Characterization of Aeromonas, Enterobacteriaceae, Pseudomonas, Bacillus and Lactobacillus spontaneously growing to high numbers in milk, minced meat, fish or cheese

Sammanfattning: The spoilage flora of refrigerated raw and pasteurized milk was classified and identified by numerical phenotypic analysis. Isolations of bacteria were made at a defined spoilage stage, when the total aerobic count was 106-107 cfu per ml. The secondary flora of ripened Swedish and Norwegian cheeses was investigated by numerical analysis. Aeromonas and Enterobacteriaceae spp. isolated from the spoiling stage of refrigerated milk, fish and minced meat were identified by numerical methods and by a commercial identification system (Biolog). Enterobacteriaceae isolates were also classified using RAPD analysis. Aeromonas isolates were tested for possible virulence factors and Enterobacteriaceae isolates for the possession of genes coding for E.coli toxins and virulence factors. Gram-negative organisms spoiled 100% of raw milk samples, and 65% and 50% of pasteurized milk products stored at 5ºC and 7ºC, respectively. Five taxa of Pseudomonas were found to be predominate and it is suggested that recontamination of pasteurized milk originates from the raw milk flora. Bacillus cereus was isolated from 21% of pasteurized milk samples stored at 7ºC but was never recovered from milk stored at 5ºC. The average shelf-life of pasteurized milk was prolonged from 9.9 d to 16.6 d by lowering the temperature from 7ºC to 5ºC. The secondary flora of ripened cheeses was found to be dominated by phenotypic groups designated "Lactobacillus paracasei/casei complex". A cluster identified as Lactococcus lactis, was found to be related to cheeses of adequate quality. Aeromonas spp., especially A.hydrophila, were found in high numbers in 69% of fishes, 7.5 % of minced meat samples and 1.2% of pasteurized milk samples. Some of the food isolates clustered closely with clinical strains. Enterobacteriaceae spp. were found in high numbers in 100% of minced meat samples, 31% of fishes, and 6% of pasteurized milk samples. The predominating species were Hafnia alvei, Rahnella aquatilis and Serratia liquefaciens. The RAPD profiles were found to facilitate the identification of H.alvei isolates. E.coli st1 gene was found in four isolates of H.alvei, lt gene in one isolate of R.aquatilis, vt1 and eaeA in one of the st1 positive isolates and vt2 in two isolates of S.liquefaciens.