The growth and/or survival of Listeria Monocytogenes, Yersinia Enterocolitica and Campylobacter jejuni under modified atmospheres at 4oC and 8oC in model food systems
Harrison, Wendy Anne
Cardiff Metropolitan University
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This thesis reports on the use of solid-phase and liquid model food systems to estimate dynamic growth data for Listeria monocytogenes and Yersinia enterocolitica and the survival of Campylobacter jejuni under modified atmospheres and refrigeration temperatures. Modified atmospheres included 5% O2: l0% CO2:85% N2 (MA1), 30% CO2:70% N2 (MA2) and aerobic conditions at 4oC and 8oC. Computer image analysis recorded macro-colony growth on agar surfaces. Correlation of bacterial colony radius to viable cell number allowed estimation of dynamic growth data for L. monocytogenes and Y. enterocolitica. Image analysis techniques were developed allowing estimation of dynamic growth data for micro-colonies under MA1 and aerobic conditions, using area to estimate colony size. Morphological change in Camp. jejuni cells on agar surfaces was determined after storage under atmospheres and temperatures of study and survival after storage was determined by incubating the cells under ideal conditions (microaerophilic; 37 oC). All solid culture models were compared with liquid culture. 30% CO2 significantly (p<0.01) suppressed the growth of L. monocytogenes and Y. enterocolitica macro-colonies especially at 4oC (3 and 2.5 fold decrease in specific growth rate, respectively compared with aerobic conditions). A significant (p<0.05) decrease was observed for Y. enterocolitica micro-colonies (approximate 1.5 fold) at 4oC compared with aerobic conditions and identical rates were obtained for L. monocytogenes (0.02 h-1). At 8oC similar growth rates were obtained under MA1 and aerobic conditions for both organisms. Growth in liquid culture showed similar findings to macro- and micro- colony growth. Survival of Carnp. jejuni was enhanced under 30% CO2 as compared with aerobic conditions. Modifying the atmosphere reduced the morphological shift of the cells to the coccoid form on agar surfaces during resuscitation at37 oC. This thesis extends the knowledge of the behaviour of foodborne pathogens in foods under MAP. The micro-colony technique coupled with image analysis improves the understanding of the change in cell morphology and growth under MAs and refrigeration temperatures.
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