In a search for novel attenuated vaccine candidates for use against Yersinia pestis, the causative agent of plague, a signature-tagged mutagenesis strategy was used and optimized for a subcutaneously infected mouse model. A library of tagged mutants of the virulent Y. pestis Kimberley53 strain was generated. Screening of 300 mutants through two consecutive cycles resulted in selection of 16 mutant strains that were undetectable in spleens 48 h postinfection. Each of these mutants was evaluated in vivo by assays for competition against the wild-type strain and for virulence following inoculation of 100 CFU (equivalent to 100 50% lethal doses [LD 50 ] of the wild type). A wide spectrum of attenuation was obtained, ranging from avirulent mutants exhibiting competition indices of 10 ؊5 to 10 ؊7 to virulent mutants exhibiting a delay in the mean time to death or mutants indistinguishable from the wild type in the two assays. Characterization of the phenotypes and genotypes of the selected mutants led to identification of virulence-associated genes coding for factors involved in global bacterial physiology (e.g., purH, purK, dnaE, and greA) or for hypothetical polypeptides, as well as for the virulence regulator gene lcrF. One of the avirulent mutant strains (LD 50 , >10 7 CFU) was found to be disrupted in the pcm locus, which is presumably involved in the bacterial response to environmental stress. This Kimberley53pcm mutant was superior to the EV76 live vaccine strain because it induced 10-to 100-fold-higher antibody titers to the protective V and F1 antigens and because it conferred efficacious protective immunity.The three pathogenic Yersinia species, Yersinia pestis, Yersinia pseudotuberculosis, and Yersinia enterocolitica, are closely related but differ in the mode of infection. Both Y. enterocolitica and Y. pseudotuberculosis are fecal-oral pathogens that cause invasive gastrointestinal diseases. On the other hand, infection with Y. pestis, the causative agent of plague, which is transmitted either by an infected-flea bite or as an inhaled aerosol, culminates in a fatal disease. Most of the documented Y. pestis virulence factors are encoded by the 70-kb plasmid common to all three Yersinia pathogenic species (5,12,15,16,36). Only a few virulence factors that are unique to Y. pestis are known to reside on the strain-specific plasmids (pMT1 and pPCP1) (35). The pMT1 virulence plasmid (19,26,34) harbors, in addition to the sequences encoding the known virulence factors murine toxin and fraction 1 (F1) capsular antigen, sequences encoding several hypothetical proteins whose relevance to Y. pestis pathogenesis remains to be determined. Moreover, the recently completed genome sequences of two Y. pestis strains, CO92 and KIM (14, 34), revealed the dynamic nature of the genome, the presence of many pseudogenes, and the existence of genes encoding hypothetical proteins residing in several putative pathogenicity islands, whose effects on Y. pestis pathogenicity remain to be carefully studied (14,34).During the last deca...

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Most U.S. medical schools offer courses in the behavioral and social sciences (BSS), but their implementation is frequently impeded by problems. First, medical students often fail to perceive the relevance of the BSS for clinical practice. Second, the BSS are vaguely defined and the multiplicity of the topics that they include creates confusion about teaching priorities. Third, there is a lack of qualified teachers, because physicians may have received little or no instruction in the BSS, while behavioral and social scientists lack experience in clinical medicine. The authors propose an approach that may be useful in overcoming these problems and in shaping a BSS curriculum according to the institutional values of various medical schools. This approach originates from insights gathered during their attempts to teach various BSS topics at four Israeli medical schools. They suggest that medical faculties (1) adopt an integrative approach to learning the biomedical, behavioral, and social sciences using Engel's "biopsychosocial model" as a link between the BSS and clinical practice, (2) define a hierarchy of learning objectives and assign the highest priority to acquisition of clinically relevant skills, and (3) develop clinical role models through teacher training programs. This approach emphasizes the clinical relevance of the BSS, defines learning priorities, and promotes cooperation between clinical faculty and behavioral scientists.

Acetylcholinesterases (AChEs) are characterized by a high net negative charge and by an uneven surface charge distribution, giving rise to a negative electrostatic potential extending over most of the molecular surface. To evaluate the contribution of these electrostatic properties to the catalytic efficiency, 20 single‐ and multiple‐site mutants of human AChE were generated by replacing up to seven acidic residues, vicinal to the rim of the active‐center gorge (Glu84, Glu285, Glu292, Asp349, Glu358, Glu389 and Asp390), by neutral amino acids. Progressive simulated replacement of these charged residues results in a gradual decrease of the negative electrostatic potential which is essentially eliminated by neutralizing six or seven charges. In marked contrast to the shrinking of the electrostatic potential, the corresponding mutations had no significant effect on the apparent bimolecular rate constants of hydrolysis for charged and non‐charged substrates, or on the Ki value for a charged active center inhibitor. Moreover, the kcat values for all 20 mutants are essentially identical to that of the wild type enzyme, and the apparent bimolecular rate constants show a moderate dependence on the ionic strength, which is invariant for all the enzymes examined. These findings suggest that the surface electrostatic properties of AChE do not contribute to the catalytic rate, that this rate is probably not diffusion‐controlled and that long‐range electrostatic interactions play no role in stabilization of the transition states of the catalytic process.

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