In vitro antagonism of the probiont Pseudomonas fluorescens strain AH2 against Aeromonas salmonicida does not confer protection of salmon against furunculosis

Pseudomonas fluorescens strain AH2 which acts as a probiont in rainbow trout reducing vibriosis-caused mortality has a strong in vitro antagonism against Aeromonas salmonicida. Strain AH2 inhibited the growth of A. salmonicida in defined glucose-casamino acid media in both agar-well-diffusion assays and in broth cultures. The inhibition was significantly enhanced by iron limited conditions as compared to iron surplus conditions (0.1 mM). The possible probiotic activity of strain AH2 against furunculosis in salmon (Salmo salar L.) infected by co-habitant infection (10% or 20% co-habitants each i.p. infected with 10(4) cfu) with A. salmonicida was investigated with three levels of probiont added to the tank water: (a) 10(5) cfu/ml added three times every other day, (b) 10(5) cfu/ml added three times per day, and (c) 10(6) cfu/ml added twice per day. With a water flow of 0.8 l/kg/min in tanks of 145 l, the levels of probiont were estimated to 10(3)-10(5) cfu/ml during days of addition in experiments (a) and (b) both with 2 kg fish diluting to < 50 cfu/ml overnight. In experiment (c) with 2.5 kg fish, levels of probiont were estimated to 10(3)-10(6) cfu/ml during day time diluting to < 50 cfu/ml overnight. Go-habitants died within 3-5 days after infection and furunculosis appeared in non-co-habitants at day 11 after start of the trial. Accumulated mortality stabilised at approx. 70-80% after 3 weeks. In neither of the trials did the treatment with P. fluorescens result in any effect on furunculosis-related mortality. The difference in in vivo effect in the vibriosis-rainbow trout system as compared with the furunculosis-salmon system shows that the selection and application of probiotic cultures must be tested for each individual host-pathogen combination and any in vivo activity cannot be predicted based on in vitro testing. (C) 2001 Elsevier Science B.V. All rights reserved.