Surface sampling methods for Bacillus anthracis spore contamination

Abstract

During an investigation conducted December 17-20, 2001, we collected environmental samples from a U.S. postal facility in Washington, D.C., known to be extensively contaminated with Bacillus anthracis spores. Because methods for collecting and analyzing B. anthracis spores have not yet been validated, our objective was to compare the relative effectiveness of sampling methods used for collecting spores from contaminated surfaces. Comparison of wipe, wet and dry swab, and HEPA vacuum sock samples on nonporous surfaces indicated good agreement between results with HEPA vacuum and wipe samples. However, results from HEPA vacuum sock and wipe samples agreed poorly with the swab samples. Dry swabs failed to detect spores >75% of the time when they were detected by wipe and HEPA vacuum samples. Wipe samples collected after HEPA vacuum samples and HEPA vacuum samples collected after wipe samples indicated that neither method completely removed spores from the sampled surfaces.

Figure 1

Sample instructions for collection of swab, wipe, and HEPA vacuum sock samples, Brentwood Mail Processing and Distribution Center, 2001. For specific location, investigator was given these instructions (exact text follows). Divide the selected space into three sections where each of the three types of surface samples (swab, wipe, HEPA vacuum sock) may be collected. Follow the random key above to designate which section will be sampled by each method and in which order the samples will be collected (follow top to bottom). Record the area of surface sampled by each method. The surface areas need not be equal, but should be sufficient to provide adequate sample collection for each method. Sample order for location was: 1) Collect the HEPA vacuum sock sample first and record surface area. After sampling, clean vacuum nozzle with alcohol and insert clean vacuum sock; remove this sock without sampling to serve as “contamination blank.” 2) Collect the WIPE sample second and record surface area. 3) Collect the SWAB samples third and record surface area. The first swab sample should be collected without moistening it. The second swab sample should be sampled pre-moistened. Take care not to overload swabs. 4) Collect an additional WIPE sample across the entire area which had been sampled by HEPA vacuum sock. 5).Collect an additional HEPA vacuum sock sample across the entire which had been sampled by WIPE.

Figure 2

Photograph of HEPA vacuum cleaner and sock sample.