457. Cutibacterium (Propionibacterium) acnes Infection Rate and Optimization of Surgical Culture Duration

Abstract Background Cutibacterium acnes is part of the normal skin and gastrointestinal tract flora that is increasingly recognized as a causative organism of surgical infections. Distinguishing between infection and contamination is difficult. The standard culture duration for C. acnes has not been determined. As a slow-growing organism, a prolonged incubation of 10–14 days is adopted in many laboratories. Ideally, only samples with high pretest probability for infection should be worked up this way, otherwise resources are overutilized with likely no benefits and potential harms to patients. We conduct a study to assess the optimal incubation duration for C. acnes. Methods We retrospectively reviewed microbiologic and clinical data of patients who underwent surgical procedures at the Veterans Affairs (VA) Hospital and the University of Utah Hospital in Salt Lake City, Utah, between 2015 and 2018 for which prolonged incubation of surgical samples was requested. Samples that grew C. acnes were divided into three groups (infection, contaminant, indeterminate) based on the quantity of growth and the number of positive samples (Figure 1). Samples in the “indeterminate” group were re-classified into the other two groups based on clinical criteria (Figure 2). Time to culture positivity (TTP) was calculated for each group. Results 741 patients contributed to a total of 909 surgical cases. There were 2,401 samples collected resulting in 4,408 bacterial cultures. C. acnes grew in 131 cases (14.41%). Fifty-five cases (44%) fulfilled the criteria for true infections and 70 cases (56%) were contaminants. 6 cases were lost to follow-up. The mean TTP of the infection and the contamination groups were 5.60 + 0.76 days and 8.67 + 0.81 days, respectively. The TTP of C. acnes from specimens of true surgical infections was significantly shorter than that of contaminants by the mean of 3.07 days (95% CI: -4.22 to -1.92); P < 0.001. Conclusion Using our microbiological and clinical criteria to differentiate infections and contaminations, this study provides evidence that surgical sample cultures should be held no longer than 7 days to limit the effect of contaminated C. acnes on cultures and reduce unnecessary antimicrobial use. Disclosures All authors: No reported disclosures.