Comparison of the Tu elongation factors from Staphylococcus aureus and Escherichia coli: possible basis for elfamycin insensitivity

In a previous study (C. C. Hall, J. D. Watkins, and N. H. Georgopapadakou, Antimicrob. Agents Chemother. 33:322-325, 1989), the elongation factor Tu (EF-Tu) from Staphylococcus aureus was found to be insensitive to a series of kirromycin analogs which were inhibitory to the EF-Tu from Escherichia coli. In the present study, the EF-Tu from S. aureus was partially purified and characterized. Its apparent molecular mass was approximately 41,000 Da, and the enzyme copurified with EF-Ts (molecular mass, 34,000 Da). S. aureus EF-Tu differed from its E. coli counterpart in that it bound negligible amounts of [3H]GDP, in addition to being insensitive to pulvomycin and aurodox (50% inhibitory concentrations, approximately 100 and 1,000 microM, respectively, versus 2 and 0.2 microM, respectively, for E. coli). The results are consistent with the formation of a stable EF-Tu.EF-Ts complex that affects the interaction of EF-Tu with guanine nucleotides and inhibitors.

Dual-action cephalosporins: cephalosporin 3'-quinolone carbamates

A series of cephalosporins has been prepared in which the 3'-position was linked to the nitrogen of the antibacterial quinolone ciprofloxacin through a carbamate function. Like the ester-linked and quaternary-linked dual-action cephalosporins reported earlier, these carbamate-linked compounds exhibited a broad antibacterial spectrum derived from both cephalosporin-like and quinolone-like activities, suggesting a dual mode of action. Studies to elucidate details of the mechanism of action have been inconclusive. Ciprofloxacin liberated as a consequence of bacterial enzyme-mediated reactions may contribute to the second mode of action, although some evidence indicates that the intact carbamate-linked bifunctional molecules may possess intrinsically both beta-lactam and quinolone activities.

Escherichia coli resistant to cephalosporins and quinolones is still susceptible to the cephalosporin-quinolone ester Ro 23-9424

Ro 23-9424 is a broad-spectrum antibacterial agent consisting of a cephalosporin (desacetylcefotaxime) linked through an ester bond to a fluoroquinolone (fleroxacin). Its activity against mutants of Escherichia coli TE18 resistant to both antibacterial components was examined. E. coli TE18 overproduces the AmpC beta-lactamase and is resistant to several cephalosporins, including desacetylcefotaxime (MIC, 50 micrograms/ml), although it is still susceptible to Ro 23-9424 (MIC, 0.2 microgram/ml). Thirty-five spontaneous, two-step mutants of E. coli TE18 which were resistant to fleroxacin (MIC, 50 micrograms/ml) were isolated. In the mutants, replicative DNA biosynthesis (permeabilized cells) was resistant to fleroxacin, and some mutants had porin abnormalities. However, all remained susceptible to Ro 23-9424 (MIC, 0.5 microgram/ml). Examination of beta-lactamase activity in the parent strain revealed that it hydrolyzes desacetylcefotaxime more rapidly than it does Ro 23-9424. Thus, Ro 23-9424 may be less susceptible to the gram-negative, chromosomal beta-lactamases that hydrolyze several broad-spectrum cephalosporins and may be effective in cases in which neither of its two components is active.

Dual-action cephalosporins: cephalosporin 3'-quaternary ammonium quinolones

When cephalosporins exert their biological activity by reacting with bacterial enzymes, opening of the beta-lactam ring can lead to expulsion of the 3'-substituent. A series of cephalosporins was prepared in which antibacterial quinolones were linked to the 3'-position through a quaternary nitrogen. Like the 3'-ester-linked dual-action cephalosporins reported earlier, these compounds demonstrated a broad spectrum of antibacterial activity derived from cephalosporin-like and quinolone-like components, suggesting a dual mode of action.

