David G. Weeks

Linear structural equations with latent variables

An interdependent multivariate linear relations model based on manifest, measured variables as well as unmeasured and unmeasurable latent variables is developed. The latent variables include primary or residual common factors of any order as well as unique factors. The model has a simpler parametric structure than previous models, but it is designed to accommodate a wider range of applications via its structural equations, mean structure, covariance structure, and constraints on parameters. The parameters of the model may be estimated by gradient and quasi-Newton methods, or a Gauss-Newton algorithm that obtains least-squares, generalized least-squares, or maximum likelihood estimates. Large sample standard errors and goodness of fit tests are provided. The approach is illustrated by a test theory model and a longitudinal study of intelligence.

Influence of pH on metabolism and urease activity of Helicobacter pylori

BACKGROUND & AIMS The metabolic and urease responses of Helicobacter pylori to variations in gastric acidity are unknown. The aim of this study was to determine effects of changes of environmental pH on metabolism, urease activity, and survival of H. pylori in an unbuffered environment. METHODS Bacterial metabolism and urease activity were determined by measuring pH changes in perfused microphysiometer chambers over a pH range from 2.5 to 9.0 with or without urea and survival by restoration of metabolism at pH 7.4. RESULTS Glucose metabolism by acid-adapted H. pylori occurred at a perfusion pH between 3.5 and 8.6 and was highest between 7.4 and 8.2. Metabolism was irreversibly inhibited at pH <3.5 or >8.6. In the presence of 2.5 mmol/L urea, the chamber pH increased to about 6.2 during perfusion between pH 5.5 and 4.0. At pH 4.0 and below, urease activity increased several-fold without change of chamber pH. Urea in the perfusate enabled retention of metabolism after acid exposure but was toxic at pH 7.4. CONCLUSIONS The metabolic range of acid-adapted H. pylori is between an environmental pH of 3.5 and 8.6. Extracellular pH-regulated internal urease activity allows metabolism in the pH range between 4.0 and 2. 5 by maintaining periplasmic pH at 6.2. The organism is an acid-tolerant neutralophile due to internal urease activity.

Acid resistance of Helicobacter pylori depends on the UreI membrane protein and an inner membrane proton barrier

ureI encodes an inner membrane protein of Helicobacter pylori. The role of the bacterial inner membrane and UreI in acid protection and regulation of cytoplasmic urease activity in the gastric microorganism was studied. The irreversible inhibition of urease when the organism was exposed to a protonophore (3,3′,4′,5‐tetrachlorsalicylanide; TCS) at acidic pH showed that the inner membrane protected urease from acid. Isogenic ureI knockout mutants of several H. pylori strains were constructed by replacing the ureI gene of the urease gene cluster with a promoterless kanamycin resistance marker gene (kanR). Mutants carrying the modified ureAB‐kanR‐EFGH operon all showed wild‐type levels of urease activity at neutral pH in vitro. The mutants resisted media of pH > 4.0 but not of pH < 4.0. Whereas wild‐type bacteria showed high levels of urease activity below pH 4.0, this ability was not retained in the ureI mutants, resulting in inhibition of metabolism and cell death. Gene complementation experiments with plasmid‐derived H. pylori ureI restored wild‐type properties. The activation of urease activity found in structurally intact but permeabilized bacteria treated with 0.01% detergent (polyoxy‐ethylene‐8‐laurylether; C12E8), suggested a membrane‐limited access of urea to internal urease at neutral pH. Measurement of 14C‐urea uptake into Xenopus oocytes injected with ureI cRNA showed acid activation of uptake only in injected oocytes. Acceleration of urea uptake by UreI therefore mediates the increase of intracellular urease activity seen under acidic conditions. This increase of urea permeability is essential for H. pylori survival in environments below pH 4.0. ureI‐independent urease activity may be sufficient for maintenance of bacterial viability above pH 4.0.

