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The effect of growth temperature and growth rate on the susceptibility to heat and pH stress were investigated in Listeria monocytogenes grown in continuous culture where these two growth variables could be varied independently of each other, and in batch culture. After growth at 30 degrees C or 10 degrees C at constant growth rate, or at 30 degrees C at different growth rates, cells did not differ in their resistance to heat at 55 degrees C. Cells grown at 30 degrees C were more resistant to acid stress at pH 2.5 than cells grown at the same growth rates at 10 degrees C. Cells grown at low growth rate at 30 degrees C gave greater resistance to acid stress than those grown at high growth rate. Growth temperature and growth rate had independent effects on the susceptibility of L. monocytogenes to acid stress conditions. This may have implications for the survival of L. monocytogenes in acidic foods.

Two methods for the successful extraction of DNA from foods are described. The rapid lysis method uses a proteinase K buffer system to lyse cells and solubilize food samples. DNA is then precipitated using isopropanol. The second method achieves cell lysis using toluene and mutanolysin, and solubilization using guanidium thiocyanate. Following protein removal with organic solvents DNA is precipitated with isopropanol. Both methods enabled the polymerase chain reaction to be applied directly to DNA extracted from samples of cheese, coleslaw and raw chicken and allowed the direct rapid, sensitive and specific detection of Yersinia enterocolitica, Aerococcus viridans and Listeria monocytogenes in these foods.

Using a novel capillary thermal cycler, randomly amplified polymorphic DNA (RAPD) generated DNA fingerprints were obtained in 3 h. The RAPD profiles were produced using a random 10-mer primer (5'-ACCGCCTGCT-3') which discriminated between different Listeria spp. Unique fragment profiles of Listeria monocytogenes serotypes were produced from serotypes 1a, 2, 3a, 4ab, 4a and 4c but serotypes, 1/2a, 4b, 4d and 7 had similar profiles.

The polymerase chain reaction was used to amplify specifically a 448 base pair internal fragment of the flaA gene from bacteria belonging to the genus Listeria. This gene encodes for the flagellin protein and allowed specific genus discrimination between other commonly found Gram-positive and Gram-negative bacteria and all Listeria spp. with the exception of Jonesia denitrificans.

An amperometric immunosensor specific to the protein A of Staphylococcus aureus, was developed using the direct electrochemical detection of phenol produced by alkaline phosphatase from phenylphosphate. The immunosensor could detect protein A at 0.01 ng/ml and could reliably detect and quantify pure cultures of protein A-bearing Staph. aureus above 10(3) cfu/ml. A similar sensitivity of detection was obtained with samples of beef and milk.