Survival of 'atypical' Aeromonas salmonicida in water and sediment microcosms of different salinities and tem~eratures

The survival of 'atypical', cytochrome oxidase-negative Aeromonas salmonicida in microcosms containing sterilized water and sediment was assessed using the plate count method. The effect of sal~nity, temperature and presence of sediment was investigated on the survival of 2 strains isolated from ulcerated flounder Platjcl~thys flesus from the northern Baltic Sea. The strains survived for only a short period (< 14 d) when inoculated into microcosms containing only sterlhzed brackish water (S = 6 4 + 0.5%0). When sterile sediment was added to the microcosms containing brackish water, the inoculated strains survived for up to 63 d . In microcosms wlth sterile sediment and fresh water or salt water (S = 30%), the strains survived for a considerably shorter period than in microcosms containing sterile bracklsh water and sediment. In sterilized water, the strains showed a better survival at 4°C than at 15'C. In water and sediment, a slightly increased survival occurred at 15'C. Adding nutnents (peptone) to the microcosms, when bacteria no longer were recovered by culture methods, did not resuscitate possible non-culturable or dormant cells. These results suggest that 'atypical' A. salmonicida, shed from ulcers of diseased flounders, may survive in the bottom sediment of brackish water environments for a long time. The sediment can thus act as a reservoir for this pathogen, facilitating the spread of the disease.