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Fresh trout fillets and salmon slices packed in trays were obtained from two multinational chain supermarkets and evaluated for freshness and bacteriological quality immediately after packaging and during storage at 3 degrees C. Initial aerobic counts at 30 and 25 degrees C were significantly (P < 0.05) lower in trout fillets (5.27 +/- 0.57 and 4.87 +/- 0.80 log CFU/g, respectively) than in salmon slices, where levels in excess of 6 log CFU/g were found. In both products, initial Enterobacteriaceae counts were slightly higher than 3 log CFU/g and increased significantly during shelf life by approximately 3 log CFU/g. Most of the enterobacteria were identified as Citrobacter freundii, Hafnia alvei, and Enterobacter cloacae. On day 0, most probable number (MPN) counts of total and fecal coliforms were not significantly different, numbers of the latter group being approximately 4 MPN/g. Escherichia coli was only detected when fish was spoiled. Although initial presumptive Staphylococcus aureus counts were approximately 3 log CFU/g, only 4 of 84 selected colonies belonged to this species. Neither Salmonella nor antimicrobial residues were detected in any sample. Ethanol content in salmon slices did not significantly (P > 0.05) increase until they became inedible. Significant correlation (r = +0.72, P < 0.05) was observed between this chemical index and viable counts at 30 degrees C only when salmon slices were inedible. Trout fillets were acceptable for 7 days, and salmon slices showed signs of spoilage after 4 days. Although public health concerns associated with packed trout and salmon appear to be minimal, data on sensory quality, shelf life, and total viable and Enterobacteriaceae counts strongly suggest the need to improve the quality control systems used by European multinational retailers, especially for imported salmon.

Aims: Development of a PCR assay for detection of aeromonads carrying the hlyA and ⁄ or aerA genes in fish. Methods and Results: The protocol involves an overnight selective enrichment step in tryptic soy broth yeast extract containing 10 lg ml -1 of ampicillin followed by extraction of DNA and PCR amplification of two haemolysin genes that contribute to the virulence of Aer. hydrophila. This procedure can detect initial populations of 1-10 cfu g )1 within 24 h in artificially contaminated samples. In naturally contaminated fish, both genes were detected in 13 out of 14 fresh fish lots (aeromonads levels between < 1 and 5AE42 log cfu g )1 ) and in 4 out of 16 lots of vacuum-packed cold-smoked fish (aeromonads levels between < 1 and 3AE37 log cfu g )1 ). Before enrichment, dominant species were Aer. hydrophila HG1 (aerA + hlyA + ), Aer. bestiarum HG2 (aerA + hlyA + ) and Aer. caviae HG4 (aerA -hlyA -). After enrichment, Aer. hydrophila HG1 (aerA + hlyA + ) was dominant. Conclusions: Fresh fish and even smoked fish carry hlyA + and ⁄ or aerA + aeromonads that can be detected by PCR within 24 h. Significance and Impact of the Study: The PCR assay described offers considerable potential as a rapid method with specificity, sensitivity and simplicity.

Expression of hemolytic and proteolytic activities throughout the growth cycle was investigated with two enterotoxic aeromonad strains assigned to the species Aeromonas hydrophila and Aeromonas veronii biovar sobria. Although growth kinetic data were dependent on strain, temperature, and substrate, maximum populations attained were higher than 9 log CFU/ml in aerated tryptone soya broth plus yeast extract (TSBYE) and salmon extract within the range 4 to 28 degrees C. For both strains in TSBYE, variable amounts of hemolytic activity were first detected at any temperature when aeromonad counts were over 9 log CFU/ml. Afterwards, this activity increased up to similar levels (109 to 112 hemolytic units per ml) without a significant increase in populations. Salmon extract supported hemolysin synthesis at 28 but not 4 degrees C. Proteolytic activity of the A. hydrophila strain was only expressed in salmon extract at 28 degrees C, whereas A. veronii biovar sobria did at 28 degrees C in both substrates and at 10 degrees C in TSBYE.