When the wetting agent was changed from distilled water to mixture 0.1%Tween 80 in distilled water, the rate of recovery of
M. luteus improved, but complete recovery could not be achieved. Increasing the number of back-and-forth swabbing motions actually
resulted in a decrease in recovery rates (see Table IV). A comparison of manual sampling to robotic sampling under the same
test conditions showed a lower recovery rate associated with manual swabbing. The recovery efficiency observed with manual
swabbing was approximately 20%. Additionally, far more variability in recovery was observed with manual swabbing as compared
with robotic swabbing (see Table V).
Wet-swab sampling often is avoided in aseptic operations because of concerns about residual moisture that might encourage
microbial proliferation. However, the experimental data clearly confirm that moistened swabs are much more effective in terms
of contamination recovery. We observed that using surfactant Tween 80 resulted in improved recovery as compared with using
plain distilled water. It has been reported that dry, agglomerated microorganisms can strongly adhere to surfaces; therefore,
it is logical that moistened swabs, particularly with the addition of a surfactant, can recover this contamination from surfaces.
Given the benefits of using moistened swabs, surface monitoring could best be executed just after the completion of an aseptic
production operation or in advance of breaking the aseptic condition of the isolator or RABS. As demonstrated in Table IV,
even when using the robot for sampling, better efficiency of recovery was observed with B. atrophaeus than with M. luteus.
The most significant difference between these two test organisms is that M. luteus is a vegetative cell and B. atrophaeus is a spore. The sizes are also different. The size of the former is ~0.7–0.8 µm, and the latter has a diameter of ~1 × 2–3
µm, although it is unknown to what degree the size of the organism plays a role in recovery efficiency. Consequently, the
authors believe the primary considerations in recovery efficiency differences between these two organisms are: spores as compared
with vegetative cells, and the agglomeration and adherence to the test coupon. The authors also believe the physical composition
of the swab tip may play a role in recovery efficiency. Additional investigation is needed to determine which factors have
the greatest influence on recovery rates.
Recovery efficiency was influenced by the time allowed for soaking the swab tip with the wetting agent. Wetting the swab immediately
before use does not appear to allow sufficient time for saturation to occur. The authors believe that wetting immediately
before use results in a swab that is completely dry before the completion of the swabbing action and therefore less efficient.
In this case, the full benefits of wet swabbing are not realized. When the swab tip is allowed to soak overnight before use,
however, the swab retains moisture from beginning to end of the swabbing action. The fully saturated swab gave a superior
recovery efficiency of microorganisms. Therefore, the degree of wetness of the swabs plays a critical role in recovery efficiency.