The scientists from London Metropolitan University (UK) have finished their work on the determination of the single cell lag phase duration of bacterial pathogens, such as Listeria monocytogenes and Escherichia coli, in various environmental conditions (deliverable N. 3.19). They have collected recovery data and developed models to predict the lag phase of pathogens after processing up to their exponential phase during the commercial distribution of fresh-cut product.The objective was to examine the effect of mild (sublethal) heat treatment and growth at sub-optimal pH values on the lag times of individual cells of Listeria monocytogenes and Escherichia coli and to fit suitable distributions to the data generated and develop models to predict probability of repair and subsequent transition from the lag to the exponential phase during distribution and shelf life of food.
The strategy adopted by scientists to achieve the objective was to use the sublethal heating and acid treatments reported in Deliverable 3.18 to prepare sublethally damaged cells of L. monocytogenes and E. coli. Single cells of treated organisms were delivered into 100-well "honeycomb" plates and incubated in a Bioscreen at sub-optimal temperatures. This instrument records growth of microorganisms by recording increases in optical density as the cells multiply. Undamaged (control) individual cells of both organisms were also incubated in the Bioscreen at the same temperatures. The temperatures for L. monocytogenes were 5°C (close to minimum for growth), 10°C and 22°C. For E. coli, the temperatures were 10°C (minimum for growth), 15°C and 22°C. The honeycomb plates were incubated in the Bioscreen for up to four weeks. After termination of the experiments, suitable distributions were fitted to the data and the models developed.The mean and standard deviation (SD) of individual cell lag times of unstressed, sublethally heat stressed and sublethally acid stressed L. monocytogenes and E. coli increased when temperature decreased. The mean and SD of individual cell lag times of L. monocytogenes increased after sublethal heat stress followed by incubation at 5, 10 and 22 °C. Individual cells of E. coli subjected to sublethal heat stress could be recovered only at 22°C; they recovered poorly at 10°C and 15°C. Acid stress resulted in an increase of individual lag time for L. monocytogenes at low temperature (5°C), which was not observed at high temperature (22°C).
Paradoxically, for E. coli, acid stress appeared to result in a decrease in individual lag at both 10°C and 22°C. The zero adjusted Gamma distribution was demonstrated as the best fit to data for the individual lag of unstressed, sublethally heated and sublethally acid stressed L. monocytogenes and unstressed and sublethally acid stressed E. coli. A single cell of L. monocytogenes which has been subjected to combined sublethal stresses of acid and heat may exhibit a risk to consumers of fresh-cut melon, if a 14 day shelf life under refrigeration is assumed. The growth capability of E. coli subjected to combined acid and heat stress was significantly affected by the subsequent recovery temperature.
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