Secretion of an Inhibitor of Follicle-Stimulating Hormone Binding to Receptor by the Bacteria Serratia, Including a Strain Isolated From Porcine Follicular Fluid

Bacterial growth in contaminated porcine follicular fluid (PFF) was associated with increased concentrations of a large molecular weight (Mr greater than 6000) inhibitor (FSH-BI) of 125I-FSH binding to calf testis membranes (Sluss and Reichert, 1983). We undertook to identify the bacteria and to determine if the inhibitor was a secretory product. Only one of 39 pure bacterial colonies isolated from PFF generated FSH-BI. The bacterium was tentatively identified as Serratia liquifaciens and was subsequently shown to also secrete FSH-BI when grown in synthetic culture media. Serratia liquifaciens from PFF secreted FSH-BI in a minimal culture medium containing only glucose as a carbon source. Other bacteria, including strains of Pseudomonas and Streptococcus did not secrete FSH-BI in either sterile PFF or synthetic culture media. Six strains of Serratia, obtained from the American Type Culture Collection, also secreted FSH-BI. FSH-BI secreted by Serratia liquifaciens was inactivated by heat (T 1/2 = 30 min at 60 degrees C), exposure to pH 2 (2 h at 25 degrees C) and was insoluble in ether, 75% acetone or 40% ammonium sulfate. Protease activity, using a casein substrate, was undetected in doses of FSH-BI which effectively (50%) inhibited 125I-FSH binding. Initial studies suggested that FSH-BI was due to effects on membranes rather than on the radioligand. These data demonstrate that Serratia liquifaciens isolated from PFF secretes a substance of Mr greater than 6000 which inhibits receptor binding of 125I-hFSH. Furthermore, the FSH-BI appears to be secreted constitutively by all (7) strains of Serratia tested.