Regularization of the Ostwald supersaturation model for Liesegang bands.

In a previous paper, we analysed the Keller-Rubinow formulation of Ostwald's supersaturation theory for the formation of Liesegang rings or Liesegang bands, and found that the model is ill-posed, in the sense that after the termination of the first crystal front growth, secondary bands form, as in the experiment, but these are numerically found to be a single grid space wide, and thus an artefact of the numerical method. This ill-posedness is due to the discontinuity in the crystal growth rate, which itself reflects the supersaturation threshold inherent in the theory. Here we show that the ill-posedness can be resolved by the inclusion of a relaxation mechanism describing an impurity coverage fraction, which physically enables the transition in heterogeneous nucleation from precipitate-free impurity to precipitate-covered impurity.

On the Keller-Rubinow model for Liesegang ring formation.

We study the model of Keller & Rubinow (Keller & Rubinow 1981 J. Chem. Phys74, 5000-5007. (doi:10.1063/1.441752)) describing the formation of Liesegang rings due to Ostwald's supersaturation mechanism. Keller and Rubinow provided an approximate solution both for the growth and equilibration of the first band, and also for the formation of secondary bands, based on a presumed asymptotic limit. However, they did not provide a parametric basis for the assumptions in their solution, nor did they provide any numerical corroboration, particularly of the secondary band formation. Here, we provide a different asymptotic solution, based on a specific parametric limit, and we show that the growth and subsequent cessation of the first band can be explained. We also show that the model is unable to explain the formation of finite width secondary bands, and we confirm this result by numerical computation. We conclude that the model is not fully posed, lacking a transition variable which can describe the hysteretic switch across the nucleation threshold.

On the formation of fold-type oscillation marks in the continuous casting of steel.

Asymptotic methods are employed to revisit an earlier model for oscillation-mark formation in the continuous casting of steel. A systematic non-dimensionalization of the governing equations, which was not carried out previously, leads to a model with 12 dimensionless parameters. Analysis is provided in the same parameter regime as for the earlier model, and surprisingly simple analytical solutions are found for the oscillation-mark profiles; these are found to agree reasonably well with the numerical solution in the earlier model and very well with fold-type oscillation marks that have been obtained in more recent experimental work. The benefits of this approach, when compared with time-consuming numerical simulations, are discussed in the context of auxiliary models for macrosegregation and thermomechanical stresses and strains.

Stability of thin liquid curtains.

We investigate the stability of thin liquid curtains with respect to two-dimensional perturbations. The dynamics of perturbations with wavelengths exceeding (or comparable to) the curtain's thickness are examined using the lubrication approximation (or a kind of geometric optics). It is shown that, contrary to the previous theoretical results, but in agreement with the experimental ones, all curtains are stable with respect to small perturbations. Large perturbations can still be unstable, however, but only if they propagate upstream and, thus, disrupt the curtain at its outlet. This circumstance enables us to obtain an effective stability criterion by deriving an existence condition for upstream propagating perturbations.

A 2D channel-clogging biofilm model.

We present a model of biofilm growth in a long channel where the biomass is assumed to have the rheology of a viscous polymer solution. We examine the competition between growth and erosion-like surface detachment due to the flow. A particular focus of our investigation is the effect of the biofilm growth on the fluid flow in the pores, and the issue of whether biomass can grow sufficiently to shut off fluid flow through the pores, thus clogging the pore space. Net biofilm growth is coupled along the pore length via flow rate and nutrient transport in the pore flow. Our 2D model extends existing results on stability of 1D steady state biofilm thicknesses to show that, in the case of flows driven by a fixed pressure drop, full clogging of the pore can indeed happen in certain cases dependent on the functional form of the detachment term.

A simple model for multicomponent etching.

