GENERAL AND FOOD MICROBIOLOGY PART II - L. V. Krasnikova - 2016

APPENDIX

Class="center">1. Culture Media Used for MICROBIOLOGICAL ANALYSIS OF Food Products

1.1. Kitt-Tarozzi medium for the isolation and cultivation of anaerobic microorganisms

Preboiled pieces of Liver are placed at the bottom of a test tube so that they cover it completely. Meat-peptone broth containing 1 % glucose (pH 7.2–7.4) is poured into the test tubes to half their volume, and a liquid paraffin seal is added. A 0.5–1.0 cm layer of liquid petrolatum is layered on top. The test tubes are sterilized at 121±1 °C for 20 min.

1.2. Lysine medium for the identification of wild Yeasts of the genera Candida, Torulopsis, Brettanomyces, etc.

To 1 dm³ of tap Water, add 50 g of glucose, 3 g of lysine, 1 g of KH₂PO₄, 1 g of MgSO₄, and 15–20 g of Agar. After melting the agar in a water bath, the medium is distributed into test tubes or flasks and sterilized at 116±1°C for 20 min.

1.3. MRS medium for the enumeration of lactic acid Bacteria

Pour 200 cm³ of water into a 1 dm³ volumetric flask and add: 20 g of glucose; 2 g of dipotassium hydrogen phosphate; 2 g of sodium acetate; 0.05 g of MnSO₄ • 4H₂O; 0.02 g of MgSO₄ • 7H₂O; 50 cm³ of Yeast autolysate; 200 mg of Cysteine.

To the prepared solution, add separately: 10 g of peptone, 1 ml of Tween-80, and 500 ml of hydrolyzed milk dissolved in a small amount of hot distilled water. Bring the liquid volume to 1 dm³ with distilled water. Adjust the pH to 6.2–6.6 (using baking soda or citric acid).

To prepare a solid medium, add 15–18 g of agar to 1 dm³ of the prepared solution. After the components are dissolved, distribute the medium into flasks or test tubes and sterilize in an autoclave at 121±1 °C for 15 min.

1.4. Wort agar with crystal violet for the detection of wild yeasts of the genus Saccharomyces

To 1 dm³ of hopped wort (dry matter mass fraction of 7 %), add 20 g of agar and 2 g of crystal violet. Melt the agar in a water bath. Filter the medium through a cotton filter, dispense into flasks, and sterilize at 116±1 °C for 20 min.

1.5. Endo agar

Endo agar is commercially manufactured as a dry powder. In the laboratory, it is prepared According to the instructions on the label. The medium must be prepared on the day of use.

1.6. Levine's medium (Eosin Methylene Blue Agar)

Levine's medium is designed for the isolation and differentiation of Enterobacteriaceae and coagulase-positive staphylococci. First, prepare the basal medium of the following composition: 10 g of peptone; 15 g of agar; 2 g of K₂HPO₄; distilled water up to 1 dm³.

Dispense the medium into 100 cm³ flasks and sterilize at 121±1 °C for 15–30 min.

To prepare the differential medium, add the following solutions—previously sterilized fractionally with flowing steam over three days—to 100 cm³ of melted basal medium: 5 cm³ of a 20 % lactose solution, 2 cm³ of alkaline eosin solution, and 1.5 cm³ of a 0.5 % methylene blue solution.

Pour the prepared medium into Petri dishes and let it dry. It has a blue-violet color.

Lactose-fermenting microorganisms (particularly coliforms) form 1.5–2.0 mm violet colonies with a metallic sheen on this medium. Lactose-negative enterobacteria (Salmonella, Shigella) form transparent, colorless colonies with a diameter of 1.0–1.5 mm. Coagulase-positive staphylococci form small colonies with a dark center on Levine's medium. Commercially available ready-to-use dry Levine's medium is widely used.

1.7. Ploskirev medium

Ploskirev medium is commercially available in dry form. Prepare the medium according to the instructions on the label.

The finished medium is transparent and has a pinkish-yellow color. Lactose-fermenting enterobacteria form deep red (lingonberry-colored) colonies on this medium. Lactose-negative bacteria grow as colorless or faintly colored colonies.

1.8. Wilson-Blair medium

Wilson-Blair iron-sulfite medium is designed for the isolation of sulfite-reducing anaerobes. To prepare this medium, add 10 cm³ of a freshly prepared, sterile 20% sodium sulfite solution (steamed in flowing steam for 1 h) and 1 cm³ of an 8% iron(III) chloride solution prepared in sterile water to 100 cm³ of sterile meat-peptone agar containing 1% glucose, melted and cooled to 80 °С. Pour the medium into test tubes in deep columns and, without sterilization, place them in an incubator at 37±1 °С to check for sterility. Inoculate the material by stab inoculation into the Column or into the melted and cooled medium. Sulfite-reducing anaerobes form black colonies in the medium or cause uniform blackening of the entire medium.

1.9. Egg-yolk salt agar (EYSA) (selective medium for staphylococci identification)

First, prepare the egg-yolk suspension. To do this, aseptically add one egg yolk to 200 cm³ of sterile saline and mix well. Store the egg-yolk suspension in a refrigerator for no more than 4 days.

To prepare the medium, sterilely add 50 cm³ of the egg-yolk suspension to 150 cm³ of melted meat-peptone agar (pH 7.2–7.4) containing 7.5–10% sodium chloride, cooled to 45 °С, mix thoroughly, and pour into Petri dishes. Store in a refrigerator for up to 7 days from the date of preparation.