Uniform Suspension of Heat-Killed Staphylococcus aureus for a Positive Control Used in the Monocyte-Activation Test

Pyrogens, classified as bacterial endotoxins and non-endotoxin pyrogens (NEPs), induce fever or shock when released into the bloodstream or spinal fluid. Recently, a monocyte-activation test (MAT) involving human cell culture has been developed to detect pyrogens in injectable products. To evaluate the sensitivity of MAT, a reference standard endotoxin was used as a positive control; however, the reactivity differed between the endotoxins and NEPs, necessitating positive controls for NEPs. This study aimed to explore a preparation method for heat-killed Staphylococcus aureus (HKSA) as a positive control for NEPs in MAT. Because S. aureus forms grape-like clusters, nine types of glass filters with pore sizes of 0.5-2.7 µm were evaluated to obtain a uniform bacterial suspension. The suspension was then heat-treated to kill the bacteria, resulting in HKSA samples. Serial dilutions of HKSA were tested by MAT using peripheral blood mononuclear cells. The interleukin-6 concentrations in the culture supernatant were measured by enzyme-linked immuno-sorbent assay to assess pyrogenic activities of HKSA. The pore sizes of the glass filters affected the uniformity of HKSA, and GF/C filter was selected for HKSA preparation. Repeated filtration improved uniformity, and a uniform suspension of HKSA was obtained through double filtration using a GF/C filter. Despite the decrease in HKSA activity as filtration frequency increased, the detection limit remained consistently unchanged. This suggests that repeated filtration can adjust the activity of HKSA to a baseline level and that a uniform suspension of HKSA exhibiting low variation is suitable as a positive control in MAT.