Detection of diphtheria toxin production by toxigenic corynebacteria using an optimized Elek test

Diphtheria, still present in many countries of the world, is caused by toxigenic strains of Corynebacterium diphtheriae and the emerging zoonotic pathogen C. ulcerans. The immunoprecipitation test according to Elek is the gold standard for detection of the major virulence factor diphtheria toxin (DT) in toxigenic corynebacteria. Due to its sophisticated methodological requirements, the classical Elek test is performed mainly by specialized reference laboratories. It was revealed that the current modification of the Elek test does not detect the toxin in weakly toxigenic isolates. Thirty-one tox-positive C. ulcerans isolates with a negative standard Elek test result previously determined as NTTB (non-toxigenic tox bearing) were re-analyzed in this study using a modifed immunoprecipitation method optimized regarding different parameters including type and concentration of antitoxin, medium volume, inoculum distance from the antitoxin disk and position of controls. All the C. ulcerans strains tested positive in the optimized Elek test. Therefore, the new modification of the Elek test could be used for detecting DT in C. diphtheriae as well as C. ulcerans, which are known to produce often much lower amounts of DT.