Fluoroquinolone Resistance Prevalence Study

The goal of this observational study is to determine the prevalence of fluoroquinolone resistance in patients receiving dose-intense melphalan with autologous peripheral blood stem cell (PBSC) transplantation in the treatment of multiple myeloma (MM). The main question\[s\] it aims to answer are: * What is the prevalence of fluoroquinolone-resistant Enterobacterales (FRE) in patients undergoing autologous PBSC transplantation with dose-intense melphalan? * Does the risk of febrile neutropenia differ in FRE carriers compared to non-carriers? Participants will be tested for the presence of FRE before receiving fluoroquinolone prophylaxis at multiple points during the transplant course, including before chemotherapy mobilization (if used) using fluoroquinolone prophylaxis, at initial transplant hospitalization, at time of hospital discharge, and at or after day 84 after transplantation (day 0 is defined as day of HSC (hematopoietic stem cell) infusion). FRE colonization will not be a determining factor in the use of fluoroquinolone prophylaxis during the treatment course. This study will be open at two transplant units: Hackensack University Medical Center (HUMC) and MedStar Georgetown University Hospital (MGUH). Estimated number of subjects to be enrolled; * HUMC: 124 * MGUH: 20 Anticipated enrollment period: 12-months with monitoring of subjects for 84 days (twelve weeks) after transplantation. Data will be analyzed over a three-month period (total study period of 18 months).