The growth and/or survival of Listeria Monocytogenes, Yersinia Enterocolitica and Campylobacter jejuni under modified atmospheres at 4°C and 8°C in model food systems

  

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 4°C and 8°C. 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°C). All solid culture models were compared with liquid culture.

30% C0² significantly (p<0.01) suppressed the growth of L. monocytogenes and Y.

enterocolitica macro-colonies especially at 4°C (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 4°C compared with aerobic conditions and identical rates were obtained for L. monocytogenes (0.02 h-1). At 8°C 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°C.

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.