Sandra C. Stringer
Norwich Research Park
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Featured researches published by Sandra C. Stringer.
Food Microbiology | 2011
Michael W. Peck; Sandra C. Stringer; Andrew T. Carter
Foodborne botulism is a severe neuroparalytic disease caused by consumption of botulinum neurotoxin formed by strains of proteolytic Clostridium botulinum and non-proteolytic C. botulinum during their growth in food. The botulinum neurotoxin is the most potent substance known, with as little as 30-100 ng potentially fatal, and consumption of just a few milligrams of neurotoxin-containing food is likely to be sufficient to cause illness and potentially death. In order to minimise the foodborne botulism hazard, it is necessary to extend understanding of the biology of these bacteria. This process has been recently advanced by genome sequencing and subsequent analysis. In addition to neurotoxin formation, endospore formation is also critical to the success of proteolytic C. botulinum and non-proteolytic C. botulinum as foodborne pathogens. The endospores are highly resistant, and enable survival of adverse treatments such as heating. To better control the botulinum neurotoxin-forming clostridia, it is important to understand spore resistance mechanisms, and the physiological processes involved in germination and lag phase during recovery from this dormant state.
International Journal of Food Microbiology | 2000
Frédéric Carlin; Héléne Girardin; Michael W. Peck; Sandra C. Stringer; G. C. Barker; Antonio Martínez; Aurea Fernandez; Pablo S. Fernández; William M. Waites; Sara Movahedi; Frans van Leusden; Maarten Nauta; Roy Moezelaar; Manuela Del Torre; Sonia Litman
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.
Meat Science | 2005
Michael W. Peck; Sandra C. Stringer
There has been a substantial increase in sales of pasteurised in-pack chilled products over the last decade. It is anticipated that this trend will continue. These foods address consumer demand in being of high quality and requiring little preparation time. The microbiological safety of these foods commonly depends on a combination of a minimal heat treatment, refrigerated storage and a restricted shelf-life. The principal microbiological safety hazard for pasteurised in-pack meat products is foodborne botulism, as presented by non-proteolytic Clostridium botulinum. This review provides a summary of research that has contributed to the safe development of these foods without incidence of botulism.
Journal of Applied Microbiology | 2000
Sandra C. Stringer; Susan M. George; Michael W. Peck
Verotoxin‐producing Escherichia coli O157:H7 is a cause of serious foodborne illness. It has a very small infectious dose and so it is vital to eliminate this pathogen from food. As heat treatment is the method of bacterial destruction most frequently used in food processing, accurate prediction of thermal death rates is necessary to achieve desired safety margins whilst minimizing processing. In most studies thermal inactivation has been described using first‐order reaction kinetics and D‐values. Whilst this approach does not seem justified on a theoretical basis, and may increase inaccuracy, there is no doubt that it is convenient and in many cases provides an adequate description of thermal death. A review of published data on the measured thermal inactivation of E. coli O157:H7 shows no strong evidence that a heat treatment of 70°C for 2 min (or equivalent) fails to deliver a 6‐decimal reduction in cell numbers.
Applied and Environmental Microbiology | 2005
Sandra C. Stringer; Martin D. Webb; Susan M. George; Carmen Pin; Michael W. Peck
ABSTRACT Knowledge of the distribution of growth times from individual spores and quantification of this biovariability are important if predictions of growth in food are to be improved, particularly when, as for Clostridium botulinum, growth is likely to initiate from low numbers of spores. In this study we made a novel attempt to determine the distributions of times associated with the various stages of germination and subsequent growth from spores and the relationships between these stages. The time to germination (tgerm), time to emergence (temerg), and times to reach the lengths of one (tC1) and two (tC2) mature cells were quantified for individual spores of nonproteolytic C. botulinum Eklund 17B using phase-contrast microscopy and image analysis. The times to detection for wells inoculated with individual spores were recorded using a Bioscreen C automated turbidity reader and were compatible with the data obtained microscopically. The distributions of times to events during germination and subsequent growth showed considerable variability, and all stages contributed to the overall variability in the lag time. The times for germination (tgerm), emergence (temerg − tgerm), cell maturation (tC1 − temerg), and doubling (tC2 − tC1) were not found to be correlated. Consequently, it was not possible to predict the total duration of the lag phase from information for just one of the stages, such as germination. As the variability in postgermination stages is relatively large, the first spore to germinate will not necessarily be the first spore to produce actively dividing cells and start neurotoxin production. This information can make a substantial contribution to improved predictive modeling and better quantitative microbiological risk assessment.
