Incidence, Temporal Distribution and Microbiological Profile of Bacterial Infections After Allogeneic Hematopoietic Stem Cell Transplantation from a Bulgarian Transplant Center

Objective: To assess the incidence, timing of onset and etiological spectrum of bacterial infections in patients with hematologic malignancies after allogeneic hematopoietic stem cell transplantation (HSCT), and to compare haploidentical and non-haploidentical recipients. Material and Methods: This retrospective single-center cohort study included 115 consecutive patients with hematologic malignancies who underwent allogeneic HSCT between December 2017 and December 2025 at the Transplant Department of University Hospital “St. Marina”—Varna, Bulgaria. Of these, 38 (33%) received haploidentical and 77 (67%) non-haploidentical grafts. Only microbiologically documented bacterial infections were analyzed. The cumulative incidence (CI) of first bacterial infection of any type and first bloodstream infection (BSI) was estimated using competing-risk methodology, with death before respective infection treated as a competing event. CI curves were compared using Gray’s test and the potential risk factors for first bacterial infection were assessed by Fine–Gray competing-risk regression. Results: Overall, 65 patients (56.5%) developed at least one bacterial infection, with a median time to the first infection of 11 days (IQR: 6–36). The CI of first bacterial infection was 40.0% at day +30 and 52.7% at 1 year; the corresponding cumulative incidences of first BSI were 27.8% and 35.0%, respectively. No statistically significant differences were observed between haploidentical and non-haploidentical recipients in the CI of first bacterial infection or first BSI. A total of 105 bacterial infectious episodes were documented. Their temporal distribution showed different patterns between the two groups, with a greater proportion of early episodes among the non-haploidentical recipients and a greater proportion of later episodes among the haploidentical ones. BSIs were the predominant infectious syndrome, occurring in 46 patients (40.0%; 46/115) and accounting for 63 episodes. Gram-negative bacteria represented 66.2% of BSI isolates, of which 44.2% were multidrug-resistant (MDR). None of the evaluated patient-, disease-, or transplant-related variables was significantly associated with the CI of first bacterial infection. Conclusions: Bacterial infections were common complications after allogeneic HSCT, particularly during the early post-transplant period, with BSIs representing the predominant infectious syndrome. Gram-negative bacteria accounted for most BSI isolates, and a substantial proportion exhibited MDR. No statistically significant differences in the CI of first bacterial infection were observed between haploidentical and non-haploidentical recipients, and donor type was not significantly associated with the risk of first bacterial infection. The distribution of all infectious episodes over the studied time periods showed different patterns between the two groups. These findings should be interpreted in the context of the retrospective single-center design and the relatively small haploidentical subgroup.