THE RELATIONSHIP BETWEEN ECOPHYSIOLOGY, INDIGENOUS MICROFLORA AND GROWTH OF LISTERIA-MONOCYTOGENES IN GRASS-SILAGE

The combined effect of the physical and chemical parameters (oxygen tension, pH and dry matter) influencing Listeria monocytogenes growth and survival in silage were simultaneously studied in a model in vitro system. Ensiled grass was exposed to a range of low oxygen concentrations, 0-5% v/v, and their effect was recorded with respect to acidification and microbial population dynamics of the epiphytic microflora, i.e. lactic acid bacteria, enterobacteria, yeasts, moulds and L. monocytogenes in grasses pre-inoculated with the latter. Listeria monocytogenes survival depended on the establishment of a fine balance between the physico-chemical and microbiological characteristics, i.e. oxygen tension, dry matter, pH, grass and microbiological quality. In all grasses ensiled, an oxygen concentration of 1.0% or greater sustained L. monocytogenes growth, below this level growth was shown to be principally dependent on the rate and quality of the fermentation. In most grasses 0.5% oxygen prolonged survival, whereas 0.1% and 0% oxygen caused L. monocytogenes to die off. In very poor quality grass with a restricted fermentation L. monocytogenes survival was prolonged even under anaerobic conditions.