Prevalence of binary toxin positive Clostridium difficile in diarrhoeal humans in the absence of epidemic ribotype 027

Virulence of Clostridium difficile 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 C. difficile 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 C. difficile has been suggested previously. Most human diagnostic tests will not detect A(-)B(-)CDT(+) strains of C. difficile 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(+) C. difficile in symptomatic humans. All glutamate dehydrogenase or toxin B gene positive faecal specimens from symptomatic inpatients over 30 days (n = 43) were cultured by enrichment, and C. difficile PCR ribotypes (RTs) and toxin gene profiles determined. From 39 culture-positive specimens, 43 C. difficile 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 C. difficile 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 Xl. Previous reports infrequently describe A(-)B(-)CDT(+) C. difficile in patients and strain collections but the prevalence of human A(-)B(-)CDT(+) C. difficile is rarely investigated. This study highlights the occurrence of A(-)B(-)CDT(+) strains of C. difficile in symptomatic patients, warranting further investigations of its role in human infection.