Patients undergoing hematopoietic stem cell transplantation (HSCT) frequently receive empiric antibiotics during the neutropenic period before engraftment. Several recent studies have shown that anaerobes in the intestine are important mediators of intestinal homeostasis, and that commensal bacteria can be potent modulators of the severity of acute graft-versus-host disease (aGVHD). However, the relationships among the type of antibiotic used during the neutropenic period, changes in the intestinal microbiota, and subsequent occurrence of aGVHD are not clear. In this study, a total of 211 patients undergoing HSCT were stratified into 3 groups: patients not treated with any antibiotics during the neutropenic period (group 1; n = 43), patients treated with cefepime only (group 2; n = 87), and patients treated with carbapenem antibiotics, defined as meropenem or prepenem with or without previous cefepime therapy (group 3; n = 81). Intestinal microbiota analyses were performed on pre- and post-HSCT stool samples, and immunophenotypic analyses were performed on pre- and post-HSCT peripheral blood samples. Among the 211 patients, 95 (45%) developed aGVHD (grade ≥II), including 54 with intestinal GVHD. The incidence of intestinal GVHD was higher in group 3 compared with group 1 and group 2 (32. 1%, 11. 6%, and 26. 4%, respectively; P = . 044). After adjusting for potentially significant variables identified by univariate analysis, multivariate analyses identified broad-spectrum antibiotic use during the neutropenic period as associated with the occurrence of intestinal GVHD (hazard ratio, 3. 25; 95% confidence interval, 1. 13 to 9. 34; P = . 029). Accordingly, loss of bacterial diversity in terms of alterations in intestinal microbiota after HSCT was observed in patients who received broad-spectrum antibiotics. Moreover, alterations in the frequencies of several intestinal bacteria phyla were associated with the occurrence of intestinal GVHD. Evaluation of circulating immune cell subsets according to type of antibiotic used during the neutropenic period revealed delayed recovery of myeloid-derived suppressor cells in the broad-spectrum antibiotic use group. Our data indicate that the use of broad-spectrum antibiotics during the neutropenic period is associated with a higher incidence of intestinal GVHD via loss of microbiome diversity. Further studies are needed to determine whether maintaining bacterial diversity can help prevent the development of aGVHD VSports手机版. .