No abstract

There are four penicillin-binding proteins (PBPs) in Staphylococcus aureus, of which PBPs 2 and 3 are essential. Cefotaxime binds selectively to PBP 2, and cephalexin binds to PBP 3, each at its respective MIC. The morphology of S. aureus strains grown in the presence of the two antibiotics was examined by phase-contrast and scanning electron microscopy. Exposure of the cells to cefotaxime at concentrations at which it bound selectively to PBP 2 resulted in the extrusion of cytoplasm and cell lysis, whereas exposure to cephalexin at concentrations at which it bound exclusively to PBP 3 resulted in cell enlargement and the cessation of septation. The latter morphological response was very similar to that produced by norfloxacin. The results suggest that in S. aureus, PBP 2 may be the primary peptidoglycan transpeptidase, and PBP 3 may be involved in septation.

Ro 23-9424 is a broad-spectrum antibacterial agent composed of a cephalosporin and a quinolone moiety. Its biological properties were compared with those of its two components and structurally related cephalosporins and quinolones. Like ceftriaxone and cefotaxime but unlike its decomposition product, desacetyl cefotaxime, Ro 23-9424 bound at c2 ,ug/ml to the essential penicillin-binding proteins lb and 3 of Escherichia coli and 1, 2, and 3 of Staphylococcus aureus. In E. coli, Keith, 28th ICAAC, abstr. no. 448, 1988

The mode of action of Ro 13-5478 and Ro 14-9578, monocyclic and tricyclic quinolone analogs, respectively, was examined for Escherichia coli and Staphylococcus aureus. The compounds showed antibacterial activity and effects on cell morphology, replicative DNA biosynthesis, and gyrase-catalyzed DNA supercoiling that were comparable to those shown by nalidixic acid and by oxolinic acid compounds. The results suggest that their site of action is DNA gyrase and that a bicyclic quinolone nucleus is not essential for activity.Quinolone antibacterial agents, including nalidixic acid and oxolinic acid, act on susceptible bacteria by immediately, selectively, and reversibly inhibiting DNA synthesis (3, 7). The quinolones also inhibit DNA replication, but not DNA repair, in permeabilized cell systems (16,19). The molecular target of the quinolones is DNA gyrase, a unique and essential procaryotic enzyme involved in the negative supercoiling (unwinding) of double-stranded DNA (4, 5). DNA gyrase acts by introducing a transient double-strand break in DNA (12), which is bridged via phosphodiester bonds to a tyrosine residue in each of the two A subunits of the enzyme and the 5' ends of the two DNA strands (20). Quinolones interfere with the enzyme at this step, leading to DNA breakage (18), which may in turn trigger the SOS response (8, 9). In Escherichia coli, the SOS response manifests itself morphologically as extensive filamentation (10).This study describes the antimicrobial activity and mechanism of action of Ro 13-5478 and Ro 14-9578, which are two lead compounds from a series of novel quinolone analogs. These compounds have an aromatic substituent joined to the pyridone ring at the 6 position, rather than fused across the 5 and 6 positions as is the case with most quinolones. The synthesis and biological properties of the monocyclic and tricyclic quinolone series will be described elsewhere

A simple radiochemical method for evaluating the action of antibiotics on Escherichia coli cells in biofilms is reported. After growth, biofilms of E. coli ATCC 25922 on disks of urinary catheter material were suspended in fresh medium containing or lacking an antibiotic, incubated for 4 h at 37°C, and pulse-labeled with [3Hjleucine for 5 min. Radioactivity in trichloroacetic acid-precipitable material in the biofilm and in the surrounding medium (planktonic E. colt) was then measured. (Difco). E. coli cells were removed from the disks by incubating the disks in wells containing 0.5 ml of 5% sodium dodecyl sulfate for 3 h at 37°C. After incubation of the disks, the sodium dodecyl sulfate was transferred to tubes containing 0.5 ml of 10% TCA supplemented with 1% Casamino Acids. Each TCA precipitate was filtered through a prewetted Gelman Metricel membrane filter (pore size, 0.45 ,um; Fisher Scientific Co., Pittsburgh, Pa.). Membranes were washed three times with 5 ml of 5% TCA containing 0.1% Casamino acids and dried under an infrared lanip, and radioactivity was counted in 10 ml of Filtron-X scintillation fluid (National Diagnostics In some experiments, viable bacterial counts of the biofilms were determined as described previously (6)