A Primer on Sampling for Biological Contaminants-Part 3: Surface Sampling Equipment and Techniques. (Tools for Environmental Health)

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There are certain topics and technologies relating to our profession that everyone seems inherently to know everything about; surface sampling is one of them. Still, without being too harsh on ourselves, we find that, when faced with the task of performing a microbiological or particulate surface survey, and, given the choice between the right and wrong way of doing things, a lot of sanitarians somehow gravitate toward the latter. If that's the case with you, you are not alone. We know that all too well, having learned from experience.

This part of our practice has never been more important. Microbial contamination has recently taken on a new significance for two reasons. Immediately after 9/11, the demands on those of us who work at local health departments to sample white powders, which might or might not have contained Bacillus anthracis spores, had us scrambling. The second reason is that for those of us who work in food quality control, the attempt to comply with the zero Listeria tolerance has had us searching for an easy method with good repeatability. So, with commercial kits freshly designed for the task, we take our samples and rely on our laboratorian to sort things out-hoping for the best.

Sampling Rationale

To help put a rational spin on this topic, let us first say that surface sampling for microbial contaminants is as diverse and as straightforward as air sampling. The method and equipment we choose should be predicated on the specific situation under evaluation. For instance, the methodology should take into account the types of microbial contaminants expected, predicted conditions of survival and dormancy, and their estimated concentration. The factors that influence the selection of the sampling tool are primarily the geometry (curved, flat, grainy, smooth, etc.) and the chemical condition (disinfectant residues, oils, salts, etc.) of the surface to be sampled. Also, somewhere in this process, we need to provide guidance to the laboratory so that the technician may select an analytical method that complements the objectives of the sampling test.

As with any sampling, some structured protocol should be followed. Ideally, random sampling eliminates the greatest bias. With surface sampling, a random scheme consists of laying out a grid pattern and following the statistical outline given in ANSI/ANSQC Z 1.4-1993. If, however, the contamination site is known, or the purpose of the sampling is to determine the degree of decontamination achieved by a cleaning and disinfection, either judgmental sampling methods (clean versus soiled) or stratified sampling methods (timed sequence or measuring the degree of cleanliness) may be considered, with appropriate controls.

Sampling Methodology

Surface sampling consists of four basic methods for enumerating microorganisms: contact plates, swabs and wipes, direct surface agar plating, and rinsing and vacuum collection.

Contact plates and swab/wipe techniques are both the easiest and the most frequently used. Contact plates work best on smooth, flat surfaces, but with some practice, they also can be used on surfaces with a slight curvature. Swabbing with a sterile cotton-tipped applicator or sponge is most useful for sampling small inaccessible areas to estimate gross contamination. Direct plating, rinsing, and vacuum recovery have limited application except where the geometry of the surface precludes the use of the other methodologies or whole-surface testing is necessary.

Contact Plates

Contact-plate sampling involves pressing a solidified nutrient agar surface against the sample surface. The plates are then incubated and a direct microbial count taken. The most widely used contact samplers are Rodac(r) and Hycon(r) plates. These surface samplers are designed so that the meniscus of the agar is raised slightly above the container, enabling ease of contact with the surface. …