Diseases of aquatic organisms, 2011, vol. 96, n. 2
Variations in the number and diversity of bacteria from the skin of brown trout Salmo
trutta L. and rainbow trout Oncorhynchus mykiss Walbaum were surveyed from different rivers
and fish farms in northern Spain. In addition to determining bacterial populations in skin samples
of healthy fish, bacterial populations were determined from skin lesions (of brown trout only)
infected with Saprolegnia parasitica, the causal agent of saprolegniosis. Mean bacterial counts
from skin lesions of brown trout suffering from saprolegniosis were nearly 1000 times greater than
from the skin of uninfected brown and rainbow trout. More than 20 different genera of bacteria
were identified, with isolates of Aeromonas and Iodobacter being the predominant genera associated
with saprolegniosis lesions. The in vitro inhibitory activity of 72 of these skin isolates was
tested against S. parasitica using 3 different assays. These included (1) assessing the inhibition by
bacteria of colony growth on agar media, (2) the inhibition of colony growth from colonized hemp
seeds in liquid media and (3) the inhibition of cyst germination in liquid media. Finally, the fungicidal
effect of the 24 most inhibitory bacterial species, and the inhibitory activity of their culture
supernatants, was tested in the same way. Isolates identified as Aeromonas piscicola, A. sobria,
Pantoea agglomerans and Pseudomonas fluorescens achieved the highest inhibition against S.
parasitica. Many of these inhibitory isolates were obtained primarily from skin lesions of fish with
saprolegniosis. It is suggested that some of these isolates might be useful in the biological control
of saprolegniosis.
