Data on the seroprevalence of antibodies to Clostridioides difficile surface proteins and toxins are scarce.

In 1983, Viscidi et al. showed that antibodies to C. difficile toxins A and B were detected in 60 to 70% of an adult population. Two-thirds of the adults tested had a serological trace, probably linked to a previous encounter with C. difficile.

One of the hypotheses raised would be that exposure to this pathogen occurs very early and regularly throughout our lives. Indeed, in this study, antibodies to C. difficile toxins were detected from early childhood and persisted over time even after 60 years. The antibody response did not appear to vary with age or terrain. However, these results were only qualitative and did not allow for inter-individual variations due to the limitations of the techniques used at the time. Finally, in this work, it was important to underline that the neutralizing character of the cytotoxic effect of toxins on cell culture was not observed in all patients.

Since this seminal work, several studies have shown that the host immune response plays a central role in the pathophysiology of C. difficile infections (CDI). In 2000, Kyne et al. showed that after colonization with a toxigenic C. difficile strain, patients with asymptomatic carriage had significantly higher serum levels of IgG directed against toxin A than patients who developed disease. Subsequently, they also showed in 2001 that a serum response directed against toxin A after a first episode of CDI was associated with less recurrence. Finally, Leav et al. showed in 2010 that a serum response directed against C. difficile toxin B was also associated with protection against recurrent forms.

Several studies have also suggested that the host immune response, this time directed against colonization factors, could also play a major role in the evolution and prognosis of CDI. In a previous study, investigators showed a significant difference in the level of anti-SlpA antibodies (S-layer component) between CDI patients and control patients.

At the same time, the epidemiology of CDI has changed since 2003 due to the emergence of a new epidemic and hypervirulent strain (PCR ribotype 027) producing a third toxin, the binary toxin. The humoral response to this toxin remains poorly described to date.

On the basis of these numerous studies, new therapeutic immunization strategies (active or passive) aimed at neutralizing the action of C. difficile toxins and colonization factors have been or are being developed.

However, it remains to identify the patients likely to benefit from these innovative strategies. This was the main objective of the SERODIFF study (currently being finalized), which identified certain patient profiles in which no seroconversion or isotype class switching of antibodies was observed following CDI. The absence of neutralizing antibody production would seem to correlate with recurrent forms. Thus, these patients would be those who could be eligible for a passive immunization strategy such as the administration of anti-toxin B monoclonal antibodies, bezlotoxumab, recently marketed in France.

In this study, investigators aim to evaluate the seroprevalence stratified by age group, sex and by the main risk factors for CDI. Furthermore, the neutralizing and protective effect of the detected antibodies against C. difficile virulence factors will be studied.

Inclusion Criteria:

• Adult patient (≥ 18 years) hospitalized in targeted short-stay (MCO) and long-stay (LTC) services for each of the three identified regions
• French-speaking patient
• Patient affiliated to the social security system or, failing that, to another health insurance system
• Patient able to give free, informed and written consent

Exclusion Criteria:

• History of blood transfusion, vascular filling, dialysis or polyvalent immunoglobulin infusion.
• Patient deprived of liberty
• Patient under court protection
• Patient in an emergency situation
• Patients unable to give personal consent, including adults under guardianship