The medium Stuart It is a semi-solid agar or a broth without nutrients, used for the transport of biological samples. Its purpose is to keep the strains present in the sample viable for a certain time, but without increasing the microbial population.
It was created by Moffet, Young, and Stuart in 1948 and later modified by Toshach and Patsula. It is composed of sodium glycerophosphate, sodium thioglycolate, calcium chloride and agar-agar. The latter is present in the semi-solid medium and absent in the liquid. Some laboratories add methylene blue.
They are generally used when it is not possible to sow the sample immediately. In this case, the sample is placed in the transport medium, while it is transferred to the laboratory that will process the culture..
In this way, it is guaranteed that the microorganisms present remain alive until they are inoculated in the corresponding culture media..
Initially, its use was intended to transport urethral or vaginal samples where the presence of Neisseria gonorrhoeae. Later, they realized that its usefulness could be broader.
The Stuart medium was found to be very efficient in preserving labile and fastidious microorganisms such as Neisseria meningitidis, Haemophilus influenzae, Streptococcus pyogenes, Streptococcus pneumoniae, Corynebacterium diphteriae, Bordetella pertussis, among others. In addition to other non-annoying microorganisms such as Enterobacteria.
The Stuart medium basically avoids the desiccation of the sample, maintains the osmotic balance and preserves the microorganisms at an ideal pH. On the other hand, strategically this medium does not contain nutrients, thereby preventing microorganisms from multiplying.
On the market there are special sample transport devices that contain this or other means of transport, called culturette. They can also be prepared in the laboratory.
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The purpose of the Stuart means of transport is to keep the samples as close to their original state, until they are processed..
Sodium glycerophosphate and calcium chloride is a buffer system that works by maintaining pH and osmolarity..
Meanwhile, sodium thioglycollate acts as a reducing agent and in the case of containing agar, this gives it a semi-solid consistency, to delay the oxygenation of the medium. Finally, methylene blue is an oxidation indicator, that is, it detects the presence of oxygen.
The semi-solid medium with redox indicator is ideal for transporting samples where the presence of anaerobic bacteria is suspected.
The Stuart medium, like all means of transport, meets certain characteristics, including:
-Maintains viable microbial strains present without multiplying.
-Contains no nutrients; Its compounds are aimed at keeping the sample hydrated, under adequate conditions of pH and osmolarity.
-Inhibits the autolyzate of some delicate microorganisms, such as Pneumococcus.
-Its use is temporary, it should not be prolonged too long.
Sodium glycerophosphate 10 gr
Sodium thioglycollate 1.0 gr
Calcium chloride 0.1 gr
Methylene blue 0.002 gr
Agar-agar 3 gr
Distilled water 1 L
Mix the components and heat until completely dissolved. Pour into tubes or special swabs. They are introduced into the autoclave and sterilized at 121ºC for 15 minutes. Let the tubes cool in an upright position (in the shape of a plug). Keep at room temperature.
Weigh 14.1 g of the medium in 1 liter of distilled water. Heat, stirring frequently until completely dissolved, it can be boiled. The rest of the procedure as already described.
The dehydrated medium is cream-colored and prepared should be blue on its surface. This is due to the presence of methylene blue.
The pH of the medium must be adjusted to 7.4 ± 0.2
Ingredients
Sodium glycerophosphate 10 gr
Sodium thioglycollate 1 gr
Calcium chloride dihydrate 0.1 gr
Distilled water 1000 ml
It has the same ingredients as the semi-solid, but it does not have agar or methylene blue. In this case, the components are mixed without heating and 0.5 ml is distributed in 16 x 125 mm tubes with screw caps..
It is sterilized in an autoclave at 121 ° C for 10 minutes. For sampling, dacron swabs should be used if the sample is for PCR or rayon if it is for culture.
Tubes are stored at room temperature.
Commercially there are special sample transport devices called culturette; This device is an elongated and thin tube that has a swab inside and at the end there is a segment that contains the semi-solid or liquid Stuart medium.
The device is opened and the sample is taken with the swab. It is then introduced back into the tube until the tip of the swab is inserted into the semi-solid transport medium; some bring a polyurethane sponge impregnated with liquid Stuart medium, with which the sample is preserved by keeping it wet.
In the event that the presence of delicate microorganisms is suspected, cotton swabs with charcoal should be used to collect the sample..
The samples that can be taken with swabs or swabs and transferred in this medium are pharyngeal exudates, rectal swabs and various secretions, including: vaginal, urethral, wound, ocular, otic and abscess..
During the transfer of the sample in the transport medium to the laboratory, subjecting the medium to extreme environmental conditions (too cold or too hot) must be avoided. Sudden changes in atmospheric pressure should also be avoided.
The time in which the medium is capable of keeping a microorganism viable will depend on the strain in question. For example, for microorganisms of the genus Neisserias and Haemophilus, they can remain viable for up to 24 hours, but strains such as Salmonella or Shigella can last days and even weeks..
If it is estimated that the processing of the sample may take more than 4 days, it is preferable to freeze the sample at -70 ° C.
The prepared means of transport or the commercial culturette must have a label where the following information will be placed:
Patient's first and last name: avoid sample confusion.
Identification number: it is convenient to keep an order at work.
Sample collection source: helps the bacteriologist to choose the appropriate culture media.
Treating physician: Sometimes it is necessary for possible consultations.
Date and time of sampling: It is necessary for the acceptance or rejection of the sample according to the time elapsed between taking the sample and receiving it in the laboratory..
Other information that would be very useful to attach is:
The presumptive diagnosis: guides in the addition of special culture media according to the diagnostic suspicion.
Antibiotic treatment: it is useful to be attached to the antibiogram.
All this information is vital to guarantee the good processing of the sample..
To evaluate the performance of the Stuart medium, known strains can be inoculated and kept at 25 ° C, such as: Bordetella pertussis ATCC 9340, Haemophilus influenzae ATCC 19418, Neisseria gonorrhoeae ATCC 19424, Neisseria meningitidis ATCC 13090, Streptococcus pneumoniae ATCC 6301.
In all cases, it is expected to recover the microorganism in question, preserved for 24 hours..
On the other hand, other strains can be used such as: Shigella flexneri ATCC 12022, Staphylococcus aureus ATCC 6538, Streptococcus pyogenes ATCC 12344, Enterococcus faecalis ATCC 33186, Pseudomonas aeruginosa ATCC 27853 and Escherichia coli ATCC 11775.
In all cases, abundant recovery is expected for up to 96 hours..
Some studies assure that the glycerophosphate present in the Stuart medium can be metabolized by some coliforms, among other Gram negative bacteria, and therefore multiply in this medium..
This risk increases the longer the sample processing is delayed. Likewise, the exposure of the Stuart medium to high temperatures during its transfer influences this particular.
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