Hygiena International wraps up its series of weekly articles that discuss monitoring microbiological contamination on surfaces. This week's fourth and final piece provides insight into the topic of 'Measuring swab performance'.
There are many factors affecting the recovery of bacteria from environmental surfaces that have a significant impact on detection and method validation. Most researchers measure what has been collected on the swab; however an alternative approach is to measure what is left behind on a surface.
The ‘swab and rinse’ method for measuring environmental contamination has remained relatively unchanged since 1917.
As discussed in previous issues, there are a large number of variables that affect swab methods giving low recovery and detection rates of 1-25%. Enumeration is therefore very imprecise so as to be virtually meaningless, particularly at low contamination levels.
Integral to the overall recovery of contamination is the release of bacteria from the swab so as to become available to the final detection method. Bacteria will bind to the material of the swab bud (>50%) and not be released although some modern materials may have a lower retention rate. The greater the perturbation of the swab in the rinse solution, the more contamination is liberated from the swab material but it is rarely 100%.
Methods for environmental pathogens frequently use sponge swabs where the release of contamination from the swab itself is of less importance provided that the whole swab and wetting solution are part of the overall test procedure.
Method validation studies for pathogens of surfaces usually require inocula to be dried on to surfaces, however this causes a large loss of viability (typically 4-6 logs). Consequently, the actual residual inoculum is not known (or no attempt is made to measure it because it is too difficult), and factorial design is used to compare detection methods in a presence/absence test format. A model system was designed to assess and differentiate swab sample recovery from the overall detection methods by measuring the contamination on the foil surface before and after swabbing. An inoculum dried on to aluminium foil was used as the surface material and two surface test methods were compared.
MicroSnap Surface Express (MSX) is designed to swab large areas (up to 12” x 12”) and to retain the sample on the swab for subsequent enrichment and detection of the entire sample. MSX was compared to contact plates.
The table below shows the results of five replicate tests for each method compared with the control (no swabbing) and conducted several times over a nine day period, and demonstrates the efficacy of the MSX device.
Swabbing a large surface area requires a swab bud that is premoistened with sufficient swab wetting agent to cover the desired surface area to collect a representative sample, and is convenient to use. This does not necessarily mean it has to be a large foam swab with associated volumes of liquid.
A high swab capture and recovery rate together with 100% detection on the swab with a rapid end point detection method is a major step forward for microbial environmental measurement.