Thrombin-induced platelet microbicidal protein (tPMP) is a small, cationic peptide released from rabbit platelets following exposure to thrombin in vitro. This peptide exerts potent in vitro microbicidal activity against a broad spectrum of bloodstream pathogens, including Staphylococcus aureus. It is known that the microbicidal actions of other cationic antimicrobial peptides (e.g., neutrophil defensins) are influenced by environmental factors and target cell growth phase. However, whether these parameters affect tPMP microbicidal activity has not been studied. Thus, we assessed the in vitro bactericidal activity of tPMP against two tPMP-susceptible strains, Bacillus subtilis ATCC 6633 and S. aureus 502A, in various target cell growth phases or under various microenvironmental conditions. The conditions studied included differing bacterial growth phase (logarithmic versus stationary), temperature (range, 4 to 42 degrees C), pH (range, 4.5 to 8.5), cationicity (range, 0.1 mM to 2 M), anionicity (range, 0.08 to 5 microM), and neutral carbohydrates ranging in molecular weight (MW) from 180 to 37,700 (range, 50 to 500 mM) as well as rabbit platelet-free plasma and serum. tPMP staphylocidal activity was greater against logarithmic- than stationary-phase cells. tPMP bactericidal activity against both B. subtilis and S. aureus was directly correlated with temperature and pH, with microbicidal activity exhibited near the physiological range (37 to 42 degrees C and pH 7.2 to 8.5, respectively). The presence of cations (Na+, K+, Ca2+, and Mg2+) decreased tPMP bactericidal activity in a time- and concentration-dependent manner, with complete inhibition at monovalent or divalent cation concentrations of > or = 250 or > or = 10 mM, respectively. Staphylocidal activity of tPMP was also inhibited by the polyanions polyanetholsulfonic acid and polyaspartic acid, at 0.1 and 0.4 microM, respectively. Coincident exposure with low-MW carbohydrates (glucose, sucrose, and melezitose) did not affect tPMP staphylocidal activity. However, higher-MW carbohydrates (raffinose and dextrans) decreased tPMP activity in a manner directly proportional to their concentration and MW. Solute-mediated inhibition of tPMP bactericidal activity was independent of solute osmolality but directly related to the duration of tPMP-solute coexposure. tPMP enhanced the staphylocidal activities of platelet-free plasma and heat-inactivated serum, while the activity of normal serum was not affected. These collective observations suggest that tPMP retains antimicrobial activities under physiological conditions which are likely to be relevant to host defense in vivo.