Prevalence of binary toxin positive <i>Clostridium difficile</i> in diarrhoeal humans in the absence of epidemic ribotype 027

<div><p>Virulence of <i>Clostridium difficile</i> is primarily attributed to the large clostridial toxins A and B while the role of binary toxin (CDT) remains unclear. The prevalence of human strains of <i>C</i>. <i>difficile</i> possessing only CDT genes (A⁻B⁻CDT⁺) is generally low (< 5%), however, this genotype is commonly found in neonatal livestock both in Australia and elsewhere. Zoonotic transmission of <i>C</i>. <i>difficile</i> has been suggested previously. Most human diagnostic tests will not detect A⁻B⁻CDT⁺ strains of <i>C</i>. <i>difficile</i> because they focus on detection of toxin A and/or B. We performed a prospective investigation into the prevalence and genetic characteristics of A⁻B⁻CDT⁺ <i>C</i>. <i>difficile</i> in symptomatic humans. All glutamate dehydrogenase or toxin B gene positive faecal specimens from symptomatic inpatients over 30 days (<i>n</i> = 43) were cultured by enrichment, and <i>C</i>. <i>difficile</i> PCR ribotypes (RTs) and toxin gene profiles determined. From 39 culture-positive specimens, 43 <i>C</i>. <i>difficile</i> isolates were recovered, including two A⁻B⁻CDT⁺ isolates. This corresponded to an A⁻B⁻CDT⁺ prevalence of 2/35 (5.7%) isolates possessing at least one toxin, 2/10 (20%) A⁻B⁻ isolates, 2/3 CDT⁺ isolates and 1/28 (3.6%) presumed true CDI cases. No link to Australian livestock-associated <i>C</i>. <i>difficile</i> was found. Neither A⁻B⁻CDT⁺ isolate was the predominant A⁻B⁻CDT⁺ strain found in Australia, RT 033, nor did they belong to toxinotype XI. Previous reports infrequently describe A⁻B⁻CDT⁺ <i>C</i>. <i>difficile</i> in patients and strain collections but the prevalence of human A⁻B⁻CDT⁺ <i>C</i>. <i>difficile</i> is rarely investigated. This study highlights the occurrence of A⁻B⁻CDT⁺ strains of <i>C</i>. <i>difficile</i> in symptomatic patients, warranting further investigations of its role in human infection.</p>