Tobin Robinson

Predicting growth of Brochothrix thermosphacta at changing temperature

A dynamic growth model was tested using Brochothrix thermosphacta incubated in broth at changing temperatures. The model successfully predicted growth in the temperature range 5-25 degrees C when temperature increased or decreased gradually and also when temperature underwent frequent sudden changes. When the temperature profile contained step changes from 20-25 degrees C to 3 degrees C the observed growth curve deviated from that predicted by the model.

The effect of the growth environment on the lag phase of Listeria monocytogenes

The duration of lag in Listeria monocytogenes was examined in relation to the physico-chemical properties of the growth environment. It was supposed that lag would be determined by two hypothetical quantities, the amount of work that a cell has to perform to adapt to new conditions and the rate at which it can perform that work. If the rate at which the cell can perform the necessary work is a function of the maximum specific growth rate in the new environment, the hypothesis predicts that lag time should be related in some way to growth rate, provided cells are initially in approximately the same physiological state. Literature data suggest this is true for many organisms when temperature is the sole growth limiting factor. However, lag times of L. monocytogenes displayed an unusual response to temperature in which lag times of cells precultured at 37 degrees C were shorter at 15 degrees C than at 20 degrees C or 25 degrees C. Analysis of data from the Food Micromodel in which growth of L. monocytogenes was controlled by combinations of pH, NaCl concentration and temperature, showed that there was a linear relationship between lag time and mean generation time although there was much scatter in the data. When the effects of pH, solute type and concentration were investigated individually in this work the correlation between lag time and mean generation time was often poor. It would thus appear that the relationship between growth environment and lag time is more complex than the corresponding relationship between growth environment and maximum specific growth rate.

The effect of inoculum size on the lag phase of Listeria monocytogenes.

The effect of inoculum size on population lag times of Listeria monocytogenes was investigated using the Bioscreen automated microtitre plate incubator and reader. Under optimum conditions, lag times were little affected by inoculum size and there was little variation between replicate inocula even at very low cell numbers. However, in media containing inhibitory concentrations of NaCl, both the mean lag time and variation between replicate inocula increased as the inoculum size became smaller. The variation in lag time of cells within a population was investigated in more detail by measuring the distribution of detection times from 64 replicate inocula containing only one or two cells capable of initiating growth. The variance of the lag time distribution increased with increasing salt concentration and was greater in exponential than in stationary phase inocula. The number of cells required to initiate growth increased from one cell under optimum conditions to 10(5) cells in medium with 1.8 M NaCl. The addition of spent medium from a stationary phase culture reduced the variance and decreased lag times. The ability to initiate growth under severe salt stress appears to depend on the presence of a resistant sub-fraction of the population, although high cell densities assist adaptation of those resistant cells to the unfavourable growth conditions by some unspecified medium conditioning effect. These results are relevant to the prediction of lag times and probability of growth from low numbers of stressed cells in food.

Editorial: OpenFoodTox: EFSA's open source toxicological database on chemical hazards in food and feed

Since its inception in 2002, the European Food safety Authority (EFSA) has produced risk assessments for more than 4,400 substances in over 1,650 Scientific Opinions, Statements and Conclusions through the work of its Scientific Panels, Units and Scientific Committee. For each individual substance, a summary of human health, animal health and ecological hazard assessments has been collected and structured into EFSA’s Chemical Hazards Database: OpenFoodTox. OpenFoodTox provides open source data for substance characterisation, links to the relevant EFSA output, background regulations and summaries of critical toxicological endpoints. An online MicroStrategy tool enables the downloading of summary data sheets for each individual substance in PDF or Excel format. OpenFoodTox is a valuable tool and source of information for scientific advisory bodies and stakeholders with an interest in chemical risk assessment. This editorial provides a snapshot description of OpenFoodTox as an open source toxicological database for chemical risk assessment. EFSA provides scientific advice to risk managers and decision makers through risk assessment and risk communication on all issues related to ‘food and feed safety, animal health and welfare, plant health, nutrition, and environmental issues’ (Regulation EC No 178/2002). Risk assessment has been defined as ‘a scientifically based process consisting of four steps: hazard identification, hazard characterisation, exposure assessment and risk characterisation’ (Regulation EC No 178/2002; WHO, 2009). In food safety, hazard identification and characterisation aims to determine safe levels of exposure for substances for human health, animal health or the environment from pivotal toxicology studies that provide the basis for a reference point. The reference points are then divided by uncertainty factors to derive reference values. Examples of reference points/points of departure for human health and animal