Table of Contents
- Introduction
- Composition of Charcoal Selective Medium
- Principle of Charcoal Selective Medium
- Preparation of Charcoal Selective Medium
- Result Interpretation on Charcoal Selective Medium
- Uses of Charcoal Selective Medium
- Limitations of Charcoal Selective Medium
Introduction
In 1984, Bolton et al. suggested that charcoal could be an effective substitute for blood in culture media used for isolating Campylobacter spp. This claim was later validated by Endtz et al., who confirmed that the isolation rate for Campylobacter was higher when using the charcoal selective medium.
Composition of Charcoal Selective Medium
Principle of Charcoal Selective Medium
The medium contains beef extract, which provides essential nitrogen, carbohydrates, vitamins, and other nutrients necessary for the growth of Campylobacter. Gelatin and casein peptones contribute amino acids and peptides as additional nutrients. Charcoal, hematin, sodium pyruvate, and ferrous sulfate enhance the aerotolerance of Campylobacter species; these supplements are believed to act as quenching agents for photochemically-produced toxic oxygen derivatives. Charcoal serves as a detoxifying agent and reduces oxygen tension. Sodium chloride supplies crucial electrolytes, maintaining osmotic equilibrium and preserving cell integrity. Sodium desoxycholate is included as a selective agent to inhibit certain bacteria. Cefoperazone, a cephalosporin antibiotic, suppresses the growth of gram-negative enteric bacilli and some gram-positive species. Vancomycin, a glycopeptide antibiotic, inhibits many gram-positive bacteria. Cycloheximide, an antifungal agent, blocks the growth of yeast and mold.
Preparation of Charcoal Selective Medium
- Add the components to distilled or deionized water and adjust the volume to 1000 mL.
- Mix thoroughly, then gently heat the mixture, bringing it to a boil while stirring frequently.
- Autoclave the solution for 15 minutes at 15 psi pressure and 121°C.
- Allow the solution to cool to 45-50°C.
- Pour the medium into sterile Petri dishes or distribute it into tubes.
- Shake the flask while dispensing to ensure the charcoal remains in suspension.
Result Interpretation on Charcoal Selective Medium
Campylobacter jejuni produces two distinct types of colonies:
- Small, raised colonies that are grayish-brown, smooth, glistening, with a clear, translucent edge.
- Flat colonies that are mucoid, translucent, grayish, and have an irregular edge.
On fresh medium, colonies may appear flat, irregular, or spreading. Some strains may form a thin film on the agar or develop colonies that extend along the streaking line.
On less fresh medium, colonies are typically 1-2 mm in diameter, round, convex, and glistening. These colonies can range in color from yellowish to gray or pinkish and are non-hemolytic.
Uses of Charcoal Selective Medium
It is utilized for the selective isolation and presumptive identification of Campylobacter species from food and human fecal specimens.
Limitations of Charcoal Selective Medium
- It is recommended to perform biochemical, immunological, molecular, or mass spectrometry testing on colonies from pure culture for comprehensive identification.
- Extending the incubation period to 72 hours may enhance the isolation rate of Campylobacter.
- Campylobacter coli, Campylobacter fetus, and some strains of Campylobacter jejuni may be inhibited by cephalosporins.
- Campylobacter jejuni should be incubated at 42°C, as it is a thermophilic organism. The higher temperature provides selectivity by inhibiting accompanying microflora and promoting the growth of Campylobacter jejuni.
- Agents in selective media might inhibit some strains of the target species or allow the growth of species that the medium is designed to inhibit, especially if the species is present in high numbers in the specimens. Therefore, specimens cultured on selective media should also be cultured on nonselective media to obtain additional information and ensure the recovery of potential pathogens.
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