Isolation and identification of Listeria monocytogenes in vegetable byproduct silages containing preservative additives and destined for animal feeding

Abstract

Abortion outbreaks in sheep and goats have been linked with the consumption of silaged oranges and artichokes, whereas traditionally only silage of lucerne and corn had been considered for the propagation of Listeria. In the present work 24 silage samples were analysed. Materials used were byproducts derived from the orange and artichoke canning industry. The silage contained as additives formic acid, beet pulp and sodium chloride; a control was retained for each type of silage. Temperature, pH and moisture values were recorded and the presence of Listeria monocytogenes investigated. The presence of L. monocytogenes was corroborated by means of morphological, cultural, biochemical and serological methods at pH values of 2.7, 2.7, 3.0 which are lower than those usually reported for Listeria. The absence of the microorganism from silage containing 2.5% NaCl may be related to the inhibitory action of this additive which may therefore be considered as a good preservative for silage. The high percentage of moisture that characterizes this kind of silage could be associated with a mould growth observed in it, and probably with the presence of Listeria. It is concluded that the presence of L. monocytogenes in silage made of vegetable byproducts can play a new and important role in the epidemiology of listeriosis in ruminants.