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Background Infant C. difficile colonization is common, but the molecular epidemiology and immunologic consequences of colonization are poorly understood. Methods In this prospective cohort study of healthy infants, serial stools collected between 1–2 and 9–12 month olds were tested for glutamate dehydrogenase (detects nontoxigenic or toxigenic C. difficile [TCD]), tcdB PCR (detects TCD), and cultured for C. difficile. Isolates underwent whole genome sequencing and multilocus sequence typing (MLST). Clonal strains were identified by single nucleotide variant (SNV) analysis. TCD was confirmed by BLAST identification of tcdA/tcdB. Serum collected at 9–12 month olds underwent ELISA for measurement of IgA, IgG, and IgM against TCD toxins A and B. For comparison, anti-toxin IgG was measured in cord blood of 50 consecutive full-term deliveries (unrelated to study infants). Arbitrary ELISA units were compared by Wilcoxon rank-sum test. Results Among 32 infants, 16 (50%) had at least one TCD+ stool, 12 of whom were colonized at least 1 m prior to serology measurements (Figures 1 and 2). A variety of STs were identified, and evidence of putative in-home (enrolled siblings) and outpatient clinic transmission was identified (Figure 3). Infants with TCD colonization had significantly greater levels of anti-toxin IgA and IgG compared with non-colonized infants and IgG compared with unrelated cord blood (Table 1).Table 1: Anti-toxin Serology (Arbitrary ELISA Units) Group Tox A IgA Tox A IgG Tox A IgM Tox B IgA Tox B IgG Tox B IgM Not colonized with TCD (n = 16) 1.37 10.14 2.74 0.8 6.50 6.14 Colonized with TCD for at least 1 month (n = 12) 4.23* 37.87* 2.54 1.73* 20.76* 5.96 Cord Blood (n = 50) 10.81^ 8.34^ *P < 0.05 (colonized vs. non-colonized); ^P < 0.05 (colonized vs. cord blood)Figure 1: Infant ClassificationFigure 2: Chronology and Results of Infant Stool C. difficile TestingFigure 3: Molecular Epidemiology of Infant C. difficile Isolates Conclusion Infant C. difficile colonization is a dynamic process with variable strain types and duration. Outpatient clinics may be a C. difficile reservoir for some patients. TCD colonization is associated with a humoral immune response against toxins A and B, but whether natural TCD immunization protects against CDI later in life requires further investigation. Disclosures L. Kociolek, Alere/Techlab: Investigator, Research support. C. P. Kelly, Actelion: Consultant, Consulting fee. Artugen: Consultant, Consulting fee. Facile: Consultant, Consulting fee. GSK: Consultant, Consulting fee. MSD: Consultant, Consulting fee. Seres: Consultant, Consulting fee. Summit: Consultant, Consulting fee. Vedanta: Consultant, Consulting fee. D. N. Gerding, Merck: Scientific Advisor, Consulting fee. Actelion: Scientific Advisor, Consulting fee. DaVolterra: Scientific Advisor, Consulting fee. Summit: Scientific Advisor, Consulting fee. Rebiotix: Medical Officer and Scientific Advisor, Consulting fee. Pfizer: Consultant, Consulting fee. MGB Pharma: Consultant, Consulting fee. sanofi pasteur: Consultant, Consulting fee. Seres: Investigator, Research grant. CDC: Investigator, Research grant. US Dept VA: Investigator, Research grant. Treatment/Prevention of C. difficile: Patent Holder, no license or royalties.

976. Clostridium difficile Colonization Molecular Epidemiology and Anti-toxin Serological Responses in Healthy Infants: A Prospective Cohort Study