The expression of most bacterial genes is controlled at the level of transcription via promoter control mechanisms that permit a graded response. However, an increasing number of bacterial genes are found to exhibit an ‘all‐or‐none’ control mechanism that adapts the bacterium to more than one environment. One such mechanism is phase variation, traditionally defined as the high‐frequency ON↔OFF switching of phenotype expression. Phase variation events are usually random, but may be modulated by environmental conditions. The mechanisms of phase variation events and their significance within the microbial community are discussed here.

Chief Scientist Office of the Scottish Government Health Directorate.

Uterine artery Doppler flow velocity has limited diagnostic accuracy in predicting pre-eclampsia, intrauterine growth retardation and perinatal death.

Cell-wall-associated proteins from Streptococcus equi subsp. equi, the causative agent of strangles, were analysed with a view to identifying a potential protective antigen. Preparations of these proteins, isolated from mutanolysin extracts of cell walls, were shown to contain one major high-M r protein species (apparent M r 220000 and 550000 when analysed by SDS-PAGE and gel-filtration chromatography, respectively). The high-M r protein bound horse fibrinogen and was purified under non-denaturing conditions using fibrinogen affinity chromatography. The fibrinogen-binding protein (FgBP) reacted with serum taken from horses recovering from strangles and protected mice against lethal challenge from S. equi subsp. equi. The sequence of the corresponding gene (fbp) was determined and shown to encode a mature protein (M r 54597) with predicted coiled-coil structure. An FgBP truncate, lacking the C-terminal cell wall/membrane anchor domain, was overexpressed in and purified from Escherichia coli and was shown to behave in an analogous fashion to the wild-type product in terms of M r estimation, fibrinogen binding and seroreactivity.

Objectives An ultrasound study to establish the nature and limits of fetal growth in a low risk population from 22 weeks of gestation until term. Design Prospective, longitudinal ultrasound study of 274 low risk pregnancies involving organised scanning schedules with all measurements performed by one observer using the same equipment.Results Growth velocity charts have been created for a number of ultrasound parameters including estimated fetal weight, by applying appropriate statistical methods to the serial data. The rates of growth of the biparietal diameter, femur length, abdominal area and estimated weight each have characteristic patterns demonstrating maximal growth rates at different gestations. Conclusions Appropriately derived and calculated ultrasound fetal growth velocity standards have been established. These data are suitable for the evaluation of ultrasonically estimated fetal growth rates in the prediction of adverse perinatal outcome and the further investigation of the role of the intrauterine environment in the origin of adult disease.been highlighted6. This study was designed to establish such standards of growth velocity for a number of fetal measurements in a low risk obstetric population. METHODSThree hundred and thirteen women attending the antenatal clinic at our hospital were enrolled into the study. Entry criteria were gestational age of less than 85 days confirmed by crown-rump length measurement and the absence of recognised risk factors for accelerated or retarded fetal growth (previous SGA or IUGR pregnancy, existing medical disorder or heavy smoking (> 20 cigarettes per day)). All the subjects were scanned for fetal anomaly at 18 weeks of gestation, which is routine practice in our department. Thereafter, the subjects were sequentially entered into one of the four scanning schedules (n = number continuing in the study). weeks (n = 67).These schedules gave weekly coverage of pregnancy from 22 weeks onwards. All ultrasound 60 0

Here we report the characterization of an Escherichia coli gene (agn43) which encodes the principal phase-variable outer membrane protein termed antigen 43 (Ag43). Theagn43 gene encodes a precursor protein of 107 kDa containing a 52-amino-acid signal sequence. Posttranslational processing generates an α43 subunit (predictedM r of 49,789) and a C-terminal domain (β43) with features typical of a bacterial integral outer membrane protein (predicted M r of 51,642). Secondary structure analysis predicts that β43 exists as an 18-stranded β barrel and that Ag43 shows structural organization closely resembling that of immunoglobulin A1 protease type of exoprotein produced by pathogenic Neisseria andHaemophilus spp. The correct processing of the polyprotein to α43 and β43 in OmpT, OmpP, and DegP protease-deficient E. coli strains points to an autocatalytic cleavage mechanism, a hypothesis supported by the occurrence of an aspartyl protease active site within α43. Ag43, a species-specific antigen, possesses two RGD motifs of the type implicated in binding to human integrins. The mechanism of reversible phase variation was studied by immunochemical analysis of a panel of well-defined regulatory mutants and by analysis of DNA sequences upstream of agn43. Evidence strongly suggests that phase variation is regulated by both deoxyadenosine methylase (Dam) and by OxyR. Thus, oxyR mutants are locked on for Ag43 expression, whereas dam mutants are locked off for Ag43 expression. We propose a novel mechanism for the regulation of phase switching in which OxyR competes with Dam for unmethylated GATC sites in the regulatory region of the agn43 gene.

The molecular architecture of membrane vesicles prepared from Eseherichia coli ML 308-225 has been studied by using crossed immunoelectrophoresis, and a reference pattern of 52 discrete immunoprecipitates has been established. Progressive immunoadsorption experiments conducted with untreated control vesicles and with physically disrupted vesicles demonstrate that the membrane-associated immunogens fall into two categories: (i) those immunogens typified by ATPase (ATP phosphohydrolasei EC 3.6.1.3) and NADH dehydrogenase [NADH: (acceptor) oxidoreductase, EC 1.6.99.3] whose expression is minimal unless the vesicles are disrupted; and (ih) immunogens such as Braun's lipoprotein that are expressed to similar extents in untreated an in isrted vesicles. A mathematical relationship between thepeak area subtended by an immunoprecipitate in the crossed immunelectrophoresis system and the quantity of vesicles used in the adsorption process has been derived. This relationship allows quantitation of the degree to which specific membrane immunogens partition between exposed and unexposed surfaces of the vesicle membrane. The results demonstrate conclusively that >95% of the membrane in the vesicle preparations is in the form of sealed sacculi with the same polarity as the intact cell.Moreover, the findings provide a strong indication that dislocation of immunogens from the inner to the outer surface of the membrane during vesicle preparation does not occur to an extent exceeding 11%. Topologically sealed cytoplasmic membrane vesicles have provided a useful model system for the study of certain transport phenomena in bacteria (1-3). These vesicles are essentially devoid of cytoplasmic constituents and catalyze active transport by a respiration-dependent mechanism that does not involve the generation or utilization of ATP or other high-energy phosphate compounds. In addition, recent studies with this system (3-8) provide strong confirmation of the hypothesis that chemosmotic phenomena (9-12) play a central, obligatory role in this process.Despite a considerable body of evidence (see ref. 13 for a review) demonstrating that bacterial membrane vesicles prepared by osmotic lysis (14, 15) retain the same polarity as the intact cell, doubts apparently exist regarding the molecular structure of these vesicles. It has been argued, for example, that a significant percentage of these vesicles are everted (i.e., inside-out) and thus have a polarity that is opposite to that of the intact cell (16,17). Alternatively, it has been suggested that, during vesicle preparation, certain membrane proteins become detached from the inner surface of the membrane and are readsorbed onto the outer surface (18)(19)(20)(21)(22). This dislocation process, it is proposed, results in vesicles that are "mosaics" in the sense that some membrane proteins are located on both sides of the membrane in a nonrandom fashion (21,22). Much of the data on which these conclusions are based are derived from the use of two broad experimental approaches: (i) comp...

No abstract