Listeria monocytogenes is a facultative intracellular pathogen that causes listeriosis, a highly fatal food-borne infection. After entry into host cells, L. monocytogenes escapes from the phagocytic vacuole and replicates actively in the cytosol. Although the replication phase is essential for the establishment and survival of the pathogen within the host and its further transmission to new hosts, investigations into the microbial pathogenesis have largely neglected this phase of the infection process and therefore little is known about the nutritional determinants of microbial growth in vivo.

The aim of this study was to elucidate the anti-pathogenic mechanisms of probiotic bacteria isolated from a commercial preparation which included Lactobacillus gasseri PA 16/8, Bifidobacterium longum SP 07/3 and Bifidobacterium bifidum MF 20/5. The direct anti-pathogenic activity of the probiotics was measured using agar diffusion assays. The ability of the probiotics to produce extra cellular inhibitory factors was also determined by applying fermentation supenatants to indicator lawns in well and disc diffusion assays. Of the eight pathogens and five lactobacilli used as screening organisms there was a low rate of pathogen inhibition. However L gasseri PA 16/8 displayed potential for inhibitory protein production through production of positive amplicons within polymerase chain reaction (PCR) using acido primers.

Listeriosis is a disease of relatively high mortality mainly caused by the pathogenic bacterium Listeria monocytogenes and is a danger primarily in newborn and unborn infants, pregnant women, the elderly, and the immunocompromised. L. monocytogenes is ubiquitous in the environment. It I can be isolated in soil, wood and decaying matter; the principal route of infection is through the ingestion of contaminated food products. Listeria has been isolated from prepared meat (hot dogs, deli meat), dairy products, unwashed raw vegetables, and seafood. Al present there is no rapid-culture immunoassay for detecting all pathogenic Listeria species (regardless of cell viability) due to the limitations of flagellar antigen targets of most antibody-based systems.

Listeria monocytogenes is a saprophytic food borne microorganism which is pathogenic to humans and animals. The pathogenicity and physiological features of L. monocytogenes have been studied for many years. Some characteristics of this microorganism have been described, such as its low temperature growth and adaptation, the expression of its flagellin gene, and the regulation of its virulence genes. Previous studies imply a possible reversed relationship that may exist between the regulation of the listerial flagellin gene, flaA and its virulence regulator gene, prfA.

Asteria monocytogenes is an important pathogen. It can grow over a wide range of temperature conditions (1°C ~ 45°C). The Cold Shock Response is an important part of its growth at low temperature. The Listeria major cold shock protein, encoded by cspla, has been found to act as both Csps and Caps involved in the cold shock response and the promoter of this gene has motifs lormally associated with binding of small DNA binding protein, IHF (Integration Host Factor). In this project the HU homologue, him, was shown to be a real gene by DNA sequence analysis and that hlm protein is produced in L. monocytogenes cells.

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Strains of Vitreoscilla stercoraria were isolated from the environment and characterised. Cell width, motility and requirement of each strain for sodium chloride were investigated. Two strains were selected for further study and the effect of monensin and FCCP on growth of the strains was investigated. One strain of Vitreoscilla (LB13) was chosen for further study, cells from strain LB 13 were found to be 1.38 µm ± 0.041 (± 1 SEM, n=10) wide, were motile by gliding and had an optimum requirement for sodium chloride for growth of 43 mM. The organism was grown in batch culture and respiratory membranes were isolated. Cytochrome bo was extracted from the respiratory membranes and further purification was achieved using column chromatography. A yield of 6.71 % was achieved with a purification factor of 18.5. The light sensitivity of Vitreoscilla stercoraria was investigated. Two strains of Vitreoscilla (LB13 and C1) were shown to be highly sensitive to UV-A (320-400 nm) with an LD50 of less than 20 kJmˉ². Superoxide dismutase and catalase were shown to provide protection from the effect of UV-A during exposure, either separately of together, indicating an involvement of reactive oxygen species. A photo-insensitive strain (LB13A) was isolated during an exposure experiment and originated from a culture of LB13. The possible sodium pumping activity of cytochrome bo from two strains of Vitreoscilla (LB13 and C1) was investigated. The Vmax of decylubiquinol oxidation by respiratory membranes from LB13 and C 1 were calculated and found to be 0.96 nmol sˉ¹ mg-1 and 13.33 nmol sˉ¹ mgˉ¹ respectively. The Km of decylubiquinol oxidation by LB13 membranes was found to be 9.8 µM. Quinol oxidation activity was tested for dependence on sodium chloride in both respiratory membranes and in the purified enzyme. No stimulation of activity was shown with either strain using decylubiquinol, duroquinol or menadiol as substrates. Given the lack of sodium sensitivity it is unlikely that the enzyme pumps sodium.

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