Absence of toxin gene transfer from Clostridioides difficile strain 630Δerm to nontoxigenic C. difficile strain NTCD-M3r in filter mating experiments

Nontoxigenic Clostridioides difficile strain M3 (NTCD-M3) protects hamsters and humans against C . difficile infection. Transfer in vitro of the pathogenicity locus (PaLoc) to nontoxigenic strain CD37 has been reported. We repeated these conjugations using toxigenic strain 630Δ erm as donor and NTCD-M3 and CD37 as recipients. In order to conduct these matings we induced rifampin resistance (50ug/ml) in NTCD-M3 by serial passage on rifampin-containing media to obtain strain NTCD-M3r. 630Δ erm /CD37 matings produced 21 PaLoc transconjugants in 5.5 x 10 9 recipient CFUs; a frequency of 3.8 x 10 −9 . All transconjugants carried the tcd B gene and produced toxin. 630Δ erm /NTCD-M3r matings produced no transconjugants in 5 assays with a total of 9.4 x 10 9 NTCD-M3r recipient cells. Toxin gene transfer to NTCD-M3r could not be demonstrated under conditions that demonstrated transfer to strain CD37.