G. C. Barker

A retail and consumer phase model for exposure assessment of Bacillus cereus

An exposure assessment is conducted for psychrotrophic and mesophilic Bacillus cereus in a cooked chilled vegetable product. A model is constructed that covers the retail and consumer phase of the food pathway, using the output of a similar model on the industrial process as input. Microbial growth is the predominant process in the model. Variability in time and temperature during transport and storage is included in the model and different domestic refrigerator temperature distributions are compared. As an end point, probable levels of B. cereus colony forming units (cfu) in packages of vegetable purée are predicted at the moment the consumer takes the product from its refrigerator, that is prior to a cooking process. The psychrotrophic strain is predicted to end up above a threshold level of 10(5) cfu/g in 0.9% to 6.3% of the vegetable purée packages, depending on domestic refrigerator temperature. Accounting for spoilage this reduces to 0.3% to 2.4%. Even if the purée is stored at 4 degrees C in the domestic refrigerator and use-by-date (UBD) is respected, the threshold level may be passed. For the mesophilic strain the threshold level is rarely passed, but in contrast to the total viable count, the spore load at the end point is predicted to be higher than in the psychrotrophic strain. Our study illustrates how an exposure assessment model, which may be used in quantitative risk assessment, can integrate expertise in modelling, food processing and microbiology over the food pathway, and thus evaluate food safety, identify gaps in knowledge and compare risk management measures. As important gaps in knowledge, the lack of sporulation and germination models and data, validated non-isothermal growth models and a spoilage model useful for risk assessment are identified. Knowledge of the dose-response relationship is limited and does not allow a full risk assessment. It is shown that exposure can be lowered by lowering domestic refrigerator temperatures, and less so much by monitoring and withdrawing contaminated products at the end of industrial processing.

Research on factors allowing a risk assessment of spore-forming pathogenic bacteria in cooked chilled foods containing vegetables: a FAIR collaborative project

Vegetables are frequent ingredients of cooked chilled foods and are frequently contaminated with spore-forming bacteria (SFB). Therefore, risk assessment studies have been carried out, including the following: hazard identification and characterisation--from an extensive literature review and expertise of the participants, B. cereus and C. botulinum were identified as the main hazards; exposure assessment--consisting of determination of the prevalence of hazardous SFB in cooked chilled foods containing vegetables and in unprocessed vegetables, and identification of SFB representative of the bacterial community in cooked chilled foods containing vegetables, determination of heat-resistance parameters and factors affecting heat resistance of SFB, determination of the growth kinetics of SFB in vegetable substrate and of the influence of controlling factors, validation of previous work in complex food systems and by challenge testing and information about process and storage conditions of cooked chilled foods containing vegetables. The paper illustrates some original results obtained in the course of the project. The results and information collected from scientific literature or from the expertise of the participants are integrated into the microbial risk assessment, using both a Bayesian belief network approach and a process risk model approach, previously applied to other foodborne hazards.

Relevance of microbial interactions to predictive microbiology

Microbial interaction can be ignored in predictive microbiology under most conditions. We show that interactions are only important at high population densities, using published data on inhibition of growth of Listeria monocytogenes in broth. Our analysis using growth models from predictive microbiology indicated that interactions only occur at population densities of approximately 10(8) cfu/ml of the protective cultures. Spoilage is evident at these levels, except for fermented foods. In bacterial colonies, diffusion limitation acts as a constraint to growth. We have shown that these constraints only become important after large outgrowth of colonies (in the order of 5-log growth in Lactobacillus curvatus colonies), which depends on the initial inoculation density. Intra-colony interactions play an important role under these conditions. There is no large outgrowth of colonies when the initial inoculation densities are high and broth culture growth can be used to approximate colony growth.

Risk assessment for Clostridium botulinum: a network approach

The construction and implementation of a mathematical framework for the representation of the hazards that arise from Clostridium botulinum growth, and toxin production, in food are described. Botulism has been recognised as a serious foodborne illness for over a century and, more recently, has become the subject of increased concern due to changing processing and consumption patterns associated with foods. In this respect quantitative risk assessment has an increasingly important role to play in assisting risk management and ensuring the safety of minimally processed foods and foods with extended shelf life. Bayesian Belief Networks are a type of expert system that integrates a graphical, flow diagram like, representation of a hazard domain with a powerful technique for combining probabilities. This technique facilitates the accumulation of understanding and experience, for particular hazard domains, into computer tools that can be used to inspect risks and account for decisions. Analysis of the hazards associated with foodborne botulism involves Belief Network components that represent contamination processes, thermal death kinetics for spores, germination and growth of cells, toxin production and patterns of consumer behaviour, etc. These developments are discussed and three important aspects of the food safety information supply, complexity, dependency and uncertainty highlighted. The benefits associated with a Bayesian view of food safety assessment are illustrated by a Belief Network representation which supports, and prioritises, decisions and actions that (a) minimise the chances and extent of detrimental events and (b) maximise opportunities for awareness and control.

