Malachite green pre-enrichment medium for improved salmonella isolation from heavily contaminated samples.

Large numbers of competitive bacteria may hinder the isolation of salmonellas from food and environmental samples when a pre-enrichment method is used. The addition of 0.1 g/l of malachite green (MG) to buffered peptone water (BPW) inhibited the multiplication of Gram-positive bacteria. Brilliant green had a similar effect but only when the normal recommended concentration of 0.02 g/l was raised to 0.05 g/l. Pure strains of salmonellas were inhibited by MG in BPW, but addition of non fat dried milk (NFDM) (5 g/l or more) counteracted this effect. MG did not affect the recovery of salmonellas injured by heat, freezing, low water activity or acidity in BPW with NFDM. It was concluded that addition of MG to BPW may improve the possibility of isolating salmonellas from heavily contaminated materials by limiting the competitive growth of Gram-positive bacteria and the subsequent lowering of the pH of the broth.