Swabs - Capture, Maintain and Release!
- Efficient collection and release of microorganisms
- Preserve the microbiological population from sampling to the bench
Samples can arrive at the microbiology laboratory in a variety of formats, often sub-samples of a large production batch or in a clinical setting, samples of body fluids. Swabs however, are unique in their presentation, as the target microorganisms need to have been efficiently collected from the sampling site, carried by an inert vector and then must be recovered from this for subsequent analysis.
Samples of microorganisms collected by swabbing will invariably have been subjected to a cleaning regime or antibiotic treatment designed to reduce or eliminate their presence. Therefore the numbers of cells present may be very small and they will likely be under some stress.
Taken from their original ecological niche, to which they had become adapted, and subjected to some process; physical, chemical, biological, or all three, they will either be killed or they will adapt and survive. The former outcome is ultimately that desired by the processor or clinician. The latter presents a risk to subsequent procedures within the processing environment or to clinical outcome.
During transportation of the swab the numbers and proportions of micro-organisms present should be the same when it arrives at the lab as they were when first sampled. Swab transport systems should be able to keep the more delicate and fastidious bacteria viable whilst preventing the more robust ones from multipyling and obscuring others. With increasing use of immunological and molecular testing techniques even more is required of the transport medium - it must maintain cellular material and not interfere with the diagnostic test used. This combined with use of centralised testing facilities extending the time from sampling to bench, makes the choice of swab and transport medium is more important than ever before.
So swabbing techniques, and the swabs themselves, need to provide an efficient collection of sample, its subsequent preservation, and ultimately the release of the target cells. In 2003 the Clinical Laboratory Standards Institute (CLSI) published M40-A , an approved standard for the quality control of microbiological transport systems, this meant that for the first time swab products could be compared and evaluated using set criteria.
Technology & Applications
Disposable, single-use plastics revolutionized the microbiology laboratory, lending themselves particularly to the testing of surfaces or inaccessible areas of equipment and to the collection of cellular samples from epithelial surfaces.
Traditionally the swab would comprise a flexible shaft terminating in a bud of compacted material, e.g. cotton, viscose, contained within a pre-labelled, tamper-evident tube. The tube may also contain a transport medium designed to maintain the viability of any organisms collected.
There is a wide range of transport media available e.g. Amies Stuart, Cary-Blair and Universal Transport Medium (UTM), these are intended to maintain the viability of any cells present until the swab is returned to the laboratory. In environmental sampling, transport media may contain neutralizers to conteract any antimicrobial compounds that may have been transferred to the swab from the surface under test.
Swab designs have evolved to accommodate a range of applications. ATP-free swabs were developed to facilitate the technique in non microbiological hygiene tests. RNase and DNase free swabs allowed better recovery of nucleic acids for forensic molecular techniques and PCR work. Presentation also developed to include multi-layer packaging with VHP resistance for pharmaceutical cleanrooms.
Swabs with sponge/foam heads are widely used for industrial applications. These are often pre-moistened with media for improved recovery and can be used for pre-enrichment and incubation within the transport tube, commonly presented as Swab Rinse Kits (SRK).
The most recent innovation has addressed the fundamental weakness in the design of swabs, i.e. release of collected sample. There is a recognition within the microbiological community that compacted, woven swab tips can retain too much of the collected sample. This would impact on the validity of quantitative data reported in cases where the sample was collected by swab, especially where the sample size would be known to be small, e.g. ATP and some clinical infections.
Typically between 30%-50% of the collected sample would be released by traditional swabs.
The new nylon flocked swab design has oriented strands of nylon arranged perpendicular to the shaft creating micro-capillaries that not only improve sample collection but also release that sample more efficiently. Figures above 90% have been reported.
With release efficiencies at this level the swab technique can now take its place among the truly quantitative analytical techniques and together with the use of liquid transport media, swab samples can now be processed using fully automated instrumentation.
Sample release efficiencies (>90%) allow truly quantitative results. Strengthening trending data, and allowing meaningful spot checks for hygiene applications.
Better sample collection efficiencies mean less invasive clinical applications.
Better reproducibility for low target sample environments, e.g. pharma applications.
Automated processing of swabs allows better laboratory productivity.
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