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The binding of cefonicid (SK&F 75073), a new parenteral cephalosporin, to the penicillin-binding proteins (PBPs) of Escherichia coli K-12 (strain KN-126) was determined by competitive binding studies versus benzyl[14C]penicillin. Cefonicid showed its greatest affinity for PBPs 1a greater than 3 greater than 1b, bound with low affinity to PBPs 4 greater than 2, and did not bind to PBPs 5 and 6. Provisional affinity constants (cefonicid concentration that gave 50% inhibition of [14C]penicillin binding) were determined: PBP 1a, less than 0.25 microgram/ml; PBP 3, 0.7 microgram/ml; PBP 1b, 10 micrograms/ml; PBP 4, 26 micrograms/ml; PBP 2, 90 micrograms/ml; PBPs 5 and 6 greater than 256 micrograms/ml. Direct binding studies with [14C]-cefonicid confirmed this pattern of binding. Subinhibitory concentrations of cefonicid (MIC, broth 0.2 microgram/ml, agar 0.4 microgram/ml) induced filamentation of E. coli KN-126. This implies that PBP 3 is the primary inhibitory site despite the higher affinity of PBP 1a for this cephalosporin.

The evolution of a highly targeted screening program for the discovery of antibiotics of the glycopeptide (vancomycin) class is described. A holistic approach was utilized which optimized not just screening techniques but also the selection of candidate producer cultures and their growth under conditions which enhanced production of target compounds. Two screen techniques were utilized; differential inhibition of a vancomycin-resistant strain and its susceptible parent, and a specific antagonism screen using the reversal of glycopeptide activity by a tripeptide analog of the glycopeptide receptor, diacetyl-L-lysyl-D-alanyl-D-alanine. The latter screen was 2- to 32-fold more sensitive to known glycopeptides than the former, and was absolutely specific, yielding no false positive responses. The use of the tripeptide antagonism assay, combined with optimized culture selection and growth conditions yielded novel glycopeptide antibiotics at a rate of 1 per 320 cultures screened. With a holistic approach to screening and properly optimized techniques, large numbers of cultures do not need to be examined in order to discover novel antibiotics.

The stability of cefonicid (SK&F 75073) towards representatives of six major classes of beta-lactamases was determined using a spectrophotometric assay. Cefonicid was stable to hydrolysis by the Type I enzyme from Enterobacter cloacae and by the enzyme from the anaerobe, Bacteroides fragilis. It was 6 to 7 times more stable than cefamandole to the Type IIIA and B enzymes from Escherichia coli, a little less stable than this antibiotic to the Type V enzyme from E. coli, and of equal stability to the Type IV enzyme from Klebsiella aerogenes. Cefonicid was a non-competitive inhibitor (Ki of 0.8 x 10(-6)M) of cephalothin hydrolysis by the Type I enzyme.

A new species of a new genus of the Actinomycetales was discovered, Kibdelosporangium aridum. This strain produces a new family of glycopeptide antibiotics designated aridicins, that contain an unusual glycolipid constituent. They inhibit Gram-positive bacteria, including staphylococci, enterococci and Clostridium sp.