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Al-Zeyara S.A.,University of Reading | Jarvis B.,University of Reading | Jarvis B.,Ross Biosciences Ltd | Mackey B.M.,University of Reading
International Journal of Food Microbiology

The aims of this study were to (i) compare the inhibitory effects of the natural microflora of different foods on the growth of Listeria monocytogenes during enrichment in selective and non-selective broths; (ii) to isolate and identify components of the microflora of the most inhibitory food; and (iii) to determine which of these components was most inhibitory to growth of L. monocytogenes in co-culture studies. Growth of an antibiotic-resistant marker strain of L. monocytogenes was examined during enrichment of a range of different foods in Tryptone Soya Broth (TSB), Half Fraser Broth (HFB) and Oxoid Novel Enrichment (ONE) Broth. Inhibition of L. monocytogenes was greatest in the presence of minced beef, salami and soft cheese and least with prepared fresh salad and chicken pâté. For any particular food the numbers of L. monocytogenes present after 24h enrichment in different broths increased in the order: TSB, HFB and ONE Broth. Numbers of L. monocytogenes recovered after enrichment in TSB were inversely related to the initial aerobic plate count (APC) in the food but with only a moderate coefficient of determination (R2) of 0.51 implying that microbial numbers and the composition of the microflora both influenced the degree of inhibition of L. monocytogenes. In HFB and ONE Broth the relationship between APC and final L. monocytogenes counts was weaker. The microflora of TSB after 24h enrichment of minced beef consisted of lactic acid bacteria, Brochothrix thermosphacta, Pseudomonas spp., Enterobacteriaceae, and enterococci. In co-culture studies of L. monocytogenes with different components of the microflora in TSB, the lactic acid bacteria were the most inhibitory followed by the Enterobacteriaceae. The least inhibitory organisms were Pseudomonas sp., enterococci and B. thermosphacta. In HFB and ONE Broth the growth of Gram-negative organisms was inhibited but lactic acid bacteria still reached high numbers after 24h. A more detailed study of the growth of low numbers of L. monocytogenes during enrichment of minced beef in TSB revealed that growth of L. monocytogenes ceased at a cell concentration of about 102cfu/ml when lactic acid bacteria entered stationary phase. However in ONE Broth growth of lactic acid bacteria was slower than in TSB with a longer lag time allowing L. monocytogenes to achieve much higher numbers before lactic acid bacteria reached stationary phase. This work has identified the relative inhibitory effects of different components of a natural food microflora and shown that the ability of low numbers of L. monocytogenes to achieve high cell concentrations is highly dependent on the extent to which enrichment media are able to inhibit or delay growth of the more effective competitors. © 2010 Elsevier B.V. Source

Jarvis B.,Ross Biosciences Ltd | Hedges A.J.,University of Bristol
Food Microbiology

Studies on the precision of chemical methods of analysis, and the associated 'sampling uncertainty', suggest that analysis of eight replicate sample units (the sample size) is required to ensure adequate analytical precision. The primary purpose of this work was to assess whether these findings are equally applicable in microbiological examination of foods. We examined the effect of sample size on the analytical precision of microbiological data by iteratively 're-sampling without replacement' (SNR). Using both theoretical data sets and colony counts from foods we demonstrate that SNR provides an effective and efficient guide to (a) choosing the number of samples to be examined in order to optimise precision and (b) deciding whether logarithmic transformation of the raw data is appropriate. We also discuss theoretical aspects of the procedure and their impact on the results obtained. © 2011 Elsevier Ltd. Source

The ability of the standard pre-enrichment procedure in buffered peptone water (BPW) to recover Salmonella Typhimurium from acidic marinade sauces containing spices was tested by inoculating marinade sauces with known numbers of an antibiotic-resistant marker strain of Salmonella Typhimurium DT104 prior to pre-enrichment. Viable numbers of salmonellae present in BPW after 24h incubation depended on the inoculum level. If initial cell numbers were low (below 103cfu per 250ml BPW) final cell concentrations were also low and, in some cases, no growth occurred. The problem was overcome by use of double-strength BPW that neutralised the acidity and allowed good recovery from otherwise inhibitory marinade sauces. © 2010 Elsevier Ltd. Source

Corry J.E.L.,University of Bristol | Jarvis B.,Ross Biosciences Ltd | Hedges A.J.,University of Bristol
Food Microbiology

The objective of this study was to assess whether it is possible to minimise the variance in colony counts on replicate target samples of foods by aseptic compositing of the samples or by increasing the quantity of sample examined. The results show that compositing reduces the overall variance, and hence the standard deviation, to very low levels, although in some cases the overall variance remains relatively high, reflecting the heterogeneous distribution of microorganisms in the foods. Increasing the weight of target sample examined (e.g. from 10g to 100g) had a pronounced effect on the mean log10 colony count and significantly reduced the variance of the mean. The results are discussed in relation to the quantity of sample that is recommended for examination in international and other standards. © 2010 Elsevier Ltd. Source

Jarvis B.,Ross Biosciences Ltd | Wilrich C.,BAM Federal Institute of Materials Research and Testing | Wilrich P.-T.,Free University of Berlin
Journal of Applied Microbiology

Aims: The purpose of this work was to derive a simple Excel spreadsheet and a set of standard tables of most probable number (MPN) values that can be applied by users of International Standard Methods to obtain the same output values for MPN, SD of the MPN, 95% confidence limits and test validity. With respect to the latter, it is considered that the Blodgett concept of 'rarity' is more valuable than the frequently used approach of improbability (vide de Man).Methods and Results: The paper describes the statistical procedures used in the work and the reasons for introducing a new set of conceptual and practical approaches to the determination of MPNs and their parameters. Examples of MPNs derived using these procedures are provided. The Excel spreadsheet can be downloaded from .Conclusions: The application of the revised approach to the determination of MPN parameters permits those who wish to use tabulated values, and those who require access to a simple spreadsheet to determine values for nonstandard test protocols, to obtain the same output values for any specific set of multiple test results. The concept of 'rarity' is a more easily understood parameter to describe test result combinations that are not statistically valid. Provision of the SD of the log MPN value permits derivation of uncertainty parameters that have not previously been possible.Significance and Impact of the Study: A consistent approach for the derivation of MPNs and their parameters is essential for coherence between International Standard Methods. It is intended that future microbiology standard methods will be based on the procedures described in this paper. © 2010 The Authors. Journal of Applied Microbiology © 2010 The Society for Applied Microbiology. Source

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