Food Microbiology | 2011
Sandra C. Stringer; Martin D. Webb; Michael W. Peck
Quantifying lag times from individual spores and the associated variability is an important part of understanding the hazard associated with spore-forming pathogens such as Clostridium botulinum. Knowledge of the underlying distribution would allow greater refinement of risk assessments. To date most studies have either examined lag time indirectly by measuring time to growth or have only examined the first stage of lag, germination. Recent studies have attempted to quantify the variability of spores during the different stages of lag phase and to examine the relationships between these stages. The effect of incubation temperature (22 °C, 15 °C, 10 °C or 8 °C), heat treatment (unheated or 80 °C for 20 s) and sodium chloride concentration in both the sporulation medium (0 or 3% w/v) or growth medium (0 or 2% w/v) on growth from individual spores has been examined. These studies found spores within a single population are very heterogeneous with large variability in all stages of lag. The duration and variability of times for germination, outgrowth and first doubling depended on both the historic treatment of the spores and the prevailing growth conditions, and the stage of lag most affected was treatment dependant.
Toxins | 2017
Michael W. Peck; Theresa J. Smith; Fabrizio Anniballi; John W. Austin; Luca Bano; Marite Bradshaw; Paula Cuervo; Luisa W. Cheng; Yağmur Derman; Brigitte G. Dorner; Audrey Fisher; Karen K. Hill; Suzanne R. Kalb; Hannu Korkeala; Miia Lindström; Florigio Lista; Carolina Lúquez; Christelle Mazuet; Marco Pirazzini; Michel R. Popoff; Ornella Rossetto; Andreas Rummel; Dorothea Sesardic; Bal Ram Singh; Sandra C. Stringer
Botulinum neurotoxins are diverse proteins. They are currently represented by at least seven serotypes and more than 40 subtypes. New clostridial strains that produce novel neurotoxin variants are being identified with increasing frequency, which presents challenges when organizing the nomenclature surrounding these neurotoxins. Worldwide, researchers are faced with the possibility that toxins having identical sequences may be given different designations or novel toxins having unique sequences may be given the same designations on publication. In order to minimize these problems, an ad hoc committee consisting of over 20 researchers in the field of botulinum neurotoxin research was convened to discuss the clarification of the issues involved in botulinum neurotoxin nomenclature. This publication presents a historical overview of the issues and provides guidelines for botulinum neurotoxin subtype nomenclature in the future.
Applied and Environmental Microbiology | 2009
Sandra C. Stringer; Martin D. Webb; Michael W. Peck
ABSTRACT In this study, we determined the effects of incubation temperature and prior heat treatment on the lag-phase kinetics of individual spores of nonproteolytic Clostridium botulinum Eklund 17B. The times to germination (tgerm), one mature cell (tC1), and two mature cells (tC2) were measured for individual unheated spores incubated at 8, 10, 15, or 22°C and used to calculate the tgerm, the outgrowth time (tC1 − tgerm), and the first doubling time (tC2 − tC1). Measurements were also made at 22°C of spores that had previously been heated at 80°C for 20 s. For unheated spores, outgrowth made a greater contribution to the duration and variability of the lag phase than germination. Decreasing incubation temperature affected germination less than outgrowth; thus, the proportion of lag associated with germination was less at lower incubation temperatures. Heat treatment at 80°C for 20 s increased the median germination time of surviving spores 16-fold and greatly increased the variability of spore germination times. The shape of the lag-time (tC1) and outgrowth (tC1 − tgerm) distributions were the same for unheated spores, but heat treatment altered the shape of the lag-time distribution, so it was no longer homogeneous with the outgrowth distribution. Although heat treatment mainly extended germination, there is also evidence of damage to systems required for outgrowth. However, this damage was quickly repaired and was not evident by the time the cells started to double. The results presented here combined with previous findings show that the stage of lag most affected, and the extent of any effect in terms of duration or variability, differs with both historical treatment and the growth conditions.
