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Malachite green pre-enrichment medium for improved salmonella isolation from heavily contaminated samples
Abstract:
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.
Insights
Adding malachite green (MG) to buffered peptone water (BPW) inhibits Gram-positive bacteria, improving salmonella isolation from food. Non-fat dried milk counteracts MG inhibition of salmonella, aiding recovery from stressed cells.
Area of Science:
- Microbiology
- Food Safety
- Bacteriology
Background:
- Competitive bacteria can hinder salmonella isolation from food and environmental samples during pre-enrichment.
- Selective agents are crucial for improving the recovery of target pathogens from complex matrices.
Purpose of the Study:
- To evaluate the efficacy of malachite green (MG) as a selective agent in buffered peptone water (BPW) for salmonella isolation.
- To determine if non-fat dried milk (NFDM) can mitigate the inhibitory effects of MG on salmonella.
Main Methods:
- Testing the inhibitory effect of malachite green (MG) and brilliant green on Gram-positive bacteria in buffered peptone water (BPW).
- Assessing the impact of MG on pure salmonella strains and the counteracting effect of non-fat dried milk (NFDM).
- Evaluating the recovery of heat, freezing, low water activity, or acid-injured salmonella in BPW with NFDM and MG.
Main Results:
- Malachite green (MG) at 0.1 g/l effectively inhibited Gram-positive bacteria in buffered peptone water (BPW).
- Non-fat dried milk (NFDM) at 5 g/l or higher counteracted the inhibitory effect of MG on salmonella.
- MG in BPW with NFDM did not hinder the recovery of stressed or injured salmonella cells.
Conclusions:
- Supplementing BPW with MG can enhance salmonella isolation from highly contaminated samples by suppressing Gram-positive bacteria.
- The addition of NFDM is essential to prevent MG-induced inhibition of salmonella, particularly for injured cells.
- This method offers a promising approach for improving salmonella detection in challenging food and environmental samples.