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The ability of Listeria monocytogenes to resist many adverse environmental conditions has been attributed in part to activation of the alternative sigma factor sigma(B), encoded by the sigB gene. The ability of this pathogen to survive and grow under stress conditions varies between strains within the species. The current study was undertaken to determine whether the role played by the sigB gene in the stress response varies among strains of different serotypes. Null mutations were generated in the sigB genes of L. monocytogenes L61 (serotype 1/2a) and L99 (serotype 4c), and the survival of the resulting mutants was compared with that of the wild-type strains under osmotic, oxidative, and carbon starvation stress conditions and on exposure to bacteriocins, ethanol, acid, and heat. Except in a few cases, strain L61 displayed greater dependence on the sigB products for survival of adverse conditions than did strain L99. The results of this study indicated that the relative importance of the sigB gene in the stress response is not the same in all strains of L. monocytogenes, and this difference may be specific to serotype groupings within the species.

Steers were fed for nine weeks on either ryegrass/clover pasture or maize (six animals per group), to evaluate the effects of diet on attributes of meat and fat. Feed intake was restricted in the grain-fed group with the intention of producing equivalent growth rates in the two treatment groups, but this resulted in significantly slower weight gain, reduced carcass weight, and reduced subcutaneous and intramuscular fat. Nevertheless, the glycolytic potential (an estimate of pre-slaughter glycogen) in the M. longissimus dorsi lumborum was approximately 20% higher (P < 0.05) in the grainfed cattle, while the ultimate pH of the meat was equivalent for both groups. The results suggest that grain-based diets can increase muscle glycogen concentrations independent of the higher calorific intake normally associated with grain diets, probably in response to changes in rumen fatty acid production. Compared with grain diet, pasture diet resulted in significantly higher (P < 0.01) muscle tocopherol content, but did not influence meat colour. The dietary treatments influenced fat colour, but both carotene content and energy intake may be responsible for this effect. The microbiological profile of meat during chilled storage was not affected by diet.

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