Diphtheria toxin CRM197 is an enzymatically inactive and nontoxic form of diphtheria toxin that contains a single amino acid substitution (G52E). Being naturally nontoxic, CRM197 is an ideal carrier protein for conjugate vaccines against encapsulated bacteria and is currently used to vaccinate children globally against Haemophilus influenzae, pneumococcus, and meningococcus.
Diphtheria toxin, a single polypeptide secreted by lysogenic strains of Corynebacterium diphtheria , causes the disease diphtheria in humans by gaining entry into the cytoplasm of cells and inhibiting protein synthesis.