Fish Diseases are major problem for the fish farming industry, which currently is the fastest growing food-protein producing sector with an annual increase of approximately 9%, among those, bacterial infections are considered as the major cause of mortality in fish hatcheries (Grisez and Ollevier, 1995). The motile aeromonas group, especially A. hydrophila, affects a wide variety of freshwater fish species and occasionally marine fish (Larsen and Jensen, 1977). The prophylactic and therapeutic control of the bacterial diseases is based on oral administration of antibiotics. However, such treatment may cause the development of resistant bacteria (Aoki et al., 1985), yeild residues in fish and introduce potential hazard to public health and to the environment. Furthermore, the normal microbial flora in the digestive tract, which is beneficial to fish, may also be killed or inhibited due to oral chemotherapy (Sugita et al., 1991). Although vaccines are being developed and marketed, they cannot be used alone as a universal disease control measure in aquaculture (Amábile-Cuevas et al., 1995). A new approach method, that is gaining acceptance within the industry, is the use of probiotic bacteria to control potential pathogens (Gomez-Gil et al., 2000, Robertson et al., 2000). In recent years, there is a great interest in the use of probiotic bacteria in aquaculture to improve disease resistance, water quality and/or growth of farmed fish (Verschuere et al., 2000). Probiotic bacteria have a posible competition for nutrients with pathogens in the digestive tract (Gatesovpe, 1997) or the hypothetical stimulation of the immune system, as the activation of macrophage (Perdigón et al., 1990). The antibacterial effect of bacteria is generally due to either singly or in combination, production of antibiotics, bacteriocins, siderophores, lysozymes and proteases, and alteration of pH values by organic acids production (Sugita et al., 1998). The use of probiotics stimulates rainbow trout immunity by stimulating phagocytic activity, complement mediated bacterial killing and immunoglobulin (Ig) production (Nikoskelainen et al., 2003). Therefore, the present study was carried out to evaluate the role of M luteus and Psuedomonas sp. as growth promoters and antibacterials for Nile tilapia fry (O. niloticus) and their effect on some physiological parameters.
MATERIALS AND METHODS
Healthy O. niloticus (2.33- 2.45g) were randomly collected from the earthen pond at the Central Laboratory for Aquaculture Research (CLAR), Abbassa, Abou-Hammad, Sharkia Governorate, Egypt. Bacteriological examinations of the collected fish were done, where samples from the internal organs (liver, kidney, gonads, stomach and intestine) and gills were cultured on TSB and incubated at 30°C for 24-48 hours. Purification and identification of the isolates were done using biochemical tests according to Bergey et al. (1984), Austin and Austin (1993) and API 20 E strip system (Bio Merieux).
The isolated bacteria from the internal organs of the investigated fish (intestine, stomach and gonads) were examined for its inhibitory effects against the pathogenic A. hydrophila obtained from CLAR-Fish Diseases Department. The in vitro probiotic activity was done using agar diffusion method and the inhibition zoon determined as described by Ruiz et al., (1996).