Both cell free filtrates and extracts of disrupted cells of the pseudomonads

Both cell free filtrates and extracts of disrupted

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in liquid and on solid culture media. Both cell-free filtrates and extracts of disrupted cells of the pseudomonads caused the inhibition, beheved to be due to antimicrobial activity. Dubois-Darnaudpeys (1977a) also examined the effects of the bacterial flora commonly occurring in surface water, such as the Pseudomonas-Achromobacter and Flavobacterium groups, on the survival and growth of Aer. salmonicida. She found that the pathogen was inhibited
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Epizootiology: Gram-negative bacteria 255 at all temperatures if the experiments were run in the presence of the other bac- teria. The abihty of other micro-organisms, e.g. Acinetobacter, Aer. hydrophila, Chromobacterium, Esch. coli, Flavobacterium and Pseudomonas, and their metabo- lites, to inhibit the survival of Aer. salmonicida in non-sterile seawater was also reported by Effendi and Austin (1991). Thus, because the isolation and study of the viability of the pathogen is complicated by competition and inhibition by other organisms, it is not surprising that investigations into survival of the pathogen, which could help to establish whether or not it is capable of a free-living existence outside of fish, have invariably retreated to laboratory-based experiments using sterilised water. A filtration method tested by Maheshkumar et al. (1990) attempted, with some degree of success, to overcome the difficulties of isolation of Aer. salmonicida posed by the presence of other bacteria, i.e. the possible overgrowth by small numbers of cells of the pathogen. In their studies, up to 51 of hatchery water was seeded with Aer. salmonicida and passed through I - ^ D S electropositive filters. This technique was used in combination with removal of the filters after backwashing, soaking them in a small volume of 3% beef extract solution, and followed by the scraping of the filters to remove trapped bacteria. When all the eluates from the filters were com- bined, recovery o^ Aer. salmonicida was determined to be 35%. Thus, Maheshkumar et al. (1990) concluded that the filtration technique demonstrated greater sensitivity than the direct examination of water. Also, the enumeration o^ Aer. salmonicida was not overly affected by the presence of other bacteria because in the water samples the pathogen retained the ability to produce a brown pigment. However, these authors noted that biochemical/serological tests would be necessary for the detection of atypical non-pigmenting isolates. Nonetheless, the classic dilemma of many eco- logical studies persists, i.e. how does the response of an organism in laboratory-based experiments relate to its performance in the natural environment where it needs to interact in a diverse and heterogeneous community? It is a problem still in search of a rehable solution. It has often been stated that ecological investigations of Aer. salmonicida are hampered by the lack of an effective selective isolation medium specifically formu- lated for the pathogen. It was, for example, the opinion of Cornick et al. (1969) that the development of a selective medium for Aer. salmonicida could quite possibly change the present views on the habitat and viability of the pathogen. McCarthy
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