MICROBIOLOGY

• Spore-forming anaerobe, Gram-positive bacillus [Fig 1].
  
  • Found in human and animal feces, also in water and soils.
  • Reclassified in 2016 as molecular sequencing suggests that the organism should be in the Peptostreptococcaceae family and termed Peptoclostridium.
    • Clostridioides difficile’s name was selected to differentiate it from Clostridiaspp., which are unrelated but allow for less clinical confusion moving from the long-standing terminology of Clostridium difficile[29].
• Cycles between a hardy spore (transmissible via fecal-oral route) form and a vegetative (metabolically active) form.
  • Only strains possessing the Pathogenicity Locus (PaLoc) are capable of toxin production.[49]
  • Toxigenic strains can produce toxin A (tcdA) and/or toxin B (tcdB), which causes clinical disease (colitis) when in vegetative form.
  • The presence of secondary bile acids, synthesized by a healthy host microbiome, largely inhibits the germination of C. difficile spores in the colon and thus prevents toxin production and disease[45].
  • Without signals to germinate, spores can latently colonize the colon, though factors contributing to colonization vs clearance remain under studied.
• Occasionally grown in anaerobic cultures, it is rarely a cause of infection other than colitis.
• Resistance described to metronidazole and high MICs correlates with poorer outcomes[18].
  • Increased MICs to vancomycin and fidaxomicin have also been documented, and the former may be associated with poorer clinical outcomes[16][7].