Quantitative risk assessment for hazards that arise from non-proteolytic Clostridium botulinum in minimally processed chilled dairy-based foods

A modular process risk model has been constructed that describes the manufacture of dairy dessert products and hazards that arise from non-proteolytic Clostridium botulinum. The model describes batch manufacture and consumer storage of a family size generic dairy dessert but includes a realistic quantification that could apply to a specific food product. The dairy dessert sector is an expanding part of the UK market. The model includes modules that describe spore loads in raw materials, spore inactivation during thermal processing, volume partition and the population kinetics for non-proteolytic C. botulinum during sequential isothermal storage regimes. Where possible elements of uncertainty and variability are identified explicitly. The model is constructed as a belief network from published data and expert opinions. The model provides marginal probabilities, and associated sensitivities, for a range of endpoint measures centred on the toxicity of a single retail unit after an extended period of storage. The decimal reduction time for non-proteolytic C. botulinum spore populations at the highest (hold) temperature of the primary thermal process and the highest temperature experienced during poorly controlled (consumer) storage are dominant factors determining risks. Priorities for additional information to support risk assessments have been identified.

Structure and distribution of arches in shaken hard sphere deposits

We investigate the structure and distribution of arches formed by spherical, hard particles shaken in an external field after they come to rest. Arches (or bridges) are formed during a computer-simulated, non-sequential deposition of the spheres after each shaking cycle. We identify these arches by means of a connectivity criterion and study their structural characteristics and spatial distribution. We find that neither the size distribution nor the shape of the arches is strongly affected by the packing fraction of the deposit. Conversely, the spatial distribution and orientation of the bridges do depend on the volume fraction occupied by the spheres.

MULTI-PARTICLE STRUCTURES IN NON-SEQUENTIALLY REORGANIZED HARD SPHERE DEPOSITS

We have examined extended structures, bridges and arches, in computer generated, non-sequentially stabilized, hard sphere deposits. The bridges and arches have well-defined distributions of sizes and shapes. The distribution functions reflect the contraints associated with hard particle packing and the details of the restructuring process. A subpopulation of string-like bridges has been identified. Bridges are fundamental microstructural elements in real granular systems and their sizes and shapes dominate considerations of structural properties and flow instabilities such as jamming.

Development and application of a new method for specific and sensitive enumeration of spores of nonproteolytic Clostridium botulinum types B, E, and F in foods and food materials.

ABSTRACT The highly potent botulinum neurotoxins are responsible for botulism, a severe neuroparalytic disease. Strains of nonproteolytic Clostridium botulinum form neurotoxins of types B, E, and F and are the main hazard associated with minimally heated refrigerated foods. Recent developments in quantitative microbiological risk assessment (QMRA) and food safety objectives (FSO) have made food safety more quantitative and include, as inputs, probability distributions for the contamination of food materials and foods. A new method that combines a selective enrichment culture with multiplex PCR has been developed and validated to enumerate specifically the spores of nonproteolytic C. botulinum. Key features of this new method include the following: (i) it is specific for nonproteolytic C. botulinum (and does not detect proteolytic C. botulinum), (ii) the detection limit has been determined for each food tested (using carefully structured control samples), and (iii) a low detection limit has been achieved by the use of selective enrichment and large test samples. The method has been used to enumerate spores of nonproteolytic C. botulinum in 637 samples of 19 food materials included in pasta-based minimally heated refrigerated foods and in 7 complete foods. A total of 32 samples (5 egg pastas and 27 scallops) contained spores of nonproteolytic C. botulinum type B or F. The majority of samples contained <100 spores/kg, but one sample of scallops contained 444 spores/kg. Nonproteolytic C. botulinum type E was not detected. Importantly, for QMRA and FSO, the construction of probability distributions will enable the frequency of packs containing particular levels of contamination to be determined.

Characterisation and modelling of oscillatory behaviour related to reuterin production by Lactobacillus reuteri.

During reuterin production by Lactobacillus reuteri in a chemostat, the growth, substrate and metabolite concentrations showed oscillatory behaviour. The sensitivity of L. reuteri towards reuterin was shown to be a possible explanation of the oscillatory behaviour. A deterministic mathematical model consisting of four coupled differential equations describing the concentrations of biomass, glucose, glycerol and reuterin with time was developed. With a set of parameter values determined from batch experiments, the model was able to predict both oscillatory and steady state behaviour in a chemostat by changing the input variables. In a batch system, the model was able to give a satisfactory description of the glucose and glycerol concentrations but not of the biomass and reuterin concentrations. Mathematical modelling of the system was shown to be an effective and systematic approach in exploring a complex biological system.

Meta-Analysis of Experimental Data Concerning Antimicrobial Resistance Gene Transfer Rates during Conjugation

ABSTRACT This paper presents the results of a meta-analysis of published transfer rates of antimicrobial resistance genes. A total of 34 papers were identified, of which 28 contained rates estimated in relation to either donor or recipient bacterial counts. The published rates ranged from 10−2 to 10−9. Generalized linear modeling was conducted to identify the factors influencing this variation. Highly significant associations between transfer frequency and both the donor (P = 1.2 × 10−4) and recipient (P = 1.0 × 10−5) genera were found. Also significant was whether the donor and recipient strains were of the same genus (P = 0.023) and the nature of the genetic element (P = 0.0019). The type of experiment, in vivo or in vitro, approached statistical significance (P = 0.12). Parameter estimates from a general linear model were used to estimate the probability of transfer of antimicrobial resistance genes to potential pathogens in the intestine following oral ingestion. The mean logarithms of these probabilities are in the range of [−7.0, −3.1]. These probability distributions are suitable for use in the quantitative assessment of the risk of transfer of antimicrobial resistance genes to the intestinal flora of humans and animals.