Semisolid RV Medium Base

Composition

Final pH (at 25°C): 5.4±0.2

Formula adjusted, standardized to suit performance parameters

Directions

Suspend 15.10 grams in 500 ml distilled water. Heat with stirring to boiling to dissolve the medium completely. DO NOT AUTOCLAVE. Cool to 45°C and aseptically add 1 vial of IMRV/RV Selective Supplement (FD193). Mix well and pour into sterile Petri plates.

Note: The motility of Salmonellas can be drastically reduced when the agar surface becomes too dry. Hence the plates should be well dried before use. If visible moisture occurs on the lid of the plates or the surface of agar, it must be removed. While incubation, incubate the plates aerobically in an upright position for no longer than 24 hours at 42°C.

Principle And Interpretation

Semisolid RV Medium Base is based on the formulation described by DeSmedt et al (1) for the detection of motile Salmonella species from food and environmental specimens. This medium detects more Salmonella positive samples than the routinely used enrichment procedures (2, 3, 4).

Tryptose, casein enzymic hydrolysate provides the nitrogenous and carbonaceous substances and other essential growth nutrients. The motility of other microorganisms is largely inhibited by the selective agents (magnesium chloride, malachite green and novobiocin).

The working of medium is based on the ability of Salmonella species to migrate in the selective medium competing with the other motile organisms, thus producing opaque halos of growth. The motile bacteria will show a halo or zone of growth originating from inoculation spot.

Quality Control

Appearance: Light yellow to light blue homogeneous free flowing powder

Gelling: Semisolid, comparable with 0.27% Agar gel.

Colour and Clarity of prepared medium: Blue coloured clear to slightly opalescent semisolid gel forms in Petri plates.

Reaction: Reaction of 3.02% w/v aqueous solution at 25°C. pH : 5.4±0.2

pH: 5.20-5.60

Cultural Characteristics

M1428: Cultural characteristics observed after an incubation at 42-43°C for 18 - 24 hours with added IMRV/RV Selective Supplement (FD193)when one drop of culture is inoculated in the centre of the medium plate. (Motility is checked by inoculating a drop of culture in the centre of the medium plate).

Storage and Shelf Life

Store below 30°C in tightly closed container and the prepared medium at 2-8°C. Use before expiry date on the label.

Reference

1. De Smedt J.M., Balderdijk R., Rappold H. and Lautenschlaeger D., 1986, J. Food Prot., 49:510.
2. De Smedt J.M., Balderdijk R., 1987, J. Food Prot., 50:658.
3. De Zutter L. et al, 1991, Int. J. Food Microbiol., 13:11.
4. De Smedt J.M. et al, 1991, Int. J. Food Microbiol., 13:301.