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Polymyxin-coagulase-mannitol-agar. I. A selective isolation medium for coagulase-positive Staphylococci
Abstract:
A selective, differential plating medium was developed for the isolation and identification of coagulase-positive and mannitol-fermenting staphylococci. Coagulase produced by growing Staphylococcus aureus caused an opaque zone of fibrin to form around each colony. Several strains of S. aureus produced a visible coagulase reaction by 8 hr, and all strains gave a positive reaction before 12 hr. Mannitol fermentation was usually observed between 12 and 36 hr. Rabbit serum was filtered through Sephadex G-100 to obtain plasmin- and plasminogen-free coagulase-reacting factor (CRF). False-negative reactions, caused by staphylokinase and staphylococcal Müller factor action on plasminogen, were eliminated when this CRF was used. False-positive reactions by lipolytic, coagulase-negative staphylococci were reduced, since gel filtration removed the serum lipoprotein which served as a primary source of opacity. The addition of 75 mug of polymyxin B per ml selectively retarded the growth of S. epidermidis and minimized false-positive reactions caused by citrate-utilizing gram-negative rods. The preparation, characteristics, and use of the medium are presented.
Insights
A new plating medium effectively identifies Staphylococcus aureus by detecting coagulase and mannitol fermentation. This method reduces false positives and negatives, improving bacterial identification accuracy.
Area of Science:
- Microbiology
- Clinical Diagnostics
- Bacteriology
Background:
- Accurate identification of Staphylococcus aureus is crucial in clinical settings.
- Existing methods for staphylococcal identification can be prone to false-positive and false-negative results.
- Coagulase production and mannitol fermentation are key characteristics for differentiating S. aureus.
Purpose of the Study:
- To develop a selective and differential plating medium for isolating and identifying coagulase-positive, mannitol-fermenting staphylococci.
- To improve the accuracy of S. aureus identification by minimizing interfering reactions.
- To present the preparation, characteristics, and application of the novel medium.
Main Methods:
- Development of a selective plating medium incorporating specific reagents.
- Utilizing rabbit serum filtered through Sephadex G-100 to obtain purified coagulase-reacting factor (CRF).
- Addition of polymyxin B to selectively inhibit certain bacterial species and reduce false positives.
Main Results:
- The medium demonstrated clear opaque zones of fibrin around S. aureus colonies due to coagulase activity.
- Coagulase reactions were visible within 8-12 hours, and mannitol fermentation occurred between 12-36 hours.
- Use of purified CRF eliminated false-negative reactions caused by staphylokinase and Müller factor.
- Gel filtration and polymyxin B addition significantly reduced false-positive reactions from other staphylococci and gram-negative rods.
Conclusions:
- The developed plating medium provides a selective and differential method for accurate S. aureus identification.
- The medium effectively distinguishes S. aureus based on coagulase production and mannitol fermentation.
- This approach enhances diagnostic reliability by mitigating common sources of error in staphylococcal identification.