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N A Curtis

Publications and source records attributed to N A Curtis.

At least 19 recordsLinked to original sources
PubMed1988

Iron-regulated outer membrane proteins of Escherichia coli K-12 and mechanism of action of catechol-substituted cephalosporins.

Selected aminothiazolyl-oxime cephalosporin congeners substituted at C-3' with a catechol moiety were used to probe the basis of the enhanced antibacterial activity against Escherichia coli K-12 often associated with chemical modifications of this type. Evidence is presented for a tonB-dependent illicit transport of the compounds across the outer membrane of E. coli K-12, the process involving jointly and specifically the Fiu and Cir iron-regulated outer membrane proteins. Thus, both tonB and fiu cir mutants showed a comparably reduced susceptibility to the probe compounds, whereas mutants singularly lacking any one of the six iron-regulated outer membrane proteins (Fiu, FepA, FecA, FhuA, FhuE, and Cir) or lacking any combination of any two of these proteins (except Fiu plus Cir) did not show this resistance. Mutants devoid of all six iron-regulated outer membrane proteins were no more resistant to the probe compounds than fiu cir or tonB strains. In addition to the latter genes, the products of the exbB and possibly the exbC loci were necessary for maximal antibacterial potency. A dependence of antibacterial activity on the level of expression of the uptake system components was noted. Comparison of penicillin-binding protein target affinity with antibacterial activity suggested a possible periplasmic accumulation of active compounds by E. coli K-12. Free vicinal hydroxyl groups of the catechol residue were a primary chemical requirement for recognition by the uptake pathway and thus for high antibacterial activity.

Bacterial Outer Membrane Proteins↗
PubMed1987

Properties of spontaneous Enterobacter cloacae mutants with temperature-conditional derepression of type I beta-lactamase synthesis.

Spontaneous mutants, with temperature-conditional derepression of chromosomally-encoded Type I beta-lactamase synthesis, were derived from two independent clinical isolates of Enterobacter cloacae. At the permissive temperature (28 degrees C) the mutants' beta-lactamase activity was equivalent to that of their respective parents but at restrictive temperatures (above 40 degrees C) the activity increased many hundred-fold. The increased beta-lactamase expression correlated with reduced beta-lactam susceptibility. In temperature shift-up experiments, the initial rate of beta-lactamase synthesis closely paralleled that of the parent strains induced with cefoxitin. Maximal beta-lactamase activity in the mutants was attained after about 3 h growth at restrictive temperatures and was significantly higher than that of the cefoxitin-induced parents. However, the level was not as high as that observed in isogenic temperature-stable derepressed mutants, under the same conditions. All temperature-conditional mutants showed hyper-induction of beta-lactamase synthesis at permissive temperatures. Our findings are discussed in relation to a positive control model for regulation of Type I beta-lactamase synthesis in Ent. cloacae.

Anti-Bacterial Agents↗
PubMed1986

Inducible type I beta-lactamases of gram-negative bacteria and resistance to beta-lactam antibiotics.

Mutants, showing either constitutive (depressed) or non-inducible expression of chromosomally-mediated Type I beta-lactamase were obtained from clinical isolates of Enterobacter cloacae, Ent. aerogenes, Citrobacter freundii, Providencia stuartii, Morganella morganii, Serratia marcescens and Pseudomonas aeruginosa. The wild-type and mutant strains were compared for susceptibility to a range of beta-lactam antibiotics. Derepression of beta-lactamase synthesis generally, but not always, resulted in a marked reduction in susceptibility to the agents tested, including the '3rd generation' cephalosporins. In many cases, the observed resistance would preclude, or severely compromise, the therapeutic efficacy of the drugs. In this context, depressed mutants of Enterobacter spp., Citro. freundii and Ps. aeruginosa could be of primary concern although those of Ser. marcescens, Prov. stuartii and Morg. morganii often exhibited equally high resistance levels to older beta-lactams. Comparison of the susceptibilities of the non-inducible mutants with that of their inducible parents suggested variation in the beta-lactamase inductive potency of different compounds in different organisms. For example, cefoxitin was a powerful inducer in Ent. cloacae, Citro. freundii and one strain of Ps. aeruginosa; similarly cefazolin and cefuroxime were good beta-lactamase inducers in Ser. marcescens and Morg. morganii. Aminothiazolyl-oxime cephalosporins and ureido-penicillins were generally poor inducers. From such comparisons, the contribution of inducible Type I beta-lactamase to resistance phenotype could be ascertained.

