Diseases of aquatic organisms, 2013, vol. 104, n. 1
Biological control of saprolegniosis with bacteria might be an alternative to the use
of chemical compounds. Among criteria for the selection of such bacteria are their absence of
pathogenicity to fish and their ability to prevent adhesion of the pathogen to the skin mucus. The
pathogenicity to rainbow trout of 21 bacterial isolates with in vitro inhibitory activity against
Saprolegnia parasitica was studied. Fifteen of the isolates, identified as Aeromonas sobria, Pantoea
agglomerans, Pseudomonas fluorescens, Serratia fonticola, Xanthomonas retroflexus and
Yersinia kristensenii, were non-pathogenic when injected into rainbow trout. Their capacity to
adhere to the skin mucus of male and female brown trout and to reduce the adhesion of S. parasitica
cysts under exclusion, competition and displacement conditions was tested. The 15 bacterial
isolates showed a low adhesion rate, ranging between 1.7% (for an A. sobria isolate) and 15.3%
(a P. fluorescens isolate). This adhesion was greater in the case of mucus from male brown trout
than from females. Similarities in the adhesion to male mucus and other substrates and correlation
to that observed to polystyrene suggest that adhesion to skin mucus does not depend on the substrate.
A high percentage (88.9%) of the S. parasitica cysts adhered to the skin mucus of male
brown trout. Almost all of the bacteria reduced this adhesion ratio significantly under exclusion
and competition conditions. However, only half of the isolates displaced cysts from skin mucus,
and more bacterial cells were necessary for this effect. A novel method to study the adhesion of S.
parasitica cysts to skin mucus of trout and their interactions with inhibitory bacteria is described.
