Rappaport Vassiliadis Medium, Modified

Principle And Interpretation

Rappaport Vassiliadis Medium, Modified is a modification of the formulation by Van Schothorst et al (1) and is recommended for the selective enrichment of Salmonellae from food specimens by FDA BAM, 1998(2). Salmonella generally survive at little high osmotic pressure, grow at slightly low pH and are resistant to malachite green compared to other bacteria. Salmonellae constitute the most taxonomically complex group of bacteria among the Enterobacteriaceae (3). Human Salmonella infections are most commonly caused by ingestion of food, water or milk contaminated by human or animal excreta. Contaminated eggs or foods containing eggs have also been a source of food borne salmonellosis.

Inoculate the samples into appropriate enriched media and incubate for 24 hrs for 24 ± 2 h at 35°C. Transfer 0.1 ml mixture to 10 ml Rappaport Vassiliadis Medium, Modified (M880F) and another 1 ml into 10 ml Tetrathionate (TT) broth (M880F). Vortex and incubate at optimum temperature for 24 ± 2h depending upon the microbial load and type of the sample. These are further subcultured into XLD Agar or HE Agar, incubate the plates for 24 ± 2h at 35°C and observe for the appearance of typical salmonellae colonies. Blue-green to blue colonies will be appeared in XLD Agar and pink colonies with or without black centers on HE Agar.

The medium contains Tryptone which provides essential growth nutrients. Magnesium chloride raises the osmotic pressure in the medium. Malachite green is inhibitory to organisms other than Salmonellae. The low pH of the medium, combined with the presence of malachite green and magnesium chloride, helps to select for the highly resistant Salmonella species. Potassium phosphate buffers the medium to maintain the constant pH. Sodium chloride maintains the osmotic balance.