Quantitative Microbial Results on Environmental Swabs

What are Quantitative Microbial Results on Environmental Swabs?

Quantitative microbial results obtained on environmental swabs are actual numbers of bacterial and fungal organisms which exist in a given environment.

Environmental swabbing of production facilities occurs for various, including regulatory requirements, risk management, sanitation verification, routine monitoring, investigations, and customer requirements.  Since the purposes for collecting environmental swab data are vast, the related interpretation of the data is varied as well.

For example, it would typically be most appropriate to analyze environmental swabs for APC (aerobic plate count) if sanitation verification is the goal.  If, however, an investigation is being conducted due to product spoilage, it may be more appropriate to analyze environmental swabs for a particular spoilage or indicator organism in addition to or instead of an aerobic plate count.  Indicator organisms include, but are not limited to, yeast and molds, coliforms and E. coli, Pseudomonads, Enterobacteriaceae, Staphylococcus aureus, Lactic Acid Bacteria.

MBL provides environmental swabs to Customers per request which contain 10 mL of a buffer to neutralize any sanitizers that may be present.

After receipt at the laboratory, the environmental swab is homogenized by mixing, 1 mL from the original 10 mL is extracted and placed onto an appropriate agar plate.

MBL may further dilute environmental swabs known or expected to have high counts.

MBL calculates the final quantitative microbial result assuming that an area of 100 cm2 was sampled according to the following equation:


Colonies present on agar x 10mL neutralizer in swab

100 cm2 (area swabbed)

One hundred CFU/cm2 is 4″ x 4″.  MBL can provide templates upon request.

We recognize that other surface areas may have been swabbed instead of 100 CFU/cm2.  For example, some producers routinely swab only 50 cm2 or 25 cm2.

We have devised this resource to help you understand how to interpret environmental swab data and how to retroactively calculate the true cm2 value if you swabbed an area different than 100 cm2.

Calculating the Entire Population (CFU/Swab = CFU/100cm2)

Column A in Table 1 below highlights in grey examples of MBL reported results per 1 cm2 based upon an intial swabbing area of 100 cm2.

Column B represents the quantity of bacteria present in 1 mL from the original 10 mL sample.  This is the actual number of colonies present on the agar dish.  Since the sample is homogenized, what is found in 1 mL can be multiplied by 10 to extrapolate the total population present in 10 mL.

In order to extract a result which reflects the entire population present in environmental sponge or swab (Column C), giving you a CFU/swab or CFU/100 cm2 result, simply multiply your result by 100 (move the decimal to the right by two positions).  In other words, this represents the entire bacterial population in the initial cm2 swabbing area.

Env Swab Micro Table

Retroactively Calculating CFU/cm2 for Other Areas Sampled

If you do not swab 100 cm2, and want to calculate the true CFU/cm2 result based upon the area you did or do swab, you may do so.

First perform the above extraction to obtain the CFU/swab or CFU/100 cm2 result.

Then simply divide by the area you did swab.

Column D shows the comparative calculation of 50 cm2 and Column E shows the comparative calculation of 25cm2.

ND, not detected, means that the analyte of interest (the specific type of bacteria the sample was analyzed for) was not detected.  This does not necessarily mean the area swabbed was sterile; it is still possible that some bacterial cells of interest did exist in the area swabbed, but were below the detection limit of the method.  It is not appropriate to report “0” as a result; instead the result is result is reported as a function of the detection limit (i.e. <0.1 CFU/cm2).

Interpreting the Results

The primary goal of environmental swabbing is to identify possible problems, no matter what area you sample.  Limiting sampling to specific, measureable surface areas can be counterproductive to the primary goal.  The intent of these guidelines are not to place an overt amount of attention on the area sampled.

Depending upon your needs and your purposes for conducing environmental swabbing, the area you swab is not nearly as important as the results obtained, as long as you know the relative area sampled and are able to make sense of the corresponding results.

Producers should compile baseline data from multiple iterations of environmental swabbing data to establish what the process is currently capable of.  Then targets can be set for improvements based on realistic changes in practices, performance and equipment.

Pass/Fail Limits

There are various guidelines as to what are and what are not acceptable quantitative microbial results obtained on environmental swabs.

We have given some comparatively liberal limits in Table 1 above.  These limits may or may not be appropriate for your situation.

Limit 1) would be the maximum allowable contamination of indicator organisms observed after sanitation.  Limit 2) would be the maximum allowable contamination observed after sanitation for the total microbial population.

Over time, a company should set their own pass/fail limits based upon baseline and historical results, with the goal always being towards improvement.

High, out-of-spec results should not be ignored.  Corrective actions should be implemented, followed by resampling and verification of corrective action effectiveness.