Diphtheria carriers in Manchester: simultaneous infection with toxigenic and non-toxigenic mitis strains.

A toxigenic strain of Corynebacterium diphtheriae mitis was isolated from a 10-week-old baby with membranous tonsillitis, and over the next 6 months thirty-nine symptom-free carriers of nitrate-positive mitis strains were found. All carriers were cleared by 14 days' treatment with erythromycin, though several relapsed after a 5-day course. Four contacts carried both toxigenic and non-toxigenic mitis strains; epidemiological evidence and phage studies suggest that these had a common origin. These findings have implications for the epidemiology of diphtheria and for the routine testing of isolates for toxigenicity.