Anti-Bacterial Agents↗
PubMed1985

Inhibition of penicillin-binding protein 3 of Escherichia coli K-12. Effects upon growth, viability and outer membrane barrier function.

A temperature-conditional, cell-division mutant of Escherichia coli K-12 possessing a thermolabile penicillin-binding protein (PBP) 3 was isolated. The mutant phenotype was due to a lesion in the pbpB gene. This mutant, and leu+ pbpB co-transductants of E. coli C600 grew as rods at 30 degrees C but were converted to filaments at 42 degrees C upon denaturation of PBP3 and concomitant cessation of cell division. These strains have been used to study the consequences of the specific inhibition of PBP3 of E. coli K-12 upon growth, viability and outer membrane integrity. Our results indicate that the singular inhibition of PBP3 is bactericidal in E. coli K-12, even though the turbidimetric response of the bacteria in broth culture suggests bacteriostasis. Furthermore, filament formation is accompanied by disruption of outer membrane barrier function, as witnessed by the rapid leakage of periplasmic beta-lactamase. This latter finding was confirmed by observing the lytic effect of a sub-inhibitory concentration of cefsulodin on filaments of E. coli K-12 induced by PBP3-specific beta-lactams. The impact of these results upon the testing of beta-lactam sensitivity of E. coli K-12 is discussed.

Acyltransferases↗
PubMed1981

A role in vivo for penicillin-binding protein-4 of Staphylococcus aureus.

The degree of cross-linking of the peptidoglycan of Staphylococcus aureus H and mutants lacking penicillin-binding proteins 1 and 4 was studied. No major changes were observed in organisms lacking protein 1 whereas loss of protein 4 was accompanied by a marked reduction in the degree of cross-linking and the absence of a membrane-bound 'model' transpeptidase activity. A similar effect was achieved when cultures of the staphylococci were treated with the beta-lactam antibiotic cefoxitin. At low concentrations (0.05 microgram ml-1) cefoxitin shows highest affinity for protein 4 to which it appears to bind irreversibly. Treatment of the mutant lacking protein 4 with this concentration of the antibiotic did not affect the degree of cross-linkage. The possibility that the decrease in cross-linkage was a consequence of DD-carboxypeptidase activity on peptidoglycan precursors was investigated. Although both S. aureus H and the mutants possessed such activity it was insensitive to benzylpenicillin and cefoxitin and the role of this enzyme(s) in peptidoglycan biosynthesis remains unknown. We conclude that in vivo protein 4 acts as a transpeptidase involved in the secondary cross-linking of peptidoglycan and this activity is necessary to achieve the high degree of cross-linkage observed in the peptidoglycan of staphylococci.

Bacterial Proteins↗
PubMed1979

Inhibition of Escherichia coli K-12 by beta-lactam antibiotics with poor antibacterial activity: interaction of permeability and intrinsic activity against penicillin-binding proteins.

The effect of methicillin, cloxacillin, 1078/1/1, penicillin G, and cephaloridine upon the penicillin-binding proteins of a permeability mutant of Escherichia coli K-12 and its isogenic wild type have been investigated. Comparison of the 50% inhibition values for the antibiotics against the penicillin-binding proteins of the two strains with the minimal inhibitory concentrations for the same compounds indicates that methicillin, cloxacillin, 1078/1/1, and to a lesser extent penicillin G, owe their poor antibacterial activity to exclusion from the bacterial cell, whereas cephaloridine is not excluded and is equally active against both the mutant and its wild type. The results further suggest that the lesion in the permeability mutant E. coli DC2 allows free access of all the compounds tested to the inner membrane target proteins.

