Microbiological properties of a new cephalosporin, BL-S 339: 7-(phenylacetimidoyl-aminoacetamido)-3-(2-methyl-1,3,4-thiadiazol-5-ylthiomethyl)ceph-3-em-4-carboxylic acid.

BL-S 339 is a new broad-spectrum, parenterally effective cephalosporin whose expression of antibacterial activity in vitro is markedly affected by the nature of the assay medium. When assayed in nutrient agar, BL-S 339 was more active than cephalothin against strains of Diplococcus pneumoniae, Streptococcus pyogenes, Escherichia coli, Serratia marcescens, Klebsiella pneumoniae, Enterobacter, and indole-positive Proteus sp. However, when assayed in Mueller-Hinton medium, its activity, especially against gram-negative bacteria, was reduced substantially, whereas the activity of cephalothin was virtually unaffected by the assay medium. The in vivo activity of BL-S 339 correlated well with its activity in nutrient agar; when administered subcutaneously to mice, it was therapeutically more efficacious than cephalothin in infections caused by both gram-positive and gram-negative bacteria. When BL-S 339 was administered intramuscularly to mice, the concentrations achieved in the blood were three times those achieved with cephalothin. BL-S 339 was bound to human serum proteins to the same extent as cephalothin. Recovery of BL-S 339 in the urine within the 24-hr period after intramuscular administration to rats was three times that of cephalothin.