Antimicrobial Susceptibility of Toxin-Producing Corynebacterium diphtheriae and C. ulcerans in Belgium

<b>Background/Objectives</b>: Despite a significant reduction in diphtheria incidence and mortality due to vaccination, antitoxin therapy and antibiotic treatments, a concerning resurgence is occurring in Europe. Resistance to penicillins and macrolides is emerging, resulting in a growing challenge for diphtheria management. This retrospective study aims to evaluate and compare antibiotic susceptibilities of both toxigenic <i>Corynebacterium diphtheriae</i> and <i>C. ulcerans</i>. <b>Methods</b>: Susceptibilities were assessed using broth microdilution-the gold standard-disk diffusion and the gradient method, and analyzed on the basis of the EUCAST breakpoint tables for the interpretation of MICs and zone diameters. Antimicrobial resistance genes and mutations were detected by analyzing whole-genome sequences (WGS). <b>Results</b>: A small number of <i>C. diphtheriae</i> isolates were resistant to the first-choice antimicrobial classes, penicillins and macrolides, while higher resistance rates were observed for ciprofloxacin (29%), tetracycline (38%) and trimethoprim-sulfamethoxazole (SXT, 85%). A good correlation was found with resistance genes and mutations detected by WGS. <i>C. ulcerans</i> isolates were susceptible to all tested antibiotics, except clindamycin, to which this species is naturally resistant, and a few ciprofloxacin resistances not confirmed by WGS. Diffusion techniques were found to be acceptable alternatives, but false susceptible results were detected for ciprofloxacin and tetracycline by disk diffusion and ciprofloxacin and SXT by gradient diffusion. <b>Conclusions</b>: Penicillins and macrolides remain the first-choice antibiotics for the treatment of diphtheria. However, antimicrobial susceptibility testing is needed for all toxigenic <i>C. diphtheriae</i> and <i>C. ulcerans</i> isolates, as resistance is emerging. Antimicrobial susceptibility testing should not be limited to penicillins and macrolides, but be extended to other antibiotics. When WGS is performed for epidemiological purposes, resistance genes and mutations should be looked for.