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Journal of Clinical Microbiology, January 2005, p. 223-228, Vol. 43, No. 1
0095-1137/05/$08.00+0     doi:10.1128/JCM.43.1.223-228.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Molecular Characterization of Diphtheria Toxin Repressor (dtxR) Genes Present in Nontoxigenic Corynebacterium diphtheriae Strains Isolated in the United Kingdom

Respiratory and Systemic Infection Laboratory, Specialist and Reference Microbiology Division, Health Protection Agency, London,¹ Health Protection Agency, Heartlands Hospital, and Division of Immunity and Infection, Medical School, University of Birmingham, Birmingham United Kingdom²

Received 11 June 2004/ Returned for modification 25 July 2004/ Accepted 12 September 2004

Nontoxigenic strains of Corynebacterium diphtheriae represent a potential reservoir for the emergence of toxigenic C. diphtheriae strains if they possessed functional diphtheria toxin repressor (dtxR) genes. We studied the predominant strain of nontoxigenic C. diphtheriae circulating in the United Kingdom to see if they possessed dtxR genes and ascertain whether they were functional. A total of 26 nontoxigenic C. diphtheriae strains isolated in the United Kingdom during 1995 and 4 nontoxigenic strains isolated in other countries were analyzed by PCR and direct sequencing to determine the presence and intactness of the dtxR genes. The functionality of the DtxR proteins was assayed by testing for the production of siderophore in medium containing high and low concentrations of iron. PCR amplification and sequence analysis of the dtxR genes revealed four variants of the predicted DtxR protein among the nontoxigenic strains isolated in the United Kingdom. Production of siderophore in medium containing a low concentration of iron and repression of siderophore production in medium containing a high concentration of iron demonstrated that in all the strains the dtxR genes were functional. These findings demonstrate that, if lysogenised by a bacteriophage, nontoxigenic strains circulating in the United Kingdom could produce toxin and therefore represent a potential reservoir for toxigenic C. diphtheriae.

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