyne were detected better at pH 6.0, also B. subtilis BGA is more sensitive than E. coli ATCC 11303 to both drugs. Blood serum samples produce bigger inhibitory zones for both antibiotics than the muscle sample. Moreover better residue retention time for drugs was in blood serum.