Peptone Water is a type of broth medium. This media is mainly used to grow the organism and is also used as a base for determining carbohydrate fermentation patterns of non-fastidious organisms. It is also used to detect indole production by the organism.
Composition of Peptone Water
| Ingredients | Gms / L |
| Peptone | 10.0 |
| Sodium chloride | 5.0 |
Final pH should be (at 25°C) 7.2±0.2
Preparation of Peptone Water
- Suspend 15.0 grams in 1000 ml distilled water.
- Add the test carbohydrate in desired quantity and dissolve completely.
- Dispense in tubes with or without inverted Durhams tubes and sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes.
Principle of Peptone Water
Peptone Water is particularly suitable as a substrate in the study of indole production. Peptone used in Peptone Water is rich in tryptophan content. The presence of indole can be demonstrated using either Kovacs or Ehlrich reagent. Peptone Water is also utilized as a base for carbohydrate fermentation studies with the addition of sugar and indicators such as bromocresol purple, phenol red, or bromothymol blue.
Peptone Water is recommended for studying the ability of an organism to ferment a specific carbohydrate which aid in the differentiation of genera and species. Peptone water is formulated as per Shread, Donovan and Lee. Peptone Water with pH adjusted to 8.4 is suitable for the cultivation and enrichment of Vibrio species. Peptone provides nitrogenous and carbonaceous compounds, long-chain amino acids, vitamins provide essential nutrients.
Sodium chloride maintains the osmotic balance of the medium. To study the fermentation ability of carbohydrates, saccharose, rhamnose, salicin are generally added in 0.5% amount separately to the basal medium before or after sterilization. The acidity formed during fermentation can be detected by the addition of a phenol red indicator, which shows a color change of the medium from red to yellow under acidic conditions. If desired, Durham’s tube may be used to detect the gas production if produced.
Result Interpretation on Peptone Water
Cultural characteristics observed after an incubation at 35-37°C for 18-24 hours
| Organism | Test Result |
| Staphylococcus aureus | negative reaction, no red ring at the interface of the medium on addition of Kovac’s reagent |
| Escherichia coli | positive reaction, red ring at the interface of the medium on addition of Kovac’s reagent |
| Salmonella Typhimurium | negative reaction, no red ring at the interface of the medium on addition of Kovac’s reagent |
Uses of Peptone Water
- Used to determine the carbohydrate fermentation patterns of non-fastidious organisms.
- Used to detect indole.
- Used as a diluent or for making suspensions of non-fastidious microorganisms for microbial enumeration procedures.
- It is used as a growth medium and as a base for carbohydrate fermentation media.
- The Peptone Water in alkaline condition (pH 8.4) is used for the cultivation and enrichment of Vibrio species.
Limitations of Peptone Water
- For biochemical, immunological, molecular, or mass spectrometry testing the colonies should be taken from pure culture for complete identification.
- Due to its low nutrient content, it’s not suitable for the growth or maintenance of fastidious microorganisms.
- Some strains may exhibit poor growth due to nutritional variations.
- The pH of the Peptone Water can be changed by some sugar solutions, hence checking must be done.
- Sub-cultures may be needed to make sure the purity of the inoculant. Mixed or contaminated cultures can results in false reactions.
References
- https://himedialabs.com/TD/M028.pdf
- https://microbenotes.com/peptone-water/
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