We consider the situation where a multicomponent solid is etched using one or more acids. Of fundamental interest is the rate of surface etching but when this involves multicomponent surface reactions, it becomes unclear how the overall rate can be estimated. In this paper, we sketch a simple model designed to determine the effective etching rate by means of an atomic scale model of the etching process.

Molecular characterization of Irish E. coli O157:H7 isolates of human, bovine, ovine and porcine origin.

AIMS: To determine the degree of relatedness between isolates of Escherichia coli O157:H7 of human, bovine, ovine and porcine origin. METHODS AND RESULTS: Escherichia coli O157:H7 isolates were compared using (i) PFGE XbaI patterns, (ii) PCR profiles of virulence genes and (iii) the DNA sequences of genes reported to play a role in pathogenicity. The 77 E. coli O157:H7 isolates demonstrated 49 different PFGE patterns of which, eight were common to multiple isolates, and the remaining 41 were distinct. Isolates of different origin did not correlate, except for one cluster consisting of two human and two beef isolates. The majority of animal isolates had the same PCR profiles of virulence genes as those isolated from clinical patients. Single nucleotide polymorphisms (SNPs) were identified in the sequence of a 255-bp region of the vtx2 subunit A gene. CONCLUSIONS: Six SNPs were detected in the vtx2A gene, defining four different haplotypes. One nonsynonymous substitution encoded for an amino acid change from glutamic to aspartic acid. SIGNIFICANCE AND IMPACT OF THE STUDY: Results indicate that although E. coli O157:H7 isolates of differing origin were distinct by PFGE, the DNA sequences of the main virulence genes associated with human clinical illness were conserved.

The potential use of chilling to control the growth of Enterobacteriaceae on porcine carcasses and the incidence of E. coli O157:H7 in pigs.

AIMS: To (i) monitor the presence of Enterobacteriaceae as indicators of faecal contamination on pig carcasses, (ii) examine the potential use of chilling as a critical control point (CCP) and establish its influence on pig carcass categorization by Decision 471/EC and (iii) determine the incidence of E. coli O157:H7 in pigs. METHODS AND RESULTS: Porcine faecal samples and carcass swabs were collected before and after chilling at four Irish pig abattoirs and examined for Enterobacteriaceae and E. coli O157:H7. Chilling generally reduced Enterobacteriaceae counts on carcasses, but increases were also observed, particularly in one abattoir. E. coli O157:H7 was absent from carcasses before chilling, present on 0.21% after chilling and was recovered from 0.63% of faecal samples. All of the isolates were found to contain virulence genes associated with clinical illness in humans. CONCLUSIONS: The data show that overall chilling had the capacity to reduce the numbers of carcasses positive for the presence of Enterobacteriaceae. SIGNIFICANCE AND IMPACT OF STUDY: The influence of chilling on the categorization of pig carcasses suggests that it has the potential to improve the numbers of acceptable carcasses and the process could be used as a CCP within a HACCP plan.

A note on spin-coating with small evaporation.

We examine a simple model of spin-coating with "small" evaporation effects and using a formal asymptotic approach we improve on previous approximate models.

Assessing the hygiene of pig carcasses using whole-body carcass swabs compared with the four-site method in EC Decision 471.

An investigation was carried out in a pig abattoir to determine the microbiological status of carcasses being produced after slaughter and dressing. The carcasses were sampled in accordance with EC Decision 471 in relation to the application of hazard analysis critical control point (HACCP) criteria to the slaughter of animals. In this regard, four sites on the animals were examined on five consecutive carcasses during each of 10 visits for the presence of total viable counts and Enterobacteriaceae. A comparison of the EC four-site method, with a whole-body swab technique, as a means of measuring carcass contamination found that the two methods gave significantly different results for both groups of organisms. A comparison of the mean of the individual data from the four sites with the data from the pooled samples revealed that there was a poor relationship between the two. Samples may be taken by excision or swabbing and allocated to three categories of process control, which, in turn, are based on microbiological criteria that are different, depending on whether sampling is by excision or swabbing. The influence of these changes in microbiological criteria is discussed in relation to the categorization of samples as acceptable, marginal, or unacceptable and the influence this has on process control. Finally, the proposed introduction of Salmonella as a safety indicator in the EC HACCP system is discussed.