Restricted multidimensional scaling models

Abstract A class of multidimensional scaling models are developed wherein certain parameters may be fixed as known constants, or proportional to one another. Traditional multidimensional scaling can be obtained as a special case by fixing only the orientation and origin of a configuration. Methods of obtaining least-square estimates of the parameters via nonlinear programming are discussed, and an effective computer program is developed to implement application of the models to data. Several well-known data sets are reanalyzed under various restricted models, and the results demonstrate the possibility of achieving insight not attainable under the traditional approach. The potential distortion arising from inadequate model specification is discussed, and the importance of substantive theory to multidimensional scaling research is emphasized.

Restricted Multidimensional Scaling Models for Asymmetric Proximities.

Restricted multidimensional scaling models [Bentler & Weeks, 1978] allowing constraints on parameters, are extended to the case of asymmetric data. Separate functions are used to model the symmetric and antisymmetric parts of the data. The approach is also extended to the case in which data are presumed to be linearly related to squared distances. Examples of several models are provided, using journal citation data. Possible extensions of the models are considered.

Perceived dimensions of attributions for loneliness.

This study examined the applicability of Weiners model of causal attributions to lay explanations for the causes of loneliness. Weiner posits three dimensions (Internatily, Stability, and controllability) along which causes vary and links each dimension to distinct consequences for the actor. To test the salience of these dimensions in lay perceptions of causality, 180 college students made judgments about the causes of loneliness. As predicted, both exploratory and confirmatory multidimensional scaling analyses found that dimensions of Internality and Stability were perceived by respondents. Contrary to recent theorizing, Controllability was not independent of the other two dimensions; instead, controllable causes were both internal and unstable. Confirmation of Internality and Stability as dimensions underlying attributions for loneliness supported the extension of Weiners model to the domain of affiliative behavior.

Interrelations among Models for the Analysis of Moment Structures.

Factor analysis in several populations, covariance structure models, three-mode factor analysis, structural equation systems with measurement model, and analysis of covariance with measurement model are all shown to be specializations of a general moment structure model published previously in this journal. Some new structured linear models are also described; they may be considered either generalizations or special cases of existing models. Simple representations are developed for complex linear models, and some applications to behavioral data are cited.

A second-order longitudinal model of ability structure.

Olsson and Bergman (1977) presented a structural equation model for the development of intellectual abilities between the ages of 10 and 13. Their model proposed four correlated factors, each of which was stable over time. They also found that development on each factor was a function of that factor alone, independent of the other factors. The present paper describes a reanalysis of these data, under a structural model which assumed that a second-order general-intelligence factor (G) could account for the correlations among factors. It was found that, for the sample used, only G and the verbal and spatial ability factors were stable over time. It was concluded that much of the stability found by Olsson and Bergman could be accounted for by the stability of G, and that development on each primary factor was heavily dependent on development of G.

Intelligence, classroom behavior, and academic achievement in children at high and low risk for psychopathology: a structural equation analysis

The intelligence, academic achievement, and classroom behavior of 158 children were assessed in a sample that is being followed longitudinally. The sample included children at high risk for mental disorder by virtue of having a parent with a psychiatric diagnosis of schizophrenia or affective disorder, children at moderate risk, and children at low risk, A series of path analyses indicated that in this sample (1) classroom behavior was more likely an affect than a cause of academic achievement, and (2) the influence of parental psychopathology on classroom behavior was mediated by a childs intelligence and academic achievement. We were unable to substantiate an unmediated causal link between parental psychopathology and childrens academic achievement or classroom behavior.

Gastric habitation by Helicobacter pylori: insights into acid adaptation

. The means wherebythis bacterium resists gastric acidity isnot only of biological interest but pro-vides a natural pharmacological andtherapeutic target for its eradication.Recent studies have shown that acid-induced activation of a urea channelregulates intra-bacterial urease activity,which enables maintenance of the pHof the periplasm of the bacteria withinviable limits during gastric acidity. Thisappears to be an adaptation mechanismthat is unique to