BMC Genomics | 2013
Sandra C. Stringer; Andrew T. Carter; Martin D. Webb; Ewelina Wachnicka; Lisa Crossman; Mohammed Sebaihia; Michael W. Peck
BackgroundClostridium botulinum is a group of four physiologically and phylogenetically distinct bacteria that produce botulinum neurotoxin. While studies have characterised variability between strains of Group I (proteolytic) C. botulinum, the genetic and physiological variability and relationships between strains within Group II (non-proteolytic) C. botulinum are not well understood. In this study the genome of Group II strain C. botulinum Eklund 17B (NRP) was sequenced and used to construct a whole genome DNA microarray. This was used in a comparative genomic indexing study to compare the relatedness of 43 strains of Group II C. botulinum (14 type B, 24 type E and 5 type F). These results were compared with characteristics determined from physiological tests.ResultsWhole genome indexing showed that strains of Group II C. botulinum isolated from a wide variety of environments over more than 75 years clustered together indicating the genetic background of Group II C. botulinum is stable. Further analysis showed that strains forming type B or type F toxin are closely related with only toxin cluster genes targets being unique to either type. Strains producing type E toxin formed a separate subset. Carbohydrate fermentation tests supported the observation that type B and F strains form a separate subset to type E strains. All the type F strains and most of type B strains produced acid from amylopectin, amylose and glycogen whereas type E strains did not. However, these two subsets did not differ strongly in minimum growth temperature or maximum NaCl concentration for growth. No relationship was found between tellurite resistance and toxin type despite all the tested type B and type F strains carrying tehB, while the sequence was absent or diverged in all type E strains.ConclusionsAlthough Group II C. botulinum form a tight genetic group, genomic and physiological analysis indicates there are two distinct subsets within this group. All type B strains and type F strains are in one subset and all type E strains in the other.
Applied and Environmental Microbiology | 2007
Martin D. Webb; Carmen Pin; Michael W. Peck; Sandra C. Stringer
ABSTRACT In this study we determined the effect of NaCl concentration during sporulation (0 or 3.0% [wt/vol] added NaCl) and subsequent growth (0 or 2.0% [wt/vol] added NaCl) on the distributions of times associated with various stages of the lag phase of individual spores of nonproteolytic Clostridium botulinum strain Eklund 17B. The effects of NaCl on the probability of germination and the probability of subsequent growth were also determined. Spore populations exhibited considerable heterogeneity at all stages of lag phase for each condition tested. Germination time did not correlate strongly with the times for later stages in the lag phase, such as outgrowth and doubling time. Addition of NaCl to either the sporulation or growth media increased the mean times for, and variability of, all the measured stages of the lag phase (germination, emergence, time to one mature cell, and time to first doubling). There was a synergistic interaction between the inhibitory effects of NaCl in the sporulation medium and the inhibitory effects of NaCl in the subsequent growth medium on the total lag time and each of its stages. Addition of NaCl to either the sporulation medium or the growth medium reduced both the probability of germination and the probability of a germinated spore developing into a mature cell, but the interaction was not synergistic. Spores formed in medium with added NaCl were not better adapted to subsequent growth in suboptimal osmotic conditions than spores formed in medium with no added NaCl were. Knowledge of the distribution of lag times for individual spores and quantification of the biovariability within lag time distributions may provide insight into the underlying mechanisms and can be used to improve predictions of growth in food and to refine risk assessments.