Growth of Listeria monocytogenes at pH 5·0 did not increase growth of the organism at pH 7·0 after exposure to low pH (3·0, 3·5), compared with cells initially grown at pH 7·0. However, growth at pH 5·0 significantly increased the survival of cells at low pH as determined by plate counts compared with cells grown at neutral pH. Thus, pH adaptation not only occurs in enteric bacteria but also in this Gram‐positive organism. Alterations in the cytoplasmic membrane could be responsible.

Lectins from Helix pomatia, Canavalia ensiformis, Agaricus bisporus and Triticum vulgaris agglutinated cultures of Staphylococcus aureus, Escherichia coli, Listeria and Salmonella spp. This agglutination was specific as it was inhibited (except with A. bisporus lectin) by the competing sugar substrates. The ability of three of these lectins, immobilized on a variety of supports, to separate these micro-organisms from pure cultures was investigated. Immobilization of the lectins on magnetic microspheres was the most effective method. Immobilized T. vulgaris lectin bound 87-100% of cells from cultures of L. monocytogenes, 80-100% of Staph. aureus, 33-45% of Salmonella spp. and 42-77% of E. coli. The A. bisporus lectin bound 31-63% of cells in cultures of L. monocytogenes, 83% of Staph. aureus but only 3-5% of the salmonella cells. Similarly H. pomatia lectin bound greater than 92% of Staph. aureus and 64% of L. monocytogenes cells but was poor at binding the Gram-negative organisms. This preference for binding Gram-positive organisms was confirmed when mixed cultures were studied. The T. vulgaris lectin was effective in removing L. monocytogenes (43%) and Staph. aureus (26%) from diluted milk and Salmonella (31-54%) from raw egg. Agaricus bisporus lectin removed L. monocytogenes from undiluted milk (10-47%) or ground beef (32-50%).

The concentrations of intracellular solutes in Listeria nwnocytogenes were examined in cells grown at various concentrations of NaCl. At 5% NaCl, cells contained elevated concentrations of potassium and glycine betaine compared with concentrations in cells grown without NaCl. At 7.5% NaCl, cells contained increased concentrations of K+, glycine betaine, glycine, alanine, and proline. Only glycine betaine, choline, or glycine promoted growth on a solidified defined medium containing 4% NaCl; there was no growth at higher concentrations of NaCl in the defined medium. Listenia monocytogenes is a gram-positive, rod-shaped bacterium of widespread distribution in the environment. It is able to live as an intracellular pathogen in humans and animals (6, 7, 10, 14). It has been isolated from a range of food samples, and there have been major outbreaks of listeriosis associated with contaminated foods. The organism is of concern to the food industry, particularly as it can survive and grow at refrigeration temperatures and high salt concentrations, e.g., up to 10% (18). This tolerance to osmotic stress suggests an adaptation of the organism to the osmotic environment. The adaptation of bacteria to a high-salt environment by the intracellular accumulation of compatible solutes has been extensively studied in gram-negative bacteria. Solutes commonly accumulated include potassium ions, amino acids, imino acids, quaternary ammonium compounds, and carbohydrates. The genetics and control of the uptake mechanism have been well characterized in Escherichia coli and Salmonella typhimurium, showing an integrated response involving accumulation of K+, glutamate, betaine, and trehalose, dependent on the nature of the environment and the degree of upshock (5). The physiological response of gram-positive bacteria to external osmotic potential has received considerably less attention. Streptomyces species accumulate proline, glutamine, and alanine because of increased synthesis when grown at enhanced salt concentrations (11). Staphylococcus aureus accumulates glutamine, proline, and betaine (1, 13, 19) in saline conditions. Proline accumulation has also been observed in Bacillus subtilis, but when betaine was present, this was accumulated in preference to proline (2, 21, 24). This paper describes the intracellular solutes accumulated in L. monocytogenes in response to increased salinity and osmotic stress. MATERIALS AND METHODS Growth of bacteria. L. monocytogenes was grown in coryneform broth containing (liter-') 10 g of tryptone (Dif

The polymerase chain reaction (PCR) was used selectively to amplify specific rDNA sequences of Carnobacterium divergens, C. mobile, C. piscicola and C. gallinarum in purified DNA extracts, crude cell lysates and food samples. The PCR products were visualized by agarose gel electrophoresis and identified, at species level, by hybridization reactions with three specific oligonucleotide probes for C. divergens, C. mobile and C. piscicola/C. gallinarum designed from 16S rRNA sequence data. The PCR was sufficiently sensitive to amplify DNA from a single bacterium to detectable levels after 30 cycles of amplification. Both radioactive (32P) and non-radioactive alkaline phosphatase labelled probes was able to detect the PCR products. Detection was highly specific and the probes did not hybridize with DNA samples from any other of the bacterial species tested. These methods enabled the rapid and specific detection and identification of carnobacteria from pure cultures and samples of meat.

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