The corynebacteria are sensitive to penicillin (MIC. 0.004 to 0.02 units/m1) and to other antibiotics active against Gram-positive bacteria. The most useful of these is erythromycin, which may be preferred to penicillin for eliminating diphtheria bacilli from the throat, particularly in treatment of persistent carriers.
Antibiotic therapy can be a useful adjunct to, but not a substitute for, antitoxin in the treatment of clinical diphtheria, since it helps to eliminate the organism but has no effect on preformed toxin which rapidly diffuses from the local lesion and, unless neutralized by antitoxin, soon becomes irreversibly bound to vulnerable tissue cells.
This is why antitoxin must be given as early as possible to any patient with suspected diphtheria as the fatality rate is directly related to the period of delay before giving antitoxin, rising from nil to 20 per cent between the onset and day 5 of the infection: the average case-fatality rate is 5 to 7 per cent.
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