Anti-Bacterial Agents↗
PubMed1979

Competition of beta-lactam antibiotics for the penicillin-binding proteins of Pseudomonas aeruginosa, Enterobacter cloacae, Klebsiella aerogenes, Proteus rettgeri, and Escherichia coli: comparison with antibacterial activity and effects upon bacterial morphology.

The competition of a number of beta-lactam morphogenic probes for the penicillin-binding proteins (PBPs) of Pseudomonas aeruginosa, Enterobacter cloacae, Klebsiella aerogenes, Proteus rettgeri, and Escherichia coli has been studied. The results indicate that the various gram-negative bacteria have similar, but not identical, PBP patterns and that the individual proteins probably perform similar morphogenic functions as in E. coli K-12. Comparison of the 50% binding concentrations of the compounds for the various PBPs of the five strains with their antibacterial activity indicates that the different antibiotics are excluded to a greater or lesser degree by the outer membrane permeability barrier and that the exclusion is most pronounced in P. aeruginosa.

Anti-Bacterial Agents↗
PubMed1979

Affinities of penicillins and cephalosporins for the penicillin-binding proteins of Escherichia coli K-12 and their antibacterial activity.

The affinities of a range of penicillins and cephalosporins for ther penicillin-binding proteins of Escherichia coli K-12 have been studied, and the results were compared with the antibacterial activity of the compounds against E. coli K-12 and an isogenic permeability mutant. Different penicillins and cephalosporins exhibited different affinities for the "essential" penicillin-binding proteins of E. coli K-12, in a manner which directly correlated with their observed effects upon bacterial morphology. Furthermore, the affinities of the compounds for their "primary" lethal penicillin-binding protein targets showed close agreement with their antibacterial activities against the permeability mutant.

Bacterial Proteins↗
PubMed1978

Modified peptidoglycan transpeptidase activity in a carbenicillin-resistant mutant of Pseudomonas aeruginosa 18s.

A carbenicillin-resistant mutant of Pseudomonas aeruginosa 18s was found to possess peptidoglycan transpeptidase activity significantly more resistant to inhibition by benzyl penicillin, ampicillin, carbenicillin, and cephaloridine than that of the parent strain. The mutant was more resistant than the parent strain to all of the beta-lactam antibiotics tested, and 50% inhibition values for these compounds against membrane-bound model transpeptidase activity paralleled this increase. The resistance of the mutant to kanamycin, streptomycin, and chloramphenicol was unchanged.

Acyltransferases↗
PubMed1976

Inhibition of peptidoglycan cross-linking in growing cells of Escherichia coli by penicillins and cephalosporins, and its prevention by R factor-mediated beta-lactamase.

The degree of peptidoglycan cross-linking has been studied in growing cells of a Dap(-) Lys(-) auxotroph of Escherichia coli K-12 by following the incorporation of [(3)H]diaminopimelic acid into the lysozyme digestion products of crude, isolated peptidoglycan. The percentage of inhibition of cross-linking increases with increasing concentrations of penicillin G, cephaloridine, and cefuroxime. When the R factor R1drd 19 was introduced into the strain by conjugation, it was found that the type IIIa, beta-lactamase specified by the plasmid was able to protect the cross-linking target against inhibition by penicillin G but not against cephaloridine, even though the beta-lactamase hydrolyzes this substrate 50% faster than penicillin G. Cefuroxime, which is completely resistant to hydrolysis by the type IIIa beta-lactamase, inhibited the peptidoglycan cross-linking target in both the R(+) and R(-) variants of the assay strain. A mutant plasmid, R1drd19amp2, which specified no type IIIa beta-lactamase synthesis, could not provide protection of the cross-linking target against penicillin G. The significance of these results, in relation to the ability of the antibiotics to pass the permeability barrier of the bacterial envelope, is discussed.

Amidohydrolases↗