Cephalosporin 3'-quinolone esters with a dual mode of action

According to the generally accepted mechanism by which bacterial enzymes react with cephalosporins, opening of the beta-lactam ring can lead to the expulsion of a 3'-substituent. A series of dual-action cephalosporins was prepared in which antibacterial quinolones were linked to the cephalosporin 3'-position through an ester bond in the expectation that, in addition to exerting their own beta-lactam activity, these cephalosporins would act as prodrugs for the second antibacterial agent. Compared to parent cephalosporins in which the 3'-substituent was acetoxy, the bifunctional cephalosporins exhibited a broadened antibacterial spectrum, suggesting that a dual mode of action may indeed be operative.

Mode of action of the dual-action cephalosporin Ro 23-9424

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 less than or equal to 2 micrograms/ml to the essential penicillin-binding proteins 1b and 3 of Escherichia coli and 1, 2, and 3 of Staphylococcus aureus. In E. coli, Ro 23-9424 produced filaments exclusively and decreased cell growth; cefotaxime produced both filaments and lysis. Like its decomposition product fleroxacin but unlike quinolone esters, Ro 23-9424 also inhibited replicative DNA biosynthesis in E. coli. In an E. coli strain lacking OmpF, growth continued after addition of Ro 23-9424, decreased after addition of cefotaxime, and stopped immediately after addition of fleroxacin. The results, together with the chemical stability of Ro 23-9424 (half-life, approximately 3 h at pH 7.4 and 37 degrees C), suggest that in E. coli the compound acts initially as a cephalosporin with intrinsic activity comparable to that of cefotaxime but with poorer penetration. Subsequent to the decomposition of Ro 23-9424 to fleroxacin and desacetyl cefotaxime, quinolone activity appears. The in vitro antibacterial activity reflects both mechanisms of action.

Effects of elfamycins on elongation factor Tu from Escherichia coli and Staphylococcus aureus

Six kirromycin analogs (elfamycins) were compared on the basis of their inhibition of Escherichia coli poly(U)-directed poly(Phe) synthesis and stimulation of elongation factor Tu (EF-Tu)-associated GTPase activity. The elfamycins tested were kirromycin, aurodox, efrotomycin, phenelfamycin A, unphenelfamycin, and L-681,217. The last three lack the pyridone ring present in the other elfamycins. All the elfamycins inhibited poly(U)-dependent poly(Phe) synthesis and stimulated EF-Tu-associated GTPase activity, suggesting that the pyridone ring is not essential for activity. The six elfamycins were also examined in a poly(U)-directed, poly(Phe)-synthesizing system derived from Staphylococcus aureus and had 50% inhibitory concentrations of greater than or equal to 1 mM. When S. aureus ribosomes and E. coli elongation factors were combined in a hybrid poly(Phe)-synthesizing system, aurodox produced essentially complete inhibition of poly(Phe) synthesis with a 50% inhibitory concentration of 0.13 microM. This suggests that the observed high MICs of kirromycin and its congeners in S. aureus reflect a kirromycin-resistant EF-Tu rather than permeability constraints.

Fleroxacin resistance in Escherichia coli

Spontaneous fleroxacin-resistant mutants of Escherichia coli K-12 were isolated at a frequency of 10(-10) to 10(-11) mutants per CFU plated. All mutants exhibited quinolone-resistant replicative DNA biosynthesis, and 4 of 11 mutants also had decreased amounts of OmpF or OmpC porin. None of the mutants had changes solely in porin proteins.

Fluorometric assay for fleroxacin uptake by bacterial cells

A sensitive and convenient method for quinolone determination has been developed, based on the natural fluorescence of the quinolone nucleus. Fleroxacin (Ro 23-6240; AM 833), used as a prototype quinolone in these studies, had an excitation maximum at 282 nm and an admission maximum at 442 nm (pH 3.0). Fluorescence intensity was pH dependent, being maximal at pH 3.0 and linear at quinolone concentrations between 1 and 200 ng/ml. A protocol for the fluorometric monitoring of fleroxacin uptake in Escherichia coli was developed. Intracellular quinolone concentrations measured by the fluorometric assay correlated well with values obtained by the bioassay. The results indicate that the fluorometric assay is an attractive alternative to the more laborious bioassay.

