Background: Bacteremia is a common complication in allogeneic hematopoietic cell transplant recipients (alloHCTr), especially during the pre-engraftment period. International guidelines recommend antibacterial prophylaxis (ABP), despite potential selection for multidrug-resistant organisms (MDRO). Limited contemporary data exist on the epidemiology of pre-engraftment bacteremia in alloHCTr, who do not receive ABP.

Methods: We performed a retrospective observational single-center cohort study including all consecutive adult alloHCTr (2015-2021), investigating the incidence, risk factors, and outcomes of bacteremia during the engraftment period. Primary fluoroquinolone (FQ) ABP is not routinely administered in our center.

Results: Among 421 patients identified, 124 bacteremia episodes were observed in 121/421 (29%) alloHCTr. The median time to the 1st bacteremia episode was 9 days (IQR 6-11). Most (105/124, 85%) episodes were monomicrobial, while >1 pathogens were identified in 19/124 (15%) episodes. Overall, 152 pathogens were isolated, with a predominance of Gram-positive (118/152, 78%), including coagulase-negative staphylococci (n:47), streptococci (n:46), and enterococci (n:15), followed by Gram-negative bacteria (GNB, 30/152, 20%), and anaerobes (4/152, 3%). There were 2/152 (1%) MDRO (extended-spectrum beta-lactamase producing) GNB. Multivariable analyses identified age >40-year-old (OR 2.4, P = 0.02), male gender (OR 1.8, P = 0.02), and a haploidentical/mismatched unrelated donor (OR 2.5, P < 0.001) as independent risk factors for bacteremia. All cause 30-day mortality among alloHCTr with bacteremia was 0.8% (1/121): one patient died due to an HCT-related complication.

Conclusion: Despite lack of primary FQ ABP, low rates of bacteremia were observed during the pre-engraftment period, with low MDRO prevalence and mortality. Our findings may allow to revisit the need for primary universal FQ ABP in high-risk neutropenic hematology patients.