Probiotics, which are micro-organisms or their products with health benefit to the host, have been used in aquaculture as a means of diseases control, supplemnting or even in some cases replacing the use of antimicrobial compounds.
In the present study, The physiological and the biochemical characters of suspected probiotic bacterial isolates were identified as Micrococcus luteus and Psuedomonas sp. as identified by Bergey et al. (1984) and Austin and Austin (1993). A wide range of Gram-positive (Bacillus, Carnobacterium, Enterococcus, Lactococcus, Lactobacillus, Micrococcus and Streptococcus) and Gram-negative bacteria (Aeromonas, Alteromonas, Photorhodobacterium, Pseudomonas and Vibrio) has been evaluated as probiotics in aquaculture (Irianto and Austin 2002a).
M. luteus and Psuedomonas sp. showed inhibitory effects in vitro against A hydrophila. However, Psuedomonas sp. gave larger inhibition zone (9 cm) than M luteus (4cm). M. luteus and Psuedomonas sp. were isolated from gonads and intestine, respectively, of apparently healthy O. niloticus, but Irianto and Austin (2002b) isolated M. luteus A1-6 from digestive tract of Oncorhychus mykiss. Sugita et al. (1998) isolated M. luteus. and Psuedomonas sp from the intestine of seven fish species and recorded that the bacteria had inhibitory effect against Vibrio vulnificus. They isolated Alteromonas haloplanktis from gonads of Argopecten purpuratus broadstock. Riquelme et al. (1996) noticed that Alteromonas haloplanktis had inhibitory activity against Vibrio sp and A. hydrophila. Lewus et al. (1991) reported that the bacteriocins which produced by lactic acid bacteria had inhibitory effect against A. hydrophila. Also, they isolated Pseudomonas (C30, 217) from broadstock. Spanggaard et al. (2001) isolated Psuedomonas sp. from O. mykiss and used as probiotics in O. mykiss mixed in water. Smith and Davey (1993) reported that P. flurescence reduced diseases caused by A. salmonicida.
From the present study M. luteus had a probiotic effect in vitro and in vivo against A. hydrophila, while Psuedomonas sp had a probiotic effect in vitro only and in vivo it changed to pathogenic bacteria. Our results agree with Chang and Liu (2002b) who indicated that Bacillus toyoi suppressed the growth of Edwarsilla tarda in vitro, but did not reduce mortalities in eels due to edwardsillosis in vivo. Irianto and Austin (2002) used M. luteus with feed as a potential combating A. salmonicida infection in rainbow trout (O. mykiss).
M. luteus gave mortality rate 25% at 107 cells / g of feed among O. niloticus challenged by A. hydrophila. Lactobacillus rhamnosus was administered at a dose of 109 and 1012 cells / g of feed to rainbow trout for 51 days and reduced mortalities from 52.6 to 18.9 % (109 cells / g feed) and to 46.3% (1012 cells / g of food) following challenge with A. salmonicida (Nikoskelainen et al., 2001). The mortality rate between O. niloticus which fed on a diet containing Psuedomonas sp was 90% which did not agree with Gram et al. (1999) who reported that P. fluorescens strain (AH2) reduced the mortality of 40 g rainbow trout infected with a pathogenic V. anguillarum. The mortality rate at the treated fish was 32% compeared to control 47%.
The mode of action of the probiotics is rarely investigated, but possibilities include competitive exclusion, i.e. the probiotics actively inhibit the colonization of potential pathogens in the digestive tract by antibiosis or by competition for nutrients and / or space, alteration of microbial metabolism, and/or by the stimulation of host immunity (Irianto and Austin, 2002a).