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P H Williams

Publications and source records attributed to P H Williams.

5 recordsLinked to original sources
PubMed1979

Novel iron uptake system specified by ColV plasmids: an important component in the virulence of invasive strains of Escherichia coli.

The enhanced virulence of invasive strains of Escherichia coli carrying ColV plasmids was shown to be due to a novel plasmid-mediated iron uptake system. Possession of a ColV plasmid conferred strong selective advantage on the host bacterial strain in experimental infections unless excess iron was administered in the inoculum. Moreover, supplementation of defined minimal medium with transferrin to complex available iron caused marked limitation of the growth of plasmid-free strains but had no effect on strains carrying a ColV plasmid. The activity of an efficient iron uptake process was clearly shown by experiments with a mutant of E. coli deficient in enterochelin biosynthesis. Although the mutant was dependent on the presence of citrate in the growth medium to facilitate iron transport, colicinogenic derivatives did not require added citrate for growth. Radioactive iron was shown to be taken up rapidly by nongrowing cells of the plasmid-carrying strain. Furthermore, it was observed that repression of the synthesis of specific outer membrane proteins normally induced by conditions of iron deficit was maintained after a shift of the colicinogenic strains from a rich medium to a medium low in iron. The ColV plasmid-mediated iron uptake system was independent of the active iron transport mechanisms known in E. coli, but like them it required tonB activity as a source of energy.

Animals↗
PubMed1978

Adherence of an enteropathogenic strain of Escherichia coli to human intestinal mucosa is mediated by a colicinogenic conjugative plasmid.

The capacity of a human enteropathogenic Escherichia coli (EPEC) strain serotype O26:K60:H11, to adhere to the mucosa of the human fetal small intestine was shown to be plasmid mediated. Adherence was transferred at a high frequency in a long-term conjugal mating experiment to E. coli K-12 and was lost by treatment of the EPEC strain with the curing agent ethidium bromide. Analysis of radioactively labeled DNA from lysates of the EPEC, transconjugant, and cured strains indicated that adherence was correlated with the presence of plasmid DNA species with an approximate average molecular weight of 56 X 10(6). Resistance to the antibiotics spectinomycin, streptomycin, sulfonamides, and tetracycline and production of colicin Ib were all transferred in long-term mating and lost upon curing coordinately with the property of adherence. In conjugal mating experiments of limited duration between E. coli K-12 strains, however, segregation of colicin production and mucosal adherence from multiple drug resistance was observed. Analysis of plasmid DNA of segregant transconjugant strains confirmed the presence in the 56 X 10(6)-dalton plasmid species of two previously unresolved components, pLG101 designating the ColIb plasmid which also carries the determinant for mucosal adherence and pLG102 representing the slightly smaller multiple drug resistance plasmid.

Antigens, Bacterial↗
PubMed1977

Purification and characterization of covalently closed replicative intermediates of ColEl DNA from Escherichia coli.

Pulse-labeled ColEl DNA molecules, undergoing replication in Escherichia coli cells either in the absence or presence of chloramphenicol, were extracted and purified by neutral sucrose density gradient sedimentation and equilibrium centrifugation in an ethidium bromide-cesium chloride gradient. In the dye-buoyant density gradient, the replicating molecules were found in regions between the supercoiled and open-circular nonreplicating plasmid DNA, as well as in the open-circular region. In a neutral sucrose gradient, peaks of pulse label were found in the region of 26 to 38 S as well as at the 23 and 17 S positions corresponding to the positions of supercoiled and open-circular ColEl DNA. In alkaline sucrose gradient, nascent ColEl DNA was found to sediment as discrete peaks corresponding to 5-6, 7-9, and 14-16 S, indicating that at least one growing strand of the replicating molecule is produced discontinuously. In the electron microscope, many of the molecules appeared as partially supercoiled structures containing two open-circular branches of equal length, of less than 20% to more than 90% replicated. Branched open-circular molecules were not observed to any significant extent without prior treatment to induce single-strand scissions. The parental strands of the replicating molecules were determined to be covalently closed, but the superhelical density of the DNA was shown to be progressively decreased as replication proceeded.

Centrifugation, Density Gradient↗