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D H Walker

Publications and source records attributed to D H Walker.

At least 253 records · Page 14Linked to original sources
PubMed1976

Genetic studies of coliphage P1. III. Extended genetic map.

An extensive genetic map of coliphage P1 has been constructed for 113 amber mutants, using primarily a modification of the conventional complementation spot test. These spot tests failed to classify the mutants into cistrons, but when they were quantitated they permitted assignment of the mutants into 10 linkage clusters. Furthermore, a linear order could be deduced for most of the mutants within each cluster. This strongly suggested that recombination was the predominant event generating plaques and that, for the practical purpose of rapid genetic mapping, such spot tests could be considered as a series of two-factor crosses. Six of the 10 linkage clusters correlated with the P1 genetic map established by Scott (1968). The locations of the remaining four clusters were determined by three-factor crosses and by prophage deletion mapping. The nonrandom occurrence of termini for 14 deletion prophages, which we established previously (Walker and Walker, 1975), and the coincidence of these termini with five out of ten regions demarcating the linkage clusters are discussed. Complementation tests in liquid frequently gave ambiguous results. Therefore, cistron designations were not assigned.

Chromosome Mapping↗
PubMed1976

Ultrastructural localization of viral antigens using the unlabeled antibody-enzyme method.

Employing the unlabeled antibody enzyme method at the ultrastructural level, a comparison was made between preembedding staining and postembedding staining for the detection of viral antigens. The bacteriophage P1 absorbed to the surface of Shigella dysenteriae was used as a model system. Preembedding staining resulted in the specific deposition of peroxidase-antiperoxidase (PAP) complexes as an electron-dense coating around the viral heads. Disadvantages of the preembedding staining method included the agglutination of cells by the primary antiserum which produced a gradient of specific staining and the "bleeding" or migration of electron dense reaction product away from the sites of attached PAP complexes. The postembedding staining method had distinct advantages over the preembedding staining in that PAP complexes were deposited directly over exposed viral heads within the thin section. In addition, the specific immunostaining of viruses was uniform through the section and no artifactual migration of reaction product was observed.

Animals↗
PubMed1976

Early lymphoreticular viral tropism and antigen persistence. Tamiami virus infection in the cotton rat.

Tamiami virus was inoculated into its natural reservoir host, the cotton rat (Sigmodon hispidus), and the course of infection was followed by sequential organ titrations, frozen-section immunofluorescence, and light and electron microscopy. In animals infected at 2 days of age, there was an early lymphoreticular tropism with peak concentrations of virus and viral antigen in lymph nodes, splenic white pulp, thymus, and bone marrow at 16 days postinoculation. Megakaryocyte infection was early and pronounced. Viral antigen concentration peaked in liver and salivary glands at day 30 and in kidney, adrenal cortex, respiratory tract, and bladder epithelium at day 60-long after viral infectivity in these organs had disappeared. Central nervous system infection was only modestly productive of infectious virus, but viral antigen continued to increase in the brain until day 90 and then did not decline throughout the 360-day study. Reticuloendothelial hyperplastic foci were found late in some target organs, but there was never any histologic or ultrastructural evidence of cytonecrosis. Older animals were virtually uninfectable; therefore, this susceptibility of newborns and their slow termination of infection represent the key to virus transmission and perpetuation in nature. These aspects of viral natural history contribute to an understanding of human exposure to the pathogenic arenaviruses which exist in similar rodent niches.

Age Factors↗
PubMed1976

Lymphoreticular and myeloid pathogenesis of Venezuelan equine encephalitis in hamsters.

