Interest in Aeromonas hydrophila as a food-borne and human pathogen is increasing. Isolation media from the clinical laboratory were evaluated for food use and either did not give quantitative recovery of A. hydrophila or did not permit ready differentiation of A. hydrophila from the background microflora. A new medium was developed which permitted quantitative recovery of A. hydrophila from foods. The medium consisted of phenol red agar base (Difco Laboratories), soluble starch (10gliter), and ampicillin (10 mg/liter). All foods surveyed contained A. hydrophila. Foods sampled included red ieats, chicken, raw milk, and seafood (fish, shrimp, scallops, crab, and oysters). The count of A. hydrophila at the time of purchase ranged from 1 x 102/g (lower limit of detection) to 5 x 105/g. In most instances, the count of A. hydrophila increased during 1 week of storage at 5°C. The starch-ampicillin agar developed permitted rapid quantitative recovery ofA. hydrophila from foods in the presence of very large numbers of competing microflora.

The firefly (Photinus pyralis) luciferase (luc) gene on plasmid vector pBESTluc and the Aequorea victoria green fluorescent protein (gfp) gene on plasmid vector pGFP were introduced into strains of Escherichia coli O157:H7. The recombinant E. coli strains were indistinguishable from their parent strains in biochemical and immunological assays and in a multiplex PCR reaction. There was no significant difference in the growth kinetics of the luc-bearing recombinants and the parent strains. At 37°C all of the recombinant strains maintained the vectors and expressed luciferase and the green fluorescent protein when grown both with and without antibiotic selection. Individual colonies of luc-bearing E. coli strains were readily luminescent in the dark after being sprayed with a solution of 1 mM beetle luciferin. The recombinants containing pGFP emitted bright green fluorescence when excited with UV light and the addition of any other proteins, substrates, or cofactors was not required. The green fluorescent protein-expressing E. coli O157:H7 strains were used in studies examining the survival of the organism in apple cider and in orange juice. In apple cider the organism declined to undetectable levels in 24 days at refrigeration temperature while in orange juice the strains survived with only small decreases in number during the 24-day sampling period. These recombinant E. coli O157:H7 strains, containing readily identifiable and stable markers, could be useful as positive controls in microbial assays as well as in studies monitoring bacterial survival and the behavior of E. coli O157:H7 in foods and in a food processing environment.

Frankfurter quality was monitored to evaluate effects of frozen storage on meat components. After fresh meat controls were tested, beef, pork, and pork fat were frozen and stored at -17:8"C for l-37 wk. At 6-wk intervals, functional and quality tests were performed on thawed and control meat samples and on frankfurters made from the samples. Frozen storage significantly affected beef and pork lean (drip loss, % solids and N in drip, extractable protein, water binding, emulsifying capacity) and fat (beef-thiobarbituric acid and pork-peroxide values); frankfurters produced from these ingredients were also affected (cooking tests, penetration force, sensory panel scores).