Methods and devices are provided for pre-treatment of a sample containing microbial cells. In some embodiments, the pre-treatment of the sample is performed via the initial selective lysis, within a sample pre-treatment vessel, of non-microbial cells, such as blood cells, and the subsequent centrifugal separation of the sample to remove the resulting debris and concentrate the microbial cells. An immiscible and dense cushioning liquid may be included for collecting the microbial cells adjacent to the liquid interface formed by the cushioning liquid upon centrifugation of the pre-treatment vessel. After removal of a substantial quantity of the supernatant, resuspension of the collected microbial cells, and re-establishment of the cushioning liquid interface, at least a portion of the remaining suspension may be removed without substantially removing the cushioning liquid. One or more intermediate wash cycles may be performed prior to extraction of the remaining suspension, which provides a “pretreated” sample.