Antagonistic effect of probiotic on pathogenic bacteria in rainbow trout (O. mykiss, W) of dam lake

Rainbow trout (Oncorhynchus mykiss) culture is an important business line conducted in fish farms on rivers, dam lakes and marine cages in our country and ın the world. But bacterial diseases of this fish due to mass production, stress and unsuitable environmental conditions can sometimes cause serious economic losses. As bacterial pathogens gain resistance, the use of probiotics instead of antibiotics in the control of these diseases gain importance. The aim of this MSc. thesis study is to determine the biochemical characteristics of pathogen bacteria by using conventional bacteriological methods recovered from rainbow trout cultured in Derbent Dam Lake, Samsun and molecular identification of these isolates with DNA isolation and PCR (Polymerase Chain Reaction) techniques. Also, in-vitro antagonistic effect of the probiotic bacteria Bacillus subtilis and Lactobacillus rhamnosus against pathogenic bacteria. During the 5 field sampling studies carried out between April and October 2017, 25 moribund rainbow trout samples were examined and bacteriological inoculations from their visceral organs such as liver, kidney and spleen were streaked onto TSA. Recovered bacteria were identified by using routine biochemical tests, rapid identification kits, 16S RNA sequencing and species-specific PCR methods. During the study, 47 Lactococcus garvieae, 10 Citrobacter freundii, 8 Hafnia alvei, 2 Bacillus gibsonii, 1 Frigoribacterium faeni and 1 Sphingomonas echinoides isolates were recovered and identified from the internal organs of moribound fish samples. Among the probiotic bacteria, B. subtilis showed antagonism against field isolate pathogenic bacteria namely L. garvieae, F. faeni, B. gibsonii and S. echinoides, but did not show antagonism against C. freundii and H. alvei. On the other hand, it was determined that the other probiotic L. rhamnosus has no antagonism against any of the pathogenic bacteria recovered from moribund fish samples. As a result, it was one more time revealed that the molecular methods are more sensitive than the routine biochemical methods in the identification of pathogens recovered from rainow trouts cultured in our country. With these results, the effects of probiotic bacteria such as B. subtilis on the pathogens of rainbow trout were revealed and the effect of the use of suitable probiotics on the disease prevention and control one more time. We predict that the increase in such applications, due to the decrease in the unconscious use of antibiotics in aquaculture sector, positive contribution on the fish, human and environment health will be provided.