Ultrastructural, histopathologic, and virologic studies of adult hamsters infected with virulent Venezuelan equine encelphalomyelitis (VEE) virus (Subtype I-B) demonstrated precise chronologic and topographic progression of lesions and viral replication in extraneural sites. Thymus contained the earliest lesions and the highest initial and subsequent viral titers. No particular cytotropism was observed as highly efficient viral replication and severe cytonecrosis proceded. Early cortical necrosis of splenic periarteriolar lymphocytic sheath was followed by lymphoblastoid repopulation of the peripheral zone. Massive bone marrow necrosis was accompained by ultrastructural evidence of VEE viral particle production in reticulum cells, rubricytes, myeloid cells, lymphoblastoid cells, and megakaryocytes. Speed, efficiency, destructiveness, and relative sensitivity of virtually all lymphoreticular and hematopoetic cells were hallmarks of virulent VEE infection in the hamster.

Animals↗
PubMed1975

Ultrastructal morphology of some prokaryotic microorganisms associated with the hindgut of cockroaches.

Scanning electron microscopy and transmission electron microscopy have been used to visualize the morphology and ultrastructure of two types of microorganisms in the hindgut of the cockroach Blaberus posticus. Both organisms, designated as either short or long rods, are attached to chitinous projections from the gut wall. Micrographs suggest that the organisms are prokaryotic with a cell wall complex characteristic of gram-negative bacteria. However, certain differences were noted between the cell wall complex of the two types. Two forms of the long-rod type were noted, with one form appearing to be a "degenerate" or "transitional" cell. In the degenerate cells, vesicles are observed that often are contiguous with the cytoplasmic membrane. There are indications that the long-rod type may divide by longitudinal fission. Neither the short- nor long-rod type has been cultivated in its respective recognizable form.

Animals↗
PubMed1975

Conditions critical for optimal visualization of bacteriophage adsorbed to bacterial surfaces by scanning electron microscopy.

The potential of scanning electron microscopy as a tool for the detection of viruses on cell surfaces has been studied using bacteriophage P1 adsorbed to Shigella dysenteriae as a model system. Viral particles were readily detectable by scanning electron microscopy on the surface of infected cells which were fixed with glutaraldehyde followed by postfixation in OsO4 and prepared by critical point drying. The virus-studded surface of the infected cells differed markedly from the relatively smooth surfaces of uninfected control cells. Examination of the same preparations with transmission electron microscopy revealed numerous viral particles adsorbed to the surfaces of infected cells, whereas the control cells were free of viruses as expected. Glutaraldehyde fixation alone did not preserve the surface detail of infected cells: cells adsorbed with viruses were not distinguishable from control cells by scanning electron microscopy although by transmission electron microscopy viruses could be visualized. Air drying from water or absolute alcohol resulted in unsatisfactory preservation as compared to the appearance of infected cells prepared by the critical point method. Thus, scanning electron microscopy is capable of resolving viral particles on cell surfaces, but detection of these particles is completely dependent both on the method of fixation and on the technique of drying used.

Adsorption↗
PubMed1975

Genetic studies of coliphage P1. I. Mapping by use of prophage deletions.

One hundred and ten amber mutants of coliphage P1 were isolated and localized into groups with respect to the existing genetic map by use of nonpermissive Escherichia coli K-12 strains lysogenic for P1 with deletions. These lysogens contain one of three types of deletion prophages: P1cry and its derivatives, P1dlacs, and P1dpros. Fourteen such lysogens were tested for their ability to rescue the amber mutants which were then assigned to one of nine deletion segments of the P1 genome defined by the termini of the various prophage deletions. The relationship of the nine deletion segments with the published P1 map is described, two new segments having been added. The deletions of the 14 prophages overlapped sufficiently to indicate that the P1 genetic prophage map should be represented in circular form, which is consistent with the fact that P1 is normally a circular plasmid in the prophage state. The distribution of mutants into deletion segments is nonrandom for at least one segment. In addition, the deletion termini of the 14 defective prophages coincided in five out of nine regions separating the nine deletion segments. Various possible explanations are discussed for the nonrandom recurrence of these deletion termini, including the evidence of hot spots of recombination.

Alleles↗
PubMed1975

The pathology of human Lassa fever.