Routes of quinolone permeation in Escherichia coli

The uptake of quinolone antibiotics by Escherichia coli was investigated by using fleroxacin (RO 23-6240, AM 833) as a prototype compound. The uptake of fleroxacin was reduced and its MIC was increased in the presence of magnesium. Quinolones induced lipopolysaccharide release, increased cell-surface hydrophobicity and outer membrane permeability to B-lactams, and sensitized cells to lysis by detergents. These effects were also antagonized by magnesium and were very similar to those seen with EDTA and gentamicin. MICs of quinolones in portin-deficient strains were increased relative to those of the parent strain, consistent with a porin pathway of entry. However, MICs were further increased in the presence of magnesium; the size of the additional increase showed a positive correlation with quinolone hydrophobicity in an OmpF- OmpC- OmpA- strain. When quinolones were mixed with divalent cations in solution, changes in quinolone fluorescence suggestive of metal chelation were observed. The addition of fleroxacin to a cell suspension resulted in a rapid initial association of fluorescence with cells, followed by a brief decrease and a final time-dependent linear increase in cell-associated fluorescence. We interpret these results as representing chelation of outer membrane-bound magnesium by fleroxacin and other quinolones, dissociation of the quinolone-magnesium complex from the outer membrane, and diffusion of the quinolone through both porins and exposed lipid domains on the outer membrane. For a given quinolone, the contribution of the porin and nonporin pathways to total uptake is influenced by the hydrophobicity of the quinolone.

Outer membrane penetration by (2,3)-methylenepenams

The penetration of the Escherichia coli outer membrane by two sterically restricted analogs of penicillin G was determined. The analog corresponding to the "open" conformation of penicillin G penetrated faster than the "closed"-form analog did, and both analogs penetrated faster than penicillin G did. The results suggest that the conformation of the beta-lactam nucleus may affect penetrability via the porin-mediated pathway.

Interaction of (2,3)-methylenepenams with penicillin-binding proteins

A series of (2,3)-methylenepenams were examined with respect to binding to essential penicillin-binding proteins (PBPs) in Escherichia coli and Staphylococcus aureus. The compounds were also examined with respect to their interaction with Streptomyces strain R61 DD-carboxypeptidase. The alpha isomer of (2,3)-methylene penicillin G bound to PBP 3 of E. coli and other enterobacteria at 0.1 to 10 micrograms/ml. The beta isomer bound to PBP 3 at 100 micrograms/ml. Either isomer bound to PBPs 1b and 2 of E. coli only at 100 micrograms/ml. The alpha, but not the beta, isomer also bound to PBP 2 of S. aureus at 0.1 micrograms/ml. Binding studies with radiolabeled compounds indicated the binding to be covalent and revealed no additional binding proteins. (2,3)-Methylenepenams active against E. coli bound to PBP 3 and induced filamentation. The compounds also inhibited Streptomyces strain R61 DD-carboxypeptidase with apparent 50% inhibitory concentrations as low as 10(-7) M. The two (2,3)-methylene penicillin G isomers bound to the enzyme covalently, most likely at the same site as penicillin G since partial proteolysis after binding radiolabeled compounds produced similar peptide patterns. The bound beta isomer was released with a half-time similar to that of penicillin G (70 min at 30 degrees C), while the alpha isomer was released with a longer half-time (13 h at 30 degrees C). With either isomer, the major release product was phenylacetylglycine, suggesting C-5-C-6 cleavage.

Monocyclic and tricyclic analogs of quinolones: mechanism of action

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.

