1. 1Microbiological Chemistry Research Laboratory, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, U.K.
2. 2Microbiological Chemistry Research Laboratory, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, U.K.
3. 3Microbiological Chemistry Research Laboratory, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, U.K.