Prevalence and level of Escherichia coli O157 on beef trimmings, carcasses and boned head meat at a beef slaughter plant.

This study investigated the prevalence and level of Escherichia coli O157 on samples of beef trimmings (n=1351), beef carcasses (n=132) and bovine head meat (n=132) in a beef slaughter plant in Ireland. The survey also included an assessment of the prevalence of virulence genes in the E. coli O157 isolates obtained. Samples were examined for the presence of E. coli O157 by direct plating on SMAC-CT and by enrichment/immunomagnetic separation (IMS) with plating of recovered immunobeads onto SMAC-CT agar. Presumptive E. coli O157 isolates were confirmed by PCR targeting a range of genes i.e. vt1, vt2, eaeA, hlyA, fliC(h7) and portions of the rfb (O-antigen encoding) region of E. coli O157. Enterobacteriaceae on head meat samples were estimated by direct plating onto Violet Red Bile Glucose agar. E. coli O157 was recovered from 2.4% (32/1351) of beef trimmings samples, at concentrations ranging from<0.70-1.61 log10 cfu g(-1). Of the 32 positive isolates, 31 contained the eaeA and hylA genes while 30/32 contained the fliC(h7) gene and 31/32 contained vt1 or vt2, or both vt genes. E. coli O157 was recovered from 3.0% (4/132) of carcass samples, at concentrations ranging from <0.70-1.41 log10 cfu g(-1). All of the carcass isolates contained the eaeA, hylA and fliC(h7) genes. E. coli O157 was recovered from 3.0% (3/100) of head meat samples, at concentrations of 0.7-1.0 log10 cfu g(-1). All of the head meat isolates contained the eaeA, hylA, fliC(h7) and vt2 genes. No head meat isolates contained the vt1 gene. Head meat samples (n=100) contained Enterobacteriaceae, at concentrations ranging from 0.70-3.0 log10 cfu g(-1). Overall, the qualitative and quantitative data obtained for E. coli O157 on beef trimming samples in this study could be employed as part of a quantitative risk assessment model.

Detection and recovery rates achieved using direct plate and enrichment/immunomagnetic separation methods for Escherichia coli O157:H7 in minced beef and on bovine hide.

AIMS: To assess the detection and recovery rates achieved with commonly used cultural methods for the enumeration and recovery of Escherichia coli O157:H7 from minced beef and bovine hide. METHODS AND RESULTS: Minced beef and bovine hide were inoculated with varying concentrations (log(10) 1.58-2.58 CFU g(-1) and log(10) 2.42-4.49 CFU 100 cm(2) respectively) of E. coli O157:H7 and recovered using a direct plate method or an enrichment/immunomagnetic separation (IMS) method and then plated onto SMAC or SMAC-CT in both cases. The direct plate method detected the pathogen consistently from minced beef samples with an average recovery of 69.2-91.2%. From faecal material on the bovine hide the recovery of the pathogen ranged from 1.80 to 64.5% with fresh faeces depending on the inocula while from dried faeces on hide the results ranged from no recovery at all to 25.1%. Enrichment/IMS recovered E. coli O157:H7 at all inocula levels tested in minced beef while the pathogen was only detected consistently at an average inocula level of log(10) 2.73 CFU 100 cm(2) from fresh faeces and log(10) 4.49 CFU 100 cm(2) from dried faeces on bovine hide. CONCLUSIONS: The direct count enumeration method for E. coli O157:H7 underestimated the numbers of pathogens present. The enrichment/IMS procedure consistently detected the pathogen from minced beef but did not always detect E. coli O157:H7 from faeces on bovine hide. SIGNIFICANCE AND IMPACT OF THE STUDY: Overall this study highlights that any microbial data, used in either predictive microbiology or risk assessment, must take account of the sensitivity and associated performance of the methods employed, in order to make an accurate reflection of the true microbiology of the examined sample.

