An Investigation Into the Recovery Rate of Different Swab Types

The term 'microbial recovery rate' refers to the efficiency with which a swab collects and yields microbial growth from surfaces during environmental monitoring to ensure the production area is free from contamination.

We see that different swab substrates can demonstrate variation in recovery rates, largely influenced by the material composition of the swab. Environmental factors, including the type of surface being swabbed, also play a critical role in the performance of swabs.

We wanted to learn more about this. So, to investigate the variable performance, we performed an internal study in our quality control laboratory.

Bacillus subtilis spores were used to create a stable controlled contaminated surface. We swabbed a standard (25cm³) area of contaminated benchtop, filtered the sample elution through a membrane filter, transferred the filter to a media cassette, and began incubation in the Growth Direct^® System for 3 days at 30-35°C.

For this study, we used traditional rayon, fibers manufactured from wood pulp, and two types of polyester swabs, manufactured from synthetic material. Flocked polyester is a unique swab design claimed to exhibit improved microbial recovery. In addition to testing the performance of swab types, we also studied the microbial recovery rates on rough versus smooth benchtop surfaces.

Here’s what we found.

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How the swabs stacked up

Table 1: CFU recovery rates (%) of rayon, traditional polyester and flocked polyester swabs when sampled from smooth versus rough benchtop surfaces.

Our results (Table 1) highlight a marked difference in microbial recovery between swab tip materials, with rayon swabs displaying a substantially lower microbial recovery rate versus polyester swabs. In line with the literature and manufacturer claims, we found that flocked tips improved swab performance yielding twice as much microbial organism recovery than traditional polyester.

We noted further differences between the different sampling surfaces, with rayon tips showing wide variability in CFU recovery between rough and smooth surface sampling. Both traditional and flocked polyester swabs showed insignificant fluctuations in microbial recovery between rough and smooth surfaces.

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Conclusion

These findings emphasize the need to be mindful when interpreting microbial enumeration data from swab sampling. The results show significant variability in microbial colony enumeration depending on swab fiber type. The unique fiber design of the polyester swabs showed superior microbial recovery compared to traditional rayon and polyester swabs.

Additionally, the variability observed in microbial recovery between different surface types—especially noted with rayon swabs—highlights the need for careful consideration of the sample test surface when selecting swabs for environmental monitoring.

If swab sampling is part of your environmental monitoring program, we suggest evaluating swab fiber options, as well as using contact plates when applicable, taking into account the potential variation in microbial recovery.

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