Prevalence of Binary Toxin Positive Clostridium difficile in Symptomatic Humans in the Absence of Epidemic Ribotype 027. Prevalence of LCT negative binary toxin positive Clostridium difficile

Virulence of Clostridium difficile is primarily attributed to toxins A and B while the role of binary toxin remains unclear. Binary toxin often occurs simultaneously with toxins A and B and the prevalence of human strains possessing only binary toxin genes (A−B−CDT+) is generally low (< 5%). Conversely, A−B−CDT+ C. difficile appears highly prevalent in Australian neonatal livestock. Zoonotic transmission of C. difficile has been suggested previously and many diagnostics may miss human A−B−CDT+ cases due to the focus on detection of toxin A and/or B. We performed a brief prospective investigation into the prevalence and genetic characteristics of A−B−CDT+ C. difficile in symptomatic humans. All faecal specimens from symptomatic inpatients over 30 days in early 2015 (n = 592) were screened for the toxin B gene or GDH, a non-toxin marker of C. difficile, before enrichment culture. C. difficile ribotypes and presence of toxin genes were determined by PCR. We recovered 43 C. difficile isolates 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 between Australian livestock-associated C. difficile was found. Both A−B−CDT+ isolates were not the dominant A−B−CDT+ strain; PCR ribotype 033, nor did they belong to toxinotype XI. Previous reports infrequently describe A−B−CDT+ C. difficile in human patients and in strain collections but the in situ prevalence of human A−B−CDT+ C. difficile is rarely investigated. This study suggests A−B−CDT+ C. difficile to be more common than previously thought, and at least warrants larger investigations.