Inducible gene expression by nonculturable bacteria in milk after pasteurization.
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 degrees 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.
Gunasekera
Thusitha S
TS
Centre for Fluorimetric Applications in Biotechnology, Department of Biological Sciences, Macquarie University, Sydney, New South Wales 2109, Australia. tgunasek@rna.bio.mq.edu.au
Sørensen
Anders
A
Attfield
Paul V
PV
Sørensen
Søren J
SJ
Veal
Duncan A
DA
D000818:Animals; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D015169:Colony Count, Microbial; D004926:Escherichia coli / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D015870:Gene Expression; D049452:Green Fluorescent Proteins; D008164:Luminescent Proteins / Q000235:genetics / Q000378:metabolism; D008892:Milk / Q000382:microbiology*; D011419:Propidium / Q000378:metabolism; D016958:Pseudomonas putida / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D013242:Sterilization / Q000379:methods*
D001426:Bacterial Proteins; D008164:Luminescent Proteins; D049452:Green Fluorescent Proteins; D011419:Propidium