Listeria monocytogenes and Australian smallgoods : detection and control

Despite its relatively low incidence, Listeria monocyto genes is an important food borne pathogen due to the seriousness of symptoms in susceptible population groups. Listeria spp. are ubiquitous and are difficult to eradicate from the smallgoods environment. The pathogen is destroyed during heat treatment, however post-processing contamination can occur. Because L. monocyto genes can grow at 4°C on vacuum- and/or modified atmosphere packed (VP, MAP) smallgoods that typically have a shelf life of six to eight weeks, dangerous growth of the organism can occur in the absence of control measures. Several international listeriosis outbreaks involving smallgoods attest to this. This thesis investigated the incidence of Listeria spp. in Australian smallgoods, and the influence of interspecies bacterial competition on growth of L. monocytogenes. The first part of this study comprised a survey of 100 smallgoods products to investigate the incidence of L. monocyto genes in Australian processed meats. Although no endogenous L. monocyto genes strains were isolated during this study, other Listeria spp. were occasionally found. This encouraging finding suggests that the increasing application of HACCP and GMP by Australian smallgoods producers has decreased the incidence of L. monocyto genes in processed meats. A substantial part of this thesis investigated the potential control of L. monocyto genes by competition with endogenous lactic acid bacteria (LAB). The study included exploring the basis of the \Jameson Effect\" a phenomenon that occurs when bacteria are grown in mixed cultures and inhibition of growth of all species is observed when one species reaches stationary phase (i.e. 'maximum population density' MPD or 'maximum carrying capacity' MCC of the environment). The \"Jameson Effect\" may be caused by a number of factors including; for example bacteriocins organic acids and nutrient depletion. This study explores the role of non-specific factors (i.e. pH and nutrient depletion) in the \"Jameson Effect\" and by inference the importance of species- specific interactions. Endogenous LAB for use in competition studies were isolated from commercial sliced ham and identified. These strains Lactobacillus sakei Leuconostoc mesenteroides and Leuconostoc carnosum are commonly found on VP/MAP processed meats. Another substantial aspect of the thesis was to determine whether nutrient depletion pH or excreted soluble metabolites could explain the \"Jameson Effect\" for inhibition of L. monocytogenes (Scott A). The results of the study suggests that competition among bacteria for nutrients is a major factor leading to the \"Jameson Effect\" and that there is little need to invoke production of species- specific inhibitors to explain the effect. Initial experiments to investigate the nature of the \"Jameson Effect\" were conducted in simple broth systems. These studies found that low pH and the presence of bacterial metabolites in the broth medium did not reduce MPD in the presence of adequate nutrients. This suggested that the \"Jameson Effect\" observed in broth systems could simply be symptomatic of nutrient depletion. In support of this conclusion consistent results were obtained in MAP ham studies undertaken to validate results of the broth studies under realistic commercial conditions. Thus it was concluded that the role of lactic acid bacteria (which dominate VP/MAP processed meats) should not be underestimated when attempting to understand and control the risk of L. monocyto genes in refrigerated VP/MAP processed meats."