Background Knowledge
The transportation of microbiological specimens from the collecting location to the laboratory is an important step in providing reliable and exact diagnostic results. Microorganism viability and integrity can be affected during shipping, resulting in false-negative results or biassed microbial profiles. To tackle these challenges, Microbiologists use specialized formulations known as transport media.
Also Read:Culture media preparation – Best ways to avoid contamination
Definition
Transport media are culture media that are used to keep pathogenic microorganisms alive in a clinical sample and inhibit the potential pathogens from being overgrown by other commensals or contaminants in the specimen during transportation to the clinical/diagnostic laboratory.
Principle of Transport media
Transport media are often specialized solutions that provide a safe environment for microorganisms during specimen transport. These solutions act as protective buffers, including just essential elements like carbohydrates, peptones, and salts while excluding nutrients such as carbon and nitrogen. Microorganisms in the specimen do not reproduce but remain viable because of the low nutritional levels and absence of carbon, nitrogen, and organic growth agents.
Purpose of Transport media
- Preservation of Viability: The major goal is to keep the microorganisms present in the collected specimens alive and unharmed. This is necessary for precise laboratory analysis and diagnostic testing.
- Prevention of Contamination: Transport media are intended to keep the specimen from becoming contaminated while in transit. They frequently include inhibitors or antibacterial compounds to prevent the growth of undesirable bacteria, fungi, or other pollutants.
- Maintenance of pH: The pH of transport media varies based on the medium type and the microorganisms it is going to represent. Transport mediums for various clinical specimens are often prepared to keep a neutral or slightly alkaline pH, as many bacteria thrive and remain stable in this pH range.
The pH of transport media is typically between 7.2 and 7.4, which is regarded physiologically normal and beneficial to the survivability of a wide variety of bacteria and other microorganisms. This pH range is near to the neutral pH of the human body, making it acceptable for clinical specimen transfer.
It is essential to note that the exact pH of transport media varies depending on the formulation and intended application.
- Nutrient Supply: Transport medium frequently contains nutrients that aid in the metabolic processes of bacteria while in transit. This is especially crucial for fastidious bacteria, which may have unique dietary needs to survive.
- Temperature Control: Certain microbes are temperature-sensitive. To avoid the growth of undesired organisms or the loss of viability owing to excessive temperatures, transport media may be constructed to maintain a specified temperature range, either through insulating qualities or by including cool packs.
Types of transport media
In microbiology, the type of transport media used depends on the type of microbe being transported and the unique needs of the downstream laboratory analysis.
- Based on their Physical State: Transport media used in microbiology research and clinical settings are available in a variety of physical states, each aimed to satisfy specific needs for preserving distinct types of specimens. Based on their physical states, the following are common types of transportation media:
- Liquid Transport Media:
- Semi-Solid or Gel Transport Media:
- Solid Transport Media:
Some examples of common modes of transportation are:
| Name of Transport Medium | Type of transport medium | Use of transport medium |
| Alkaline peptone water | Liquid bacterial transport media | Use for the transport of V. cholerae |
| Amies transport media | Use for the transport of fastidious microorganisms | Recovery of aerobes and anaerobes Transport of swab specimens |
| Amies charcoal transport Medium | Liquid or semi-solid bacterium transport media | Recovery of Neisseria gonorrhea |
| Brain Heart infusion broth | Liquid bacterial transport media | Use for the transport of enteric viruses particularly for the norovirus, adenovirus |
| PVA ( Polyvinyl Alcohol) transport media | Liquid viral transport media | Use for the transport of enteric viruses particularly for the norovirus,adenovirus |
| Thioglycollate Broth | Liquid bacterial transport medium | Use for the transport of aerobic and anaerobic microorganisms |
| Modified Thayer-Martin medium | Solid bacterial transport media | Use for the transportation of Neisseria gonorrhoeae |
| Staurt Transport medium | Semi solid bacterial type of transport media | Transportation of swabs for recovery of fastidious, non-fastidious, and anaerobic bacteria |
| Anaerobic transport medium | Semi solid bacterial transport media | Recovery of anaerobic, microaerophilic, and facultative bacteria |
| Buffered Glycerol (Buffered Glycerol Saline)medium | Liquid bacterial transport media | Transport of fecal and rectal samples for recovery of enteric pathogens Recovery of Salmonella spp., Shigella spp., and E. coli in fecal sample |
| Cary Blair medium | Semi solid bacterial transport media | Transport of fecal and rectal samples for recovery of enteric pathogens Recovery of Vibrio cholerae, V. parahaemolyticus, Salmonella spp., Shigella spp., and E. coli O157:H7 |
| M4\M5\M6 Transport media | Liquid viral transport media | Commonly used for the transport and preservation of clinical specimens containing viruses, including respiratory viruses such as influenza and SARS-CoV-2. |
| Citrate Transport Medium | Liquid bacterial transport media | Use for the transportation of Neisseria gonorrhoeae |
| Universal Transport media | Liquid bacterial and viral transport media | Designed for the transport and preservation of a broad range of microorganisms, including bacteria and viruses, for various diagnostic purposes. |
| 5%to 10%Formaline Solution | Liquid parasite transport medium | Recovery and transportation of intestinal parasites |
| Leibovitz Emory Transport Medium | Liquid viral transport medium | Recovery and transport of viruses including herpesviruses and enteroviruses |
| Mycoplasma Transport Broth | Liquid bacterial transport medium | Used for the transport and preservation of clinical specimens containing Mycoplasma species |
| Chlamydia Transport Medium | Liquid bacterial transport medium | Transport of swab for recovery of Chlamydia trachomatis and Herpes Simplex Virus |
| Regan-Lowe Transport Medium | Liquid bacterial transport medium | Used for the transport and preservation of clinical specimens containing Bordetella pertussis |
These examples demonstrate the variety of transport media, each customized to the specific needs of various bacteria and clinical specimens. The nature of the microorganism being researched, as well as the laboratory’s specific diagnostic criteria, influence the choice of a suitable transport medium.
Applications of transport media
- They are used to transport clinical specimens from collecting sites to labs for diagnosis, particularly from remote or resource-limited locations.
- If there is a delay in processing/culturing clinical specimens, they are employed to preserve them.
- They are utilized when exchanging or transferring very sensitive to environmental change microbial cultures.
- they serve to inhibit the growth of contaminants and other non-interesting microbial species in clinical samples.
- They work to keep anaerobes, fastidious organisms, and obligatory pathogens alive in the specimen.
- They also serve to transport food, water, and other biological samples for testing.
Limitations of transport media
- Because the transport mediums are nutrient-limited, organisms cannot survive for an extended period of time.
- Despite the fact that the transport medium inhibits and prevents the overgrowth of many contaminants, some contaminants with similar metabolic, physical, and chemical requirements continue to exist.
- Temperature and pressure, for example, must be maintained.
- For isolation and diagnosis, specimens must be cultivated in a different culture medium.
References and sources
- PRESCOTT’S MICROBIOLOGY, TENTH EDITION. / Joanne M. Willey, Hofstra University, Linda M. Sherwood, Montana State University, Christopher J. Woolverton, Kent State University.—Tenth edition.
- https://www.sciencedirect.com/science/article/abs/pii/S0196439983800154
- ttps://byjus.com/biology/classification-of-culture-media/
