SIM Motility Medium, Modified

SIM Medium is recommended for determination of hydrogen sulphide production, indole formation and motility of enteric bacilli in accordance with FDA BAM.

Composition**

Final pH ( at 25°C) 7.3±0.2

**Formula adjusted, standardized to suit performance parameters

Directions

Suspend 30.0 grams in 1000 ml distilled water. Heat to boiling to dissolve the medium completely. Dispense in tubes. Sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes. Allow the tubes to cool in an upright position.

Principle And Interpretation

SIM Medium is recommended by FDA BAM, 1998 (1) to differentiate enteric bacilli particularly Salmonella and Shigella on the basis of sulphide production, indole formation and motility (2). Jordan and Victorson (3) reported that Salmonella Paratyphi A and Paratyphi B can be distinguished on the basis of H2S production using lead acetate. Sulkin and Willett (4) used Triple Sugar Iron Agar with 1% agar for motility along with H2S production and carbohydrate fermentation. Sosa (5) described a peptone medium with low agar for motility and indole determination.

Motility, indole and sulphide production tests are used to differentiate Enterobacteriaceae members. SIM Medium combines these three differential characteristics in a single medium. Peptonized iron and sodium thiosulphate are the indicators of H2S production. This H2S reacts with peptonized iron to form black precipitate of ferrous sulphide. Salmonella are H2S positive and Shigella are H2S negative. Motile organisms intensify the H2S reaction. Motile organisms grow away from line of inoculation showing diffused growth while non-motile organisms grow along the stabline. Motility detection is possible due to the semisolid nature of the medium. Salmonella is motile while Shigella are non motile. Tryptophan, from peptic digest of animal tissue, is degraded by specific bacteria to produce indole. The indole is detected by the addition of chemical reagents following the incubation period.

Inoculate fresh culture with a single stab using straight needle through the center of the medium. Following incubation, observe for motility (diffuse growth outward from the stabline or turbidity throughout the medium) and for H2S production (blackening of the medium). To detect indole production, add three or four drops of Kovacs reagent and observe for development of red color (positive reaction). Determine motility and H2S production prior to determination of indole production.

Quality Control

Appearance Cream to beige homogeneous free flowing powder

Gelling Semisolid, comparable with 0.3% Agar gel.

Colour and Clarity of prepared medium Medium amber coloured slightly opalescent gel forms in tubes as butts

Reaction Reaction of 3.0% w/v aqueous solution at 25°C. pH : 7.3±0.2

pH 7.10-7.50

Cultural Response

Cultural characteristics observed after an incubation at 35-37°C for 18-24 hours.

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. FDA, U.S. 1998. Bacteriological Analytical Manual. 8 ed. Gaithersburg, MD: AOAC International.
2. MacFaddin, J. F. 1985. Media for Isolation-Cultivation-Identification-Maintenance of Medical Bacteria vol. 1. Baltimore: Williams and Wilkins.
3. Jordan, E. O. and Victorson, R 1917. J. Inf. Dis, 21.
4. Sulkin, S. E. and Willett, J. C 1940. J. Lab. Clin. Med., 25.
5. Sosa, L 1943. Rev. Inst. Bacteriol, 11.