Outbreak of Pseudomonas aeruginosa Bacteremia Infections among Stem-Cell Transplant Patients Related to Change in Prophylaxis

Background: Pseudomonas aeruginosa outbreaks can originate from various sources and can cause severe complications in posttransplant patients. Antibiotic prophylaxis can decrease posttransplant infections; however, consideration must be given to P. aeruginosa coverage as we outline an outbreak among the stem-cell transplant (SCT) population. Methods: A multidisciplinary outbreak investigation was conducted to evaluate sources of contamination and changes in clinical processes. Positive blood cultures from SCT patients and environmental isolates were analyzed using whole-genome sequencing (WGS). Incidence density rates for P. aeruginosa blood cultures from January 2019 through October 2020 were calculated per 10,000 patient days and stratified by unit, specimen, and transplant type. Statistical analysis was calculated with significance at p < 0.05. Results: A cluster of 8 SCT patients was identified between May and September 2020. Moreover, 10 environmental samples were positive for P. aeruginosa including drains, water sources prior to the point-of-use (POU) filter and blood-bank thaw machines. Phylogenetic analysis revealed 1 cluster of 2 patients who shared the same room, 5 patients with unique P. aeruginosa isolates, and 2 separate clusters of environmental isolates with relatedness only to each other. Review of clinical processes showed a change from fluoroquinolone prophylaxis to cephalosporin in the spring of 2020. Also, 5 P. aeruginosa bacteremia infections occurred prior to June (11.78 cases per 10,000 patient days). During the period of cephalosporin use, 8 infections were identified (58.27 cases per 10,000 patient days) (P = .006). Following the restart of fluoroquinolone, zero infections have occurred to date, as of January 28, 2021. Conclusions: Discontinuation of fluoroquinolone prophylaxis was associated with P. aeruginosa bacteremia infections in SCT patients. Use of fluoroquinolone prophylaxis in SCT patients is protective from P. aeruginosa bacteremia infections. There have been no further infections in the following 3 months after the change back to the use of fluoroquinolone. Additionally, WGS showed that most patient isolates did not have a common source, suggesting that P. aeruginosa gastrointestinal colonization may play a role in seeding these bacteremia infections. Funding: No Disclosures: None