Prevalence and numbers of Escherichia coli O157 on bovine hides at a beef slaughter plant.

In this study, we investigated the prevalence and numbers of Escherichia coli O157 on bovine hides. Samples (n = 1,500) were collected over a 17-month period (30 samples per week) by sponge swabbing approximately 122-cm2 areas of the bovine rump of slaughtered cattle at an early stage of carcass processing (first legging). Sponge samples (n = 1,500) were stomached in buffered peptone water supplemented with novobiocin, directly plated on sorbitol MacConkey with Cefixime tellurite (SMAC-CT), enriched for 24 h, extracted by immunomagnetic separation, and plated onto SMAC-CT agar. Presumptive E. coli O157 colonies from SMAC-CT plates were confirmed by PCR for the presence of eaeA, hlyA, fliCh7, vt1, vt2, and portions of the rfb (O-antigen encoding) region of E. coli O157. Overall, E. coli O157 was recovered from 109 samples (7.3%) at concentrations ranging from less than 0.13 to 4.24 log CFU/100 cm2. PCR analysis revealed a wide diversity of genetic profiles among recovered isolates of verocytotoxigenic E. coli. Of the isolates recovered, 99 of 109 contained the attaching and effacing gene (eaeA) and the hemolysin gene (hlyA), and 78 of 109 had the flagellar H7 antigen-encoding gene (fliCh7). Only 6 of 109 isolates contained both verotoxin-producing genes (vt1 and vt2); 91 of 109 contained the vt2 gene only, whereas 1 of 109 contained the vt1 gene only. The remaining 11 of 109 contained neither vt1 nor vt2.

Characterisation of E. coli O157 isolates from bovine hide and beef trimming in Irish abattoirs by pulsed field gel electrophoresis.

Escherichia coli O157 isolates from bovine hide (n=117) and beef trimmings (n=32) from a single abattoir were examined by pulsed field gel electrophoresis (PFGE). Using BioNumerics software, dendrograms of isolates from each sample type (i.e. hide and beef trimming) were produced. In assessing the genetic relatedness of isolates, a similarity criterion of 80% was applied. The 117 E. coli O157 hide isolates were grouped into 14 clusters, comprising of 109 different PFGE profiles. Of the 109 different PFGE profiles, 8 were common to multiple isolates (i.e. shared 100% similarity by PFGE). The 32 E. coli O157 beef trimming isolates produced 28 different PFGE profiles and 2 clusters. Of the 28 PFGE profiles, 2 were common to multiple isolates and the remaining 26 were distinct. On a number of sampling occasions, isolates displaying identical PFGE patterns were recovered from multiple isolates collected from a single sample type (i.e. hides or trimmings), suggesting cross contamination from contaminated hides/animals to uncontaminated hides/animals and from contaminated beef trimmings to uncontaminated beef trimmings during abattoir operations.

An investigation of the hammocking effect associated with interface pressure measurements using pneumatic tourniquet cuffs.

A simple experimental arrangement is used to investigate the influence of sensor height on the pressure indicated by an inherently linear sensor sandwiched between a rigid curved surface and a pneumatic tourniquet cuff. The sensor-indicated pressure is monitored for sensor heights in the range 0-3 mm and for cuff inflation pressures of 0-40 kPa ( approximately 0-300 mmHg). The sensor response is found to be non-linear with a saturation tendency at high applied pressures. A model which treats the cuff as an elastic membrane draped over the sensor is shown to be successful in accounting for the general form of the sensor characteristic particularly at cuff pressures greater than about 5 kPa. The model is of use in estimating the errors that are likely to arise in intrusive sensors used to measure interface pressures under tourniquets.