Abstract
Grass was field-dried to 3 different dry matter (DM) levels (200, 430 and 540 g/kg) and inoculated with 106–107 cfu/g of a Listeria monocytogenes strain sharing a phagovar occasionally involved in food-borne outbreaks of listeriosis. Formic acid (3 ml/kg) or lactic acid bacteria (8·105/g) with cellulolytic enzymes were applied only to forages with low and intermediate DM levels. Forages were ensiled in laboratory silos (1700 ml) and were stored at 25°C for 30 or 90 days. After 90 days of storage, L. monocytogenes could not be detected in any silo, except one with the high dry matter grass without additive. After 30 days of storage, between 102 and 106 cfu L. monocytogenes/g silage were isolated from the untreated silages. Increasing the DM content from 200 to 540 g/kg did not reduce listeria counts possibly because of the lower production of fermentation acids (higher pH). In silages treated with additives, counts of L. monocytogenes were always lower than in silages without additive. In wet silages (DM 200 g/kg) both additives were effective, but in the wilted silages (DM 430 g/kg) only the bacterial additive reduced listeria counts below detection level. Listeria counts were highly correlated to silage pH (r = 0.92), the concentration of lactic acid (r = -0.80) and the pooled amount of undissociated acids (r = -0.83).
Highlights
Listeria monocytogenes is the causative organism of the disease listeriosis which affects both man and a wide range of animals with manifestations such as septicaemia and/or affection of the central nervous system
Acidic conditions inhibit the growth of L. monocytogenes, but there is no consensus on the precise pH-level in silage at which the organism will cease to grow (Irvin (1968): no growth below pH 5.5; Fenlon (1988): slow growth at pH 4.5)
The decreasing water activity reduces the activity of all viable bacteria, including lactic acid bacteria
Summary
Listeria monocytogenes is the causative organism of the disease listeriosis which affects both man and a wide range of animals with manifestations such as septicaemia and/or affection of the central nervous system. Acidic conditions inhibit the growth of L. monocytogenes, but there is no consensus on the precise pH-level in silage at which the organism will cease to grow (Irvin (1968): no growth below pH 5.5; Fenlon (1988): slow growth at pH 4.5). This is not surprising because there are usually more environmental factors than just pH that affect the growth of listeria and other competing organisms. The decreasing water activity reduces the activity of all viable bacteria, including lactic acid bacteria
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