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Listeria monocytogenes and Australian smallgoods : detection and control

Birrell, FK 2005 , 'Listeria monocytogenes and Australian smallgoods : detection and control', Research Master thesis, University of Tasmania.

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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.

Item Type: Thesis - Research Master
Authors/Creators:Birrell, FK
Keywords: Food contamination, Foodborne diseases, Listeriosis, Food, Listeria monocytogenes
Copyright Holders: The Author
Copyright Information:

Copyright 2005 the Author – The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (MSc)--University of Tasmania, 2006. Includes bibliographical references. Ch. 1. Listeria monocytogenes and Australian smallgoods: detection and control -- Ch. 2. Survey of Listeria monocytogenes in Australian smallgoods -- 3. Isolation and growth of lactic acid bacteria in ham -- Ch. 4. Mechanisms of interactions between cells of different bacterial species in co-culture -- Ch. 5. Control of Listeria monocytogenes by lactic acid bacteria as competitors to growth -- Ch. 6. Conclusions

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