Amies Transport Medium- Composition, Principle, Preparation, Results, Uses
Amies transport medium is a type of microbiological transport medium that is used to collect, transport and preserve specimens that contain pathogenic bacteria. It was developed by Cary and Blair in 1964, based on the original formula of Stuart (1954), and modified by Amies (1967) to include charcoal.
Amies Transport Medium is a semi-solid medium that contains the following ingredients:
- Agar: A solidifying agent that provides a semi-solid consistency to the medium. It also helps to prevent the drying of the medium and the swab during transport.
- Sodium thioglycollate: A reducing agent that lowers the oxidation-reduction potential of the medium and creates a favorable environment for anaerobic bacteria. It also inhibits the growth of aerobic bacteria and fungi.
- Charcoal: An adsorbent that neutralizes toxic substances that may be present in the specimen or produced by bacterial metabolism. It also enhances the survival of fastidious pathogens such as Neisseria gonorrhoeae by removing inhibitory fatty acids and bile salts.
- Calcium chloride, magnesium chloride, potassium chloride, and sodium chloride: Salts that provide essential ions for bacterial metabolism and maintain the osmotic balance of the medium. They also help to preserve the cell membrane integrity and prevent cell lysis.
- Sodium phosphate monobasic and sodium phosphate dibasic: Buffers that maintain the pH of the medium at 7.2±0.2, which is optimal for most bacteria. They also prevent the acidification of the medium by bacterial fermentation.
- Distilled water: The solvent that dissolves all the ingredients and makes up the bulk of the medium.
Amies Transport Medium is designed to preserve the viability of microorganisms without promoting their growth. It is a non-nutritive, phosphate-buffered and semi-solid medium that provides a reduced environment for anaerobic bacteria. The medium also contains charcoal and sodium thioglycollate, which help to neutralize toxic substances that may harm sensitive pathogens such as Neisseria gonorrhoeae.
To prepare Amies Transport Medium, you will need the following ingredients and equipment:
- 20 g of Amies Transport Medium powder
- 1 litre of distilled water
- A large beaker or flask
- A magnetic stirrer and a stirring bar
- A pH meter and a buffer solution
- Small, screwcap bottles
- An autoclave
- Sterile cotton-tipped swabs on wooden sticks
- A pair of scissors or a wire cutter
Amies Transport Medium does not support the growth of microorganisms, but only preserves their viability until they can be cultured in a suitable medium. Therefore, the result interpretation on Amies Transport Medium depends on the type of specimen collected and the culture methods used.
Amies Transport Medium is a widely used medium for collecting, transporting and preserving microbiological specimens. It has several advantages over other transport media, such as:
- It is formulated to maintain the viability of microorganisms without a significant increase in growth, being nonnutritive, phosphate buffered and semi-solid. This prevents overgrowth of commensals and preserves the original ratio of pathogens in the specimen.
- It provides a reduced environment due to the presence of sodium thioglycollate and the small amount of agar. This enhances the survival of anaerobes and facultative anaerobes, such as Neisseria gonorrhoeae, Neisseria meningitidis, Haemophilus influenzae and Bacteroides fragilis.
- It contains charcoal, which helps to neutralize materials that are toxic to sensitive pathogens, such as fatty acids, bile salts and metabolic by-products. Charcoal also absorbs antibiotics that may be present in the specimen, reducing their inhibitory effect on bacterial growth.
- It contains calcium, magnesium, potassium and sodium salts, which help the survival of gonococcal cells and also control the permeability of bacterial cells. These salts also buffer the pH and osmotic pressure of the medium.
- It is easy to prepare and use. It can be sterilized by autoclaving and stored in a cool place until use. It can be inoculated with sterile cotton-tipped swabs on wooden sticks, which are then pushed down one-third of the medium depth and cut so that the swab is forced to the bottom of the medium. The cap is screwed firmly on the bottle and kept cool during transport.
Amies Transport Medium is a useful tool for collecting, transporting and preserving microbiological specimens, especially anaerobic pathogens such as Neisseria gonorrhoeae. However, it also has some limitations that should be considered before using it. Some of these limitations are:
- The medium may lose its effectiveness over time due to evaporation, oxidation or contamination. Therefore, it is important to use fresh medium and store it in a cool place until use. The old medium should be freshly steamed and the charcoal resuspended before use.
- The medium is nonnutritive and semi-solid, which means that it does not support the growth of microorganisms but only maintains their viability. Some growth of contaminants may also occur during the long period of transport. Therefore, after transportation, the specimen should be inoculated in the proper medium as soon as possible. For optimum results, the time lapse between sample collection and inoculum onto culture medium should be reduced to the minimum.
- The medium may not be suitable for the transport of fastidious organisms that require specific nutrients or environmental conditions for survival. For example, some strains of Haemophilus influenzae may not survive well in Amies Transport Medium. Therefore, it is advisable to use a more selective or enriched medium for such organisms.
- The medium may not preserve the viability of all microorganisms equally. For example, gonococci survive well in Amies Transport Medium for at least 6 to 12 hours provided they are not exposed to temperature extremes. However, by 24 hours, the numbers of gonococci decrease to an extent that may prevent their recovery if small numbers were present initially in the specimen. Therefore, it is important to transport the specimen as quickly as possible and avoid temperature fluctuations.
- The medium may interfere with some laboratory tests or procedures due to the presence of charcoal or other components. For example, charcoal may absorb some antibiotics or dyes and affect their activity or detection. Therefore, it is important to remove the charcoal before performing such tests or procedures.
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