<prompt> True statement regarding diphtheria isIt can be diagnosed by demonstration of antibodies by ELISAImmunisation prevents carrier stateTreatment of contacts is not indicatedIron has critical value in the production of toxin  <response>: Toxin production is influenced by the concentration of iron in the medium. The optimum level of iron for toxin production is 0.1 mgl, while a concentration of 0.5mgl inhibits the formation of the toxin. Reference: Textbook of Microbiology; Anathanarayan and panickers; 10th edition; Page no: 240 