Effect of antifungal agents on lipid biosynthesis and membrane integrity in Candida albicans

Eight antifungal agents were examined for effects on lipid biosynthesis and membrane integrity in Candida albicans. Lipids were labeled in vivo or in vitro with [14C]acetate and analyzed by thin-layer and gas chromatography. Membrane integrity was measured by a recently developed [14C]aminoisobutyric acid radiolabel release assay. The imidazole antifungal agents miconazole, econazole, clotrimazole, and ketoconazole, at concentrations inhibiting ergosterol biosynthesis (0.1 microM), decreased the ratio of unsaturated to saturated fatty acids in vivo but not in vitro. Similarly, naftifine, tolnaftate, and the azasterol A25822B, at concentrations inhibiting ergosterol biosynthesis (10, 100, and 1 microM, respectively), decreased the ratio of unsaturated to saturated fatty acids in vivo only. This suggests that the effect on fatty acids observed with ergosterol biosynthesis inhibitors may be secondary to the effect on ergosterol. With imidazoles, oleic acid antagonized inhibition of cell growth but not inhibition of ergosterol. This suggests that, with the C-14 demethylase inhibitors, decreased unsaturated fatty acids, rather than decreased ergosterol, are responsible for growth inhibition. Cerulenin, previously reported to be a potent inhibitor of both fatty acid and ergosterol biosynthesis, was found in the present study to inhibit the former (at 5 microM) but not the latter (up to 100 microM). Of the antifungal agents tested, econazole and miconazole (at 100 microM) produced complete release of [14C]aminoisobutyric acid, which is consistent with membrane damage.

Possible physiological functions of penicillin-binding proteins in Staphylococcus aureus

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.

Chitin synthase in Candida albicans: comparison of digitonin-permeabilized cells and spheroplast membranes

The treatment of Candida albicans (yeast form) with digitonin or dimethyl sulfoxide permeabilized cells and caused the activation of chitin synthase in situ. Endogenous activation was completely prevented by the sulfhydryl reagents N-ethylmaleimide, p-chloromercuribenzoate, and 5,5'-dithiobis(2-nitrobenzoic acid); partially prevented by the protease inhibitors antipain, leupeptin, and N alpha-tosyl-L-lysyl chloromethyl ketone; and also partially prevented by EDTA. Thus, a clostripain-like protease may be involved in the endogenous activation phenomenon. The pH activity profile, cofactor requirements, and kinetic parameters of the endogenously activated chitin synthase were identical to those of the trypsin-activated enzyme in protoplast membranes.

A modified radiometric assay for 3-hydroxy-3-methylglutaryl coenzyme A reductase

A radiometric assay for measuring the activity of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase is described. The assay is based on the separation of the mevalonate product from HMG-CoA by high-voltage electrophoresis. This method is more sensitive and more specific than the NADPH-based spectrophotometric assay, and less tedious than available radiometric assays. It has been used to measure HMG-CoA reductase activity in crude extracts of Saccharomyces cerevisiae and in human skin fibroblasts.

Streptomyces R61 DD-carboxypeptidase: hydrolysis of X-D-alanyl-D-alanine peptides measured by a fluorometric assay

A fluorometric procedure for measuring the activity of DD-carboxypeptidase is described. The method is based on the reaction of one of the products, D-alanine, with o-phthaldialdehyde to form a highly fluorescent adduct. The method has been applied in examining a series of X-D-alanyl-D-alanine peptides as substrates of the penicillin-sensitive DD-carboxypeptidase from Streptomyces R61. The effect of the third residue, X, on kinetic parameters and its implications on the steric analog model for penicillin action are also discussed.

Penicillin-binding proteins in Bacteroides fragilis

The penicillin binding proteins (PBSs) of Bacteroides fragilis, a clinically important Gram-negative rod, were studied. Four PBPs were detected by polyacrylamide gel electrophoresis/fluorography, PBP 4 (molecular weight, 35,000) being a minor PBP. The PBP pattern was thus different from that of the Enterobacteria and Pseudomonads. Antibacterial activity of beta-lactam antibiotics was associated with binding to PBP 1 (molecular weight, 100,000), 2 (molecular weight, 86,000) and 3 (molecular weight, 68,000). Binding to PBP 2 was associated with filamentation while binding to PBP 1 resulted in cell lysis.

Binding of monobactams to penicillin-binding proteins of Escherichia coli and Staphylococcus aureus: relation to antibacterial activity

A series of novel monocyclic beta-lactam antibiotics having side chains related to penicillin, piperacillin, azlocillin, and cefotaxime were examined with respect to binding to essential penicillin-binding proteins (PBPs) in Escherichia coli and Staphylococcus aureus. In the penicillin series, there was poor binding to all essential PBPs of E. coli (greater than 100 micrograms/ml) but good binding to PBPs 1, 2, and 3 of S. aureus (approximately 1 microgram/ml). In the piperacillin and azlocillin series, there was good binding to PBP 3 of E. coli (0.1 microgram/ml) and PBPs 1, 2, and 3 of S. aureus (approximately 1 microgram/ml). In the cefotaxime series, there was generally good binding to PBP 3 of E. coli (0.1 micrograms/ml) but poor binding to PBPs 1, 2, and 3 of S. aureus (greater than or equal to 100 micrograms/ml). With a few exceptions in the cefotaxime series, antibacterial activity paralleled essential PBP binding. Binding studies with radioactively labeled compounds revealed no additional essential monobactam-binding proteins in the two organisms. The studies suggest that monobactams are intrinsically active against both gram-positive and gram-negative bacteria; the activity spectrum of a given monobactam is determined by the binding to essential PBPs, which in turn is determined by the nature of the substituents on the beta-lactam nucleus.

Penicillin-binding proteins in a Staphylococcus aureus strain resistant to specific beta-lactam antibiotics

The penicillin-binding proteins (PBPs) of a clinical isolate of Staphylococcus aureus specifically resistant to oral cephalosporins were compared with those of a susceptible strain. In the resistant strain, PBP3 (75,000 molecular weight) was missing or had substantially (greater than 100-fold) reduced affinity for penicillin; PBP2 (80,000 molecular weight) was increased in amount and contained a satellite band, PBP2'; PBPs 1 and 4 were unchanged. Oral cephalosporins bound poorly to PBP2 in both susceptible and resistant strains, but only in the latter did binding correlate with antibiotic activity. The results are consistent with the suggestion that PBP2 is essential in S. aureus. PBP2 might in addition compensate for PBP3 when the latter is missing. In the susceptible strain the lack of correlation between binding to PBP2 and beta-lactam antibiotic activity is due to the very high affinity of the also essential PBP3 for beta-lactam antibiotics.

Mode of action of azthreonam

Azthreonam (SQ 26,776) is a member of a new class of monocyclic beta-lactam antibiotics. In Escherichia coli, azthreonam caused filamentation at its lowest effective concentration (0.2 microgram/ml), a morphological effect identical to that observed with cephalothin. The penicillin-binding protein (PBP) profile indicated a very high affinity for PBP3 (complete binding at 0.1 microgram/ml), a moderate affinity for PBP1a (complete binding at 10 micrograms/ml), and poor affinities for PBP1b, PBP2, PBP4, and PBP5/6 (complete binding at greater than or equal to 100 micrograms/ml). Accordingly, azthreonam had poor activity against Streptomyces R61 DD-carboxypeptidase (50% inhibition, greater than 100 micrograms/ml) and E. coli peptidoglycan transpeptidase (50% inhibition, 100 micrograms/ml). Azthreonam also showed very high affinity for PBP3 (complete binding at 0.1 microgram/ml) in Proteus vulgaris, Enterobacter cloacae, Klebsiella pneumoniae, and Pseudomonas aeruginosa. In all four organisms, its PBP profile was similar to that observed in E. coli. It is concluded that azthreonam, although of novel structure, has a mode of action similar to that of cephalosporins, affecting specifically septation in E. coli and most likely other gram-negative bacteria.

Interaction between monobactams and Streptomyces R61 DD-carboxypeptidase

The monobactams are a novel family of monocyclic beta-lactam antibiotics characterized by the 2-oxoazetidine-1-sulfonic acid moiety. A series of monobactams bind covalently to the Streptomyces R61 DD-carboxypeptidase in a manner similar to that for bicyclic beta-lactams, especially cephalosporins. The similarity of interaction was established by the following criteria: inhibition of binding by diisopropylfluorophosphate and alpha-dicarbonyls; stoichiometry of binding; similarity of partial proteolysis products of radiolabelled enzyme; rates of release of bound beta-lactams; nature of hydrolysis and hydroxylaminolysis products.