Amies transport medium is a widely used and effective semisolid medium for the transportation of swab specimens to the microbiology laboratory. Placing swabs in a moist container or transport medium prevents drying and death of bacteria.
Amies is a modification of Stuart’s medium in which glycerophosphate is replaced by an inorganic phosphate buffer and charcoal is added. This modification prolongs the viability and thus increases the isolation of anaerobes, delicate bacterial pathogens like Neisseria gonorrhoeae Haemophilus influenzae, Neisseria meningitidis, and Haemophilus ducreyi. For delays as long as 24 hours, Amies medium with charcoal can be used.
The presence of sodium thioglycollate and a small amount of agar creates a reduced environment which favors the growth of a wide variety of pathogens. The use of charcoal in transport media neutralize toxic materials in the specimen or on the swab and facilitates the growth of sensitive bacterial pathogens such as Neisseria gonorrhoeae.
Calcium, magnesium, potassium and sodium salts help the survival of the pathogens and also control the permeability of bacterial cells. Phosphate buffer maintains the pH of the medium.
Composition of Amies transport medium
Final pH (at 25°C) 7.2±0.2
Preparation of Amies Transport medium:
Amies transport medium is best prepared from ready to use dehydrated powder, available from most suppliers of culture media.
Preparation of the medium
- Suspend the desired amount of media in distilled water (2 grams in every 100 ml distilled water; concentration may vary depending on manufacturer so prepare the medium as instructed by the manufacturer).
- Heat to boiling to dissolve the medium completely.
- Dispense 6-7 ml in 10 ml capacity screw cap tubes or Bijou bottles.
- Sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes with bottle caps loosened.
- When the medium has cooled, tighten the bottle caps.
- Invert the tubes several times while agar is solidifying, to maintain uniform suspension of charcoal particles.
- Label the tubes with a lot number, preparation date and expiry date (9 months from preparation).
- Store the medium in a cool place away from direct light with the bottle tops tightly screwed
pH of medium: This should be within the range of pH 7.0-7.4 at room temperature.
Shelf-life: Up to 9 months providing there is no change in the volume or appearance of the medium to suggest contamination or an alteration of its pH.
Inoculation of the medium:
Inoculation clinic and ward staff should be advised to check the expiry date before using the medium. Specimen collected on a sterile cotton wool swab should be immersed into the medium, cutting off the swab stick to allow the bottle top to be replaced tightly.
- Insert the swab (with cotton tip downwards) into the medium to one-third of the medium depth.
- Cut off or break the swab stick if longer than the tube.
- Screw the cap firmly in a way that the swab is forced to the bottom of the medium.
- Transport to the laboratory as soon as possible or preferably within 6 hours (maximum up to 24 hours) maintaining a cold chain (do not freeze).
Uses of Amies transport medium
Amies transport medium is used for collection, transport and preservation of microbiological specimens especially throat, vaginal, and wound swab samples. It is routinely used for the transport of specimens from collection centres (or health centres) to the microbiology laboratory.
- Sputum: Transfer a purulent part of the sputum to a cotton-wool swab, and insert it in a container of Amies transport medium. Amies medium ensures the survival of pathogens such as S.pneumoniae and H. influenzae and also avoid the overgrowth of fast-multiplying commensals.
- Pus: Collect the pus specimen using a sterile cotton-wool swab. When pus is not discharged, use a sterile cotton swab to collect a sample from the infected site. Insert it in a container of Amies transport medium, breaking off the swab stick to allow the bottle top to be replaced tightly.
- Nasopharyngeal specimen: To identify carriers of N.meningitidis, a nasopharyngeal swab specimen is collected and transported to the laboratory in Amies or Staurt’s medium or plated directly onto medium and transported in a CO2 containing the system. Amies medium is also used in suspected Bordetella pertussis and Corynebacterium diphtheriae cases.
- Urogenital swab: Amies medium is the most efficient medium for transporting urethral, cervical, and vaginal swabs. It ensures the survival of Haemophilus ducreyi (which causes soft chancre) and delicate organisms like Neisseria gonorrhoeae.
Reference and further reading
- MacFaddin J.F., 1985, Media For Isolation-Cultivation-Identification-Maintenance of Medical Bacteria, Vol. 1, Williams and Wilkins, Baltimore. Revision: 04/ 2018
- Jorgensen,J.H., Pfaller , M.A., Carroll, K.C., Funke, G., Landry, M.L., Richter, S.S and Warnock., D.W. (2015) Manual of Clinical Microbiology, 11th Edition. Vol. 1
- Monica Cheesbrough District laboratory Practice in Tropical countries (Part II)