The invention provides methods of detecting bacteria in fluids, including blood, platelets and other blood products for transfusion, and urine. The methods are based on lysing the bacteria to release ATP and detecting the ATP. Eukaryotic cell contamination is a problem to be overcome, because eukaryotic cell contain large amounts of ATP. Thus, some of the methods involve separating intact eukaryotic cells (e.g., platelets) from intact bacterial cells before lysing the bacterial cells to release ATP, contacting the ATP with an ATP-consuming enzyme that catalyzes a reaction, and monitoring the enzyme-catalyzed reaction. Typically, the enzyme is luciferin, and the reaction is monitored by detecting light produced by the luciferin. Other methods of the invention involve contacting a fluid sample with a support surface that binds bacterial cells, lysing the bacterial cells to release ATP, contacting the ATP with an ATP-consuming enzyme, and monitoring the enzyme-catalyzed reaction. Apparatuses for carrying out the methods are also disclosed.