Toxin-positive <i>Clostridium difficile</i> latently infect mouse colonies and protect against highly pathogenic <i>C. difficile</i>.

Abstract

<i>Clostridium difficile</i> is a toxin-producing bacterium and a leading cause of antibiotic-associated disease. The ability of <i>C. difficile</i> to form spores and infect antibiotic-treated persons at low multiplicity of infection (MOI) underlies its large disease burden. However, <i>C. difficile</i>-induced disease might also result from long-harboured <i>C. difficile</i> that blooms in individuals administered antibiotics. Mice purchased from multiple vendors and repeatedly testing negative for this pathogen by quantitative PCR bloomed <i>C. difficile</i> following antibiotic treatment. This endogenous <i>C. difficile</i> strain, herein termed LEM1, which formed spores and produced toxin, was compared with highly pathogenic <i>C. difficile</i> strain VPI10463. Whole-genome sequencing revealed that LEM1 and VPI10463 shared 95% of their genes, including all known virulence genes. In contrast to VPI10463, LEM1 did not induce overt disease when administered to antibiotic-treated or germ-free mice, even at high doses. Rather, blooms of LEM1 correlated with survival following VPI10463 inoculation, and exogenous administration of LEM1 before or shortly following VPI10463 inoculation prevented <i>C. difficile</i>-induced death. Accordingly, despite similar growth properties in vitro, LEM1 strongly outcompeted VPI10463 in mice even at 100-fold lower inocula. These results highlight the difficulty of determining whether individual cases of <i>C. difficile</i> infection resulted from a bloom of endogenous <i>C. difficile</i> or a new exposure to this pathogen. In addition to impacting the design of studies using mouse models of <i>C. difficile</i>-induced disease, this study identified, isolated and characterised an endogenous murine spore-forming <i>C. difficile</i> strain able to decrease colonisation, associated disease and death induced by a pathogenic <i>C. difficile</i> strain.