Activity of nisin against Streptococcus pneumoniae, in vitro, and in a mouse infection model.

Sir, Nisin, a lanthionine-containing peptide from L actococcus lactis, is used as a food preservative because of its activity against spoilage bacteria. Like other cationic peptides, nisin interacts with the cell membrane and induces pore formation. Because of the increasing resistance of pathogenic bacteria to antibiotics, we are evaluating the potential use of nisin, and of variant peptides, as antimicrobial agents for human use. Nisin was obtained from Aplin & Barrett, Ltd, Beaminster, UK. Other antimicrobial agents were supplied by Sigma Chemical Co., St Louis, MO, USA. Clinical isolates of Streptococcus pneumoniae from the USA and E u r o p e were kindly provided by Dr J. D . Knudsen, Statens Seruminstitut, Copenhagen, Denmark. MICs were determined by broth microdilution in cation adjusted Mueller– Hinton broth (D ifco, Detroit, MI, U SA) with 5% fetal bovine serum (Sigma). Nisin dilutions were made in 0.2% bovine serum albumin (BSA; Sigma) to prevent adsorption to plastic microtitre trays. Swiss–Webster mice (Taconic Farms, Germantown, NY, USA) weighing approximately 20 g were infected intraperitoneally, as described previously, with 10–10 cfu of S. pneumoniae Felton, approximately ten-fold the inoculum that reproducibly killed all untreated animals within 48 h. Five or six mice per group received antimicrobial agents intravenously (in 0.1 mL 5% dextrose for injection) or subcutaneously (0.2 mL), within 10 min of infection and, in some cases, 5 h later. Serum levels of nisin were assayed, using seven mice for each time point, by an agar diffusion method with Micro coccus luteus ATCC 9341 as indicator strain. Sample and standard dilutions were in 10% mouse serum. The limit of detection was 0.45 mg nisin/L serum. Animal experiments were conducted at New York Medical College, Valhalla, NY, USA. All procedures were approved by the Institutional Animal Care and Use Committee. The MIC of nisin was 0.25 mg/L for the isolates tested,