Why Charcoal is Crucial in Amies Transport Medium for Gonorrhea Detection
Gonorrhea is a major public health issue worldwide. The bacteria that causes gonorrhea, Neisseria gonorrhoeae, has developed resistance to nearly all antimicrobials used for treatment. Being able to obtain live N. gonorrhoeae isolates through culture is essential for monitoring antimicrobial resistance trends. However, many clinics cannot perform direct on-site cultures. They must use transport media to store patient samples temporarily until they can be cultured in a laboratory. Transport media that maintain N. gonorrhoeae viability are crucial.
A study in 2021 evaluated two commercially available transport media - Amies with charcoal, and Amies without charcoal. It compared how well each medium could recover N. gonorrhoeae from clinical samples under real-world conditions in Ukraine. A total of 103 urethral/cervical samples from symptomatic gonorrhea patients were collected. The samples were randomly allocated to the two media types. After brief storage at 4°C, they were transported to the laboratory for culture.
Three aspects were compared between the two media:
1) Recovery rate - the percentage of samples that yielded viable N. gonorrhoeae isolates;
2) Time to isolation - how quickly colonies grew after plating on agar; and
3) Growth density - how much the bacteria multiplied and spread across the agar surface.
The study found charcoal-containing Amies medium performed significantly better than charcoal-free Amies. Its recovery rate was 86.3% compared to only 59.6% for the charcoal-free medium. Growth time was faster, with 84.3% isolated within 24 hours versus 42.3% for the charcoal-free swabs. Bacterial growth density was also superior, with 59.1% showing growth beyond the first quadrant of the agar plate, versus 19.4% with the charcoal-free medium.
The researchers concluded that charcoal-containing Amies transport medium was clearly preferable for maintaining N. gonorrhoeae viability. The charcoal allows larger yields of viable bacteria to be recovered from patient samples. It also enables faster isolation compared to charcoal-free medium. Faster isolation improves the chances of obtaining the fresh bacterial colonies needed for antimicrobial susceptibility testing before the bacteria die off.
This study provides solid evidence to support the use of charcoal-supplemented Amies transport media for gonorrhea diagnostics and surveillance when direct plating of patient samples is not possible. Maintaining N. gonorrhoeae viability after sample collection is challenging due to its fastidious nature. The findings suggest charcoal provides key benefits for preserving these fragile bacteria. Continuous validation is still needed to verify optimal performance of transport media across various settings. But charcoal-containing Amies medium appears to be an excellent option for facilitating culture-based detection and antimicrobial resistance monitoring of N. gonorrhoeae, particularly in resource-limited clinics and laboratories. Routine validation of commercially available transport systems will help ensure good quality N. gonorrhoeae isolates are available for public health surveillance worldwide.
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Boiko I, Krynytska I. Comparative performance of commercial Amies transport media with and without charcoal for Neisseria gonorrhoeae culture for gonococcal isolation and antimicrobial resistance monitoring in Ukraine. Germs. 2021 Jun 2;11(2):246-254. doi: 10.18683/germs.2021.1261. PMID: 34422696; PMCID: PMC8373411.
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