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Two species of microorganisms, Micrococcus luteus NBRC13867, a vegetative Gram-positive coccus, and Bacillus atrophaeus ATCC 9372, a spore, were selected as test organisms. These microorganisms were selected because they are representative of common human
associated organisms, which are often Gram-positive cocci, and also of spore-bearing organisms that may enter on supplies
or apparatus that might not have been properly decontaminated. Small stainless-steel coupons were used as carriers of the
test microorganisms. The carrier surface was polished to 320 grit and a predetermined number of CFU was inoculated on the
polished surface of the carrier. A metal surface finish of 320 grit was chosen because it is broadly representative of a level
of polish applied to critical surfaces of the filing and component feed machinery. In general, the surfaces of isolator enclosures
are given a similar surface finish. The inoculated carriers were air-dried under HEPA-filtered uni-directional airflow.
Table II: Dry swab efficacy (robot).
After some preliminary experimentation with commercial swabs (Becton, Dickinson and Company in Franklin Lakes, NJ, and ITW
Texwipe in Mahwah, NJ), the authors developed specifications for a swab apparatus that was then custom made. After test operation,
all swab rods were incubated in Soya Bean Casein Digest broth at 30° C for two weeks and then inspected for the presence of
colonies indicative of microbial growth. The relatively long incubation time was chosen to ensure maximal recovery.
Swab sampling conditions
Table III: Wet swab efficiency with distilled water as wetting agent (robot).
The swab sampling experiments were conducted using the test parameters listed in Table I. The authors hypothesized that when
the wet-swab method was used, the microbial recovery rate might vary as a function of moisture content in the swab tip. Therefore,
two moisture levels were evaluated in this study. To ensure a very moist swab in one study, the tip was moistened immediately
before use. To achieve a higher moisture level, swabs were soaked one day before use to ensure the full saturation of the
swab tip.
Results
Table IV:Wet swab efficiency with distilled water 10.1%Tween80 as wetting agent (robot) .
When the dry-swab method was used, the observed recovery (CFU) correlated well with the inoculation level for B. atrophaeus spores. No M. luteus colonies were observed after incubation. A high swabbing force resulted in better recovery of spores (see Table II). In addition,
the wet method, in which distilled water was used as the wetting agent, gave much better recovery efficiency than was observed
using dry swabs. Sufficient time for absorption of water into the swab tip resulted in more efficient recovery. In addition,
recovery efficiency was significantly affected by the speed at which the swab moved across the surface being sampled (see
Table III).
