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Applied and Environmental Microbiology, July 2003, p. 4214-4218, Vol. 69, No. 7
0099-2240/03/$08.00+0     DOI: 10.1128/AEM.69.7.4214-4218.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Microplate Bioassay for Nisin in Foods, Based on Nisin-Induced Green Fluorescent Protein Fluorescence

J. Reunanen and P. E. J. Saris*

Applied Chemistry and Microbiology, University of Helsinki, Helsinki, Finland

Received 22 October 2002/ Accepted 7 April 2003

A plasmid coding for the nisin two-component regulatory proteins, NisK and NisR, was constructed; in this plasmid a gfp gene (encoding the green fluorescent protein) was placed under control of the nisin-inducible nisF promoter. The plasmid was transformed into non-nisin-producing Lactococcus lactis strain MG1614. The new strain could sense extracellular nisin and transduce it to green fluorescent protein fluorescence. The amount of fluorescence was dependent on the nisin concentration, and it could be measured easily. By using this strain, an assay for quantification of nisin was developed. With this method it was possible to measure as little as 2.5 ng of pure nisin per ml in culture supernatant, 45 ng of nisin per ml in milk, 0.9 µg of nisin in cheese, and 1 µg of nisin per ml in salad dressings.

* Corresponding author. Mailing address: Applied Chemistry and Microbiology, P.O. Box 56, FIN-00014 University of Helsinki, Finland. Phone: 358 9 19159369. Fax: 358 9 19159322. E-mail: per.saris{at}helsinski.fi.

Applied and Environmental Microbiology, July 2003, p. 4214-4218, Vol. 69, No. 7
0099-2240/03/$08.00+0     DOI: 10.1128/AEM.69.7.4214-4218.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.



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