Inducible Gene Expression by Nonculturable Bacteria in Milk after Pasteurization |
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Authors: | Thusitha S. Gunasekera, Anders S rensen, Paul V. Attfield, S ren J. S rensen, Duncan A. Veal |
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Affiliation: | Thusitha S. Gunasekera, Anders Sørensen, Paul V. Attfield, Søren J. Sørensen, and Duncan A. Veal |
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Abstract: | The viability of bacteria in milk after heat treatments was assessed by using three different viability indicators: (i) CFU on plate count agar, (ii) de novo expression of a gfp reporter gene, and (iii) membrane integrity based on propidium iodide exclusion. In commercially available pasteurized milk, direct viable counts, based on dye exclusion, were significantly (P < 0.05) higher than viable cell counts determined from CFU, suggesting that a significant subpopulation of cells in pasteurized milk are viable but nonculturable. Heating milk at 63.5°C for 30 min resulted in a >4-log-unit reduction in the number of CFU of Escherichia coli and Pseudomonas putida that were marked with lac-inducible gfp. However, the reduction in the number of gfp-expressing cells of both organisms under the same conditions was <2.5 log units. These results demonstrate that a substantial portion of cells rendered incapable of forming colonies by heat treatment are metabolically active and are able to transcribe and translate genes de novo. |
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