Sepsis is “a life-threatening organ dysfunction caused by a dysregulated host response to infection”. The global incidence of sepsis is difficult to ascertain due to lack of or insufficient reporting. The global incidence of sepsis was estimated at 31 million cases per year (437 per 100,000) and its mortality, around 5 million/year. The microbiological diagnostics of bacteremia consists of blood culture. The advantage of this method is its simplicity and cost effectiveness. The disadvantage is that this method is time-consuming and has low sensitivity. Bacterial growth is detected in 15-61%. Time needed to initiate a targeted therapy has a direct impact on patient’s survival. Molecular methods based on the genetic analysis are costly and available only in some research centers. The goal of this doctoral thesis was to assess if screening methods of Gram staining and Fluorescent Hybridization In Situ (FISH) can increase a rate of detection of bacteria in blood. Study results confirm the effectiveness of both methods in microbiological testing of whole blood - significantly higher percentage of positive results in comparison to blood culture. Additionally, both methods enabled the detection of bacteria in negative media culture, as well as in the blood of healthy volunteers, which may confirm the reports to date regarding the constant ‘physiological’ bacteremia.