Paper I — 2015 September (Supplementary) (2010 Scheme) — Q6
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Paper I
2015 September (Supplementary) (2010 Scheme) · 40 marks · 120 min
Question
Transport media
Q62 marksShort Notes
Answer
Transport media are used to preserve the viability of pathogens present in a clinical specimen during transit from the point of collection to the laboratory, without allowing multiplication of the organism (which would distort the true bacterial load) or overgrowth of contaminating/commensal organisms, while preventing the fastidious/fragile pathogen from dying due to desiccation or unfavourable conditions before it can be cultured.
Key properties: non-nutritive (or minimally nutritive) — buffered to maintain a stable pH and osmolarity, and reduced in oxygen content where anaerobes need to be preserved.
Examples:
Cary-Blair medium — the most widely used general-purpose transport medium for enteric bacterial pathogens (e.g., Vibrio cholerae, Shigella, Salmonella) in stool specimens; maintains viability for prolonged periods without allowing overgrowth.
Stuart’s/Amies transport medium — semisolid, non-nutritive media used for swabs (throat, wound, genital) intended for general bacterial (including Neisseria gonorrhoeae) culture; Amies medium (a modification of Stuart’s) additionally contains charcoal to absorb toxic/inhibitory substances.
Venkatraman-Ramakrishnan (VR) medium — an alkaline transport medium specifically used for Vibrio cholerae in stool/rectal swab specimens.
Sach’s buffered glycerol saline — used for stool specimens when Salmonella/Shigella is suspected (glycerol is toxic to Vibrio cholerae, so unsuitable when cholera is suspected).
Viral transport medium (VTM) — contains antibiotics/antifungals to suppress bacterial/fungal contamination, along with protein stabilizers, used for respiratory/other swabs intended for viral culture or PCR.
Clinical importance: correct choice and prompt use of transport medium (with appropriate temperature control during transit) is essential for reliable culture-based diagnosis, particularly for fastidious/fragile organisms like Neisseria gonorrhoeae and anaerobes.
