Aeromonas salmonicida survival studies The survival of Aer salmonicida in water

Aeromonas salmonicida survival studies the survival

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Aeromonas salmonicida survival studies The survival of Aer. salmonicida in water has been thoroughly examined by numerous investigators (WilHamson, 1929; Smith, 1962; Lund, 1967; McCarthy, 1980; Sakai, 1986a, b; Rose et al., 1990a, b; Morgan et al., 1991; Effendi and Austin, 1991, 1994; Table 8.1). Unfortunately, caution must be used in the interpretation of some of the
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Epizootiology: Gram-negative bacteria 249 data as many of the studies employed pre-sterilised water or types of water in which Aer. salmonicida would not normally be present, e.g. distilled or tap water. Thus, the information gleaned from such studies does not necessarily reflect the behaviour of the pathogen in the natural aquatic environment. However, enough work has been done to allow tentative conclusions to be drawn. Based on the survival data accu- mulated, it appears that Aer. salmonicida is capable of surviving for a prolonged period in fresh, brackish and seawater, although contradictory results as to the exact time interval involved abound. Thus, for unsterilised freshwater, including river water, recovery of the pathogen from as Httle as 24 h to as long as 19 days had been reported by different studies. Survival in unsteriHsed brackish water was between 16 and 25 days (Smith, 1962; McCarthy, 1980). In unsteriHsed seawater, the organism could be recovered from between 24 h and 8 days (McCarthy, 1980). If steriHsed water samples were used, survival time in the absence of competing—antagonistic— organisms was invariably greatly increased. For example, survival times in freshwater of up to 63 days were reported (Cornick et al, 1969), and in seawater up to 24 days (Lund, 1967). Using whole cells and DNA released into lake water micocosms, with media and PCR for detection, Deere et al. (1996a) cultured Aer. salmonicida for <4 weeks, but found the DNA remained intact for >13 weeks. This discrepancy between the results of culturing and other techniques opens a veritable Pandora's Box. Why should intact DNA be found 2+ months after culturing techniques indicated that the population of Aer. salmonicida had disappeared? Using a laboratory-based microcosm and culturing, direct counts, respiratory activity (the reduction of tetrazoliums to coloured formazans; after Effendi and Austin, 1993), iFAT, epifluorescence microscopy and the direct viable count tech- niques (by incorporating yeast extract and naUdixic acid; after Kogure et al, 1979), Effendi and Austin (1994) confirmed the emerging view that cells o^ Aer. salmonicida remained after plate counts reached zero. These workers determined that survival was maximal in brackish conditions, i.e. saHnity = 25%o, notably on substrates—especially on wood but also in sediment—rather than in the water column. Similarly, using an oxidase-negative atypical isolate, sterilised microcosms and culturing techniques, Wiklund (1995a) deduced that survival was better at 4°C than 15°C in brackish rather than sea or freshwater, and in the presence of particulates, i.e. sand. The addition of nutrients did not resuscitate cells after colony counts decHned to zero.
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