Listeria Fraser Supplement is a specialized product designed for the detection and enumeration of Listeria monocytogenes and Listeria spp. in food and environmental samples. This supplement is essential in microbiological analysis, especially in the food safety industry, complying with ISO 11290 standards. It is used in combination with Listeria Fraser Broth and Demi Fraser Broth to provide a selective enrichment environment that aids in the presumptive identification of Listeria species.
**Composition and Principle**
Each kit contains 10 vials of the supplement. The critical component of this supplement is Ammonium Ferric Citrate, present at a concentration of 0.25g per vial for the liquid form and 0.25g which becomes 0.5g per liter of medium for the freeze-dried form.
The principle of this supplement is rooted in its ability to aid in differentiation. The presence of Ammonium Ferric Citrate in the final medium allows for the hydrolysis of aesculin to aesculetin by Listeria spp. This aesculetin then reacts with ferric ions, resulting in a blackening of the medium. This reaction not only facilitates the differentiation of Listeria spp. but also enhances the growth of Listeria monocytogenes.
The procedure for using the Listeria Fraser Supplement involves a few crucial steps:
1. Preparation of the base culture medium according to its instructions and cooling it to 45-50°C before adding the supplement.
2. Aseptically adding one vial (5 ml) of the Listeria Fraser Supplement to 500 ml of medium.
3. Thorough mixing and transferring into final containers.
It is important to note that the liquid form of the supplement is ready-to-use, while the freeze-dried form needs reconstitution with 5 ml of sterile distilled water.
**Quality Control and Interpretation**
Quality control involves both visual inspection and microbiological control. The liquid form should appear as a homogeneous dark brown suspension, and the freeze-dried form should yield a brownish, limpid solution once reconstituted. For microbiological control, the medium is prepared according to label directions and inoculated with specific microbial strains like Listeria monocytogenes, Escherichia coli, and Enterococcus faecalis. Growth and blackening of the medium are observed as indicators of the supplement’s effectiveness.
**Storage and Transport**
Listeria Fraser Supplement should be stored at 2-8°C, away from light, until its expiry date. However, stability studies have shown that storage or transport at 18-25°C for up to 4 days or at 35-39°C for 48 hours does not affect the product’s performance. Any signs of deterioration or contamination necessitate the disposal of the product.