air sampling for the food, pharmaceutical, clean room industries

Technical Data < The Science of Sampling

There are four cardinal premises in environmental microbiology. Understanding the rationale of these premises will have a significant and positive impact on our choice of sampling techniques, our approaches to analysis, and, ultimately, accuracy in our interpretation of data.

The first, and probably most important, premise: Most microbes do not survive well outside of their natural environment or growth site. For instance, a human pathogen that thrives in the gut, where it has an enriched, warm, moist environment, does not do well on a dry, cool surface or in low-humidity air. Organisms found in the latter types of environments are usually stressed. We therefore expect to find a greater number of microorganisms associated with humans in a room with a lot of activity and considerably fewer—and different types—of organisms in a room that has been vacant for some time.
Premise 2: Microbes are found everywhere, in virtually every environment, both natural and man-made. Therefore, when sampling, expect to find non-target organisms that may be far more robust than those you are looking for. Expect the unexpected.
Premise 3: There is no uniformity of distribution. Different environments differ with respect to quality and quantity of microbes. The greater the proximity to an aerosolizing source or portal of exit (if you are dealing with the chain of infection), the greater will be the number of microorganisms. The same holds true for growth conditions. More microbes will be found where conditions favor their growth and reproduction. Because microorganisms have mass, they behave as particulates. More organisms will be on horizontal than on vertical surfaces, and more at the floor than at the ceiling.
Finally, Premise 4: Each environment can be considered a separate biosphere, each with a characteristic bioburden. Therefore, we need to estimate the bioload, both qualitatively and quantitatively, for each environment or portion thereof we want to sample or evaluate. The estimate will guide us to the appropriate collection and analytical methodology, and serve as a template for interpretation of the data.

Microbial air samplers are characterized by mode of capture, flow rate and flow characteristics, and collection efficiency as a function of particle size and shape. As a rule, aerosol collection devices that exhibit the lowest shear forces collect samples in which microorganisms have the highest viability. Conversely, these samplers usually have the lowest physical efficiencies in terms of numbers of airborne particles collected. Therefore, the efficiency of microbiological collection depends largely on the sampling method used.

The primary objective of any sampling program is to produce a set of samples that are representative of the source under investigation and that are suitable for subsequent analysis. Because the air is not homogeneous in any environment, there can be no duplicate samples. We therefore need to consider sampling conditions, sampling time, and sample size as limitations in our data collection scheme.

To further complicate this issue, consider the following: Air is not a natural environment for most microbes. Survival of microorganisms in air is affected by a large number of environmental factors, the most important of which are temperature and humidity. Under natural conditions, these numerous factors operate simultaneously. Consider also that force is required to generate an aerosol and, likewise, to capture particles within that aerosol. These forces can damage or even fracture fragile structures such as microbes in their vegetative state. The fragile nature of airborne microorganisms is largely species dependent and is determined by physiological condition. Once airborne, microbes become stressed through desiccation or hydration, depending upon the condition of their natural growth site. Radiation, oxygen, ozone, and various other gaseous and particulate pollutants, if not lethal, may further stress the organisms. Some stressed and injured microorganisms may, however, fully recover when given a suitable environment. This property of reversible injury or repair in microorganisms is widespread, and the implications of it are important in development of the testing protocol.

The outcome of a well-planned sampling strategy depends on good science, logic, and, to a lesser degree, a measure of good luck. Taking the time to estimate the types of organisms that may be present; describing the static, dynamic, and physical characteristics of the area under test and its air; and conducting a viable/nonviable–particulate profile of the space to be sampled will yield data that become the basis for the entire microbiological sampling scheme.