Pathological findings have been described in only a small number of cases of Lassa fever since the virus was first isolated in 1969. Morphologically, eosinophilic necrosis of hepatocytes was the most frequent finding and focal necroses, often extensive, were present in most cases. These findings are similar to the lesions previously described in Argentinian and Bolivian haemorrhagic fever. Focal interstitial pneumonitis, focal tubular necrosis in the kidney, lymphocytic infiltration of the splenic veins, and partial replacement of the splenic follicles by amorphous eosinophilic material have been described, but the significance of these findings is unclear. More detailed and sophisticated investigations are required in the future if pathogenetic mechanisms are to be unravelled.

Humans↗
PubMed1975

Experimental Lassa virus infection in the squirrel monkey.

Experimental Lassa virus infection was investigated in a nonhuman primate in order to elucidate the target organs of the viral infection and the course of pathologic events. Four squirrel monkeys (Saimiri scirreus) were inoculated intramuscularly with Lassa virus and sacrificed for organ titrations and histopathology, one each day, on Days 7, 12, 14, and 28 after inoculation. The animals showed a variable clinical course, with an incubation period of 8 to 18 days. The virus was demonstrated to be virtually pantropic; however, lymph node, liver, and kidney were key early targets. After the onset of overt disease, patterns of lymphoreticulotropism, hepatotropism, nephrotropism, adrenotropism, and persistent viremia were evident. Complement-fixing antibody failed to develop after 28 days of infection. Histopathologic findings included germinal center necrosis in spleen and lymph node; myocarditis; acute arteritis; renal tubular necrosis and regeneration; hepatocytic regeneration; chronic inflammation of choroid plexus, ependyma, and meninges; and cerebral perivascular cuffing. There is a relationship between many of these lesions and certain features of other arenavirus infections. The model offers the opportunity to pursue investigations of experimental pathogenesis, transmissibility, and efficacy of immunotherapy.

Animals↗
PubMed1975

Comparative pathology of Lassa virus infection in monkeys, guinea-pigs, and Mastomys natalensis.

Experimental Lassa virus infections of squirrel monkeys, guinea-pigs, and the African multimammate rat, Mastomys natalensis, were studied virologically and pathologically. In the monkeys, early viral lymphoreticulotropism, hepatotropism, nephrotropism, and viraemia were noted. At the time of death, viral titres in nearly all target organs were associated with necrotic changes: splenic lymphoid necrosis, renal tubular necrosis, myocarditis, arteritis, and hepatocytic regeneration. In convalescent monkeys, organ titres diminished slowly, and viraemia persisted at 28 days. At this time, renal and splenic regeneration was occurring and a new lesion, choriomeningitis, was present.Guinea-pigs infected with Lassa virus developed respiratory insufficiency with pulmonary oedema, alveolar hyaline membranes, myocarditis, and focal calcification of myocardial fibres and hepatocytes. Dying animals contained Lassa virus in virtually every organ tested, whereas survivors at 56 days were free of virus and had high complement-fixing antibody titres.Infection of neonatal Mastomys did not cause any clinical disease or pathological lesions despite the presence of virus in the blood, lymph nodes, liver, spleen, lung, brain, urine, and throat secretions throughout the 74-day study. Infected adult Mastomys also remained normal but had virus in many organs. In one animal, virus persisted until the termination of the study at 103 days. Several animals developed a mild meningoencephalitis. The pattern of infection and virus shedding in M. natalensis is ideal for maintenance of the virus in nature; together with the epidemiological field data this emphasizes the incidental nature of the exposure and infection of man.

Animals↗
PubMed1972

Bacteriophage T4 head models based on icosahedral symmetry.

In an accompanying paper, the coordinate variation in lengths of deoxyribonucleic acid molecules and head lengths in morphological variants of bacteriophage T4 is discussed. As an appendix to that report, models for the T4 phage heads, based on previously developed architectural principles for assembly of icosahedral virus, are compared with the data for the